While the cat's away...

[azure]

It goes into heat created by the resistance in the cell, also the mineral deposits will build "bridges" between the plates in the cells creating a short circuit. Whether or not this is detected on the Ammeter depends on how the system is designed.

My copy of the airplane's circuit diagram is (was) tucked into the service manual -- my chances of finding it sometime in the next week are next to nil as it's buried somewhere in the disaster my mechanic left in my hangar after he finished my annual. Anyway this is from memory, but I'm pretty sure the circuit is designed to detect if there's current passing through the battery. Granted it's never a good idea to put blind trust in Cessna's gauges -- but I don't see any way to directly check the ammeter. So while what you're saying makes sense, I don't see how I can be sure it's not the charging system that's to blame for allowing sulfate to build up by never developing a high enough voltage to fully charge the battery (what I meant by "abusing").

I don't have a known good battery to try as a test, and I hesitate to keep applying the "throw money at it" method and just replace the battery, when if I don't know the source of the problem I can't be sure the new battery won't end up in the same condition as this one in a few months.

Anything I should know about electrolyte checking lead acid aircraft batteries? (safety-related that is)...

 

[Henning]

My copy of the airplane's circuit diagram is (was) tucked into the service manual -- my chances of finding it sometime in the next week are next to nil as it's buried somewhere in the disaster my mechanic left in my hangar after he finished my annual. Anyway this is from memory, but I'm pretty sure the circuit is designed to detect if there's current passing through the battery. Granted it's never a good idea to put blind trust in Cessna's gauges -- but I don't see any way to directly check the ammeter. So while what you're saying makes sense, I don't see how I can be sure it's not the charging system that's to blame for allowing sulfate to build up by never developing a high enough voltage to fully charge the battery (what I meant by "abusing").

I don't have a known good battery to try as a test, and I hesitate to keep applying the "throw money at it" method and just replace the battery, when if I don't know the source of the problem I can't be sure the new battery won't end up in the same condition as this one in a few months.

Anything I should know about electrolyte checking lead acid aircraft batteries? (safety-related that is)...

Well then the ammeter would not detect it then because the energy is being lost inside the battery and never makes it through. It gets changed to heat.

As for checking the battery acid, safety glasses are a good thing and mix up a liter of water with 1/4 cup of baking soda to pour on whatever you may spill/squirt/dribble the acid on. Shouldn't happen though.

As for not "just throwing money at it" I agree which is why I suggest dropping the battery a couple of times. It has an incredibly good effective rate of getting a battery working again as it breaks the bridges and knocks some of the mineral accumulation off. This is a temp fix though because you still have these piles of crap in the bottoms of the cells that will eventually bridge plates again. It's a "get home" technique for the most part that I also use it as a partial test (a positive result is proof, but a negative one is not a clearance) in diagnostics since it's so quick and easy to do. Drop it a couple times, battery now takes a proper charge, "yep, that was a problem" costs nothing.

You may as well try using one off the desulfinator charger/maintainers Bruce was pointing out since you want one regardless, they are good and you should use one whether it fixes this problem or not. Get one and plug it in for a few days/week and see what happens, it may help.

 

[Tom-D]

Well then the ammeter would not detect it then because the energy is being lost inside the battery and never makes it through. It gets changed to heat.

Not true, other wise the battery would boil. her's isn't.

As for checking the battery acid, safety glasses are a good thing and mix up a liter of water with 1/4 cup of baking soda to pour on whatever you may spill/squirt/dribble the acid on. Shouldn't happen though.

As for not "just throwing money at it" I agree which is why I suggest dropping the battery a couple of times.

DO NOT do that..if your battery has sulfided from setting, dropping it will do no good, and you run a 75% chance of breaking a circuit connection inside the battery. simply do a high rate discharge and a recharge of the battery by a battery shop.

It has an incredibly good effective rate of getting a battery working again as it breaks the bridges and knocks some of the mineral accumulation off. This is a temp fix though because you still have these piles of crap in the bottoms of the cells that will eventually bridge plates again. It's a "get home" technique for the most part that I also use it as a partial test (a positive result is proof, but a negative one is not a clearance) in diagnostics since it's so quick and easy to do. Drop it a couple times, battery now takes a proper charge, "yep, that was a problem" costs nothing.

that's a great way to destroy the battery and give your self an excuse to buy a new battery.

You may as well try using one off the desulfinator charger/maintainers Bruce was pointing out since you want one regardless, they are good and you should use one whether it fixes this problem or not. Get one and plug it in for a few days/week and see what happens, it may help.

I'd simply go flying and after about 30 minutes, turn off the alternator side of the battery master and turn on the landing light, wait 30 minutes and turn the landing light off and the alternator back on, and fly until the amp meter shows a very small rate of charge.

and simply fly more often.

 

[Henning]

Not true, other wise the battery would boil. her's isn't.

Your evidence of that is?

that's a great way to destroy the battery and give your self an excuse to buy a new battery.

I've never broken one, it's gotten me home more times than not, there is nothing to lose as "destroying" a junk battery is like stabbing a corpse. If here battery is reading 12.3v at the terminals with a charge on it, it's junk and needs replacing anyway. I only suggest it as a way for her to do a quick verification test that may (or may not) work. If I wasn't stranded and I tested my battery at 12.3 volts on a full charge, I'd just go buy a new battery cause this one's junk.

 

[bbchien]

Liz, I wouln't be dropping any batteries. However, you may have TWO problems- the battery and the charging system. First thing to do is repair the battery while it can be repaired- DESULFATE it.

The second is to record bus voltage in flight.....

 

[TMetzinger]

Agreed - a bad battery will make your charging system look bad as it constantly tries to charge an unchargeable load. When you have a known good battery, then you can evaluate the charging system.

 

[Henning]

Liz, I wouln't be dropping any batteries. However, you may have TWO problems- the battery and the charging system. First thing to do is repair the battery while it can be repaired- DESULFATE it.

The second is to record bus voltage in flight.....

I suspect for the length of time she's had this problem plus the sitting it did, she may be beyond the point of that working, but as I said earlier It's certainly worth a try. She should buy one to maintain her next battery on anyway so it's not like it's a one time use she'll be buying it for. I give it a 50% chance of reviving this one.

Lets see what the Fluke meter tells us of the voltage at the terminals first. We could simply be losing half a volt in resistance somewhere in the buss, loose connection starting to grow corrosion....

Without establishing the independent state of the battery static voltage, we cannot begin diagnostic. Remember, all my suggestions have been predicated on the insistence that the battery voltage was accurate at 12.3 just after receiving a full charge and subject to a light load. That is a bad battery.

At this point I am not sure anything is bad with the charging system or battery as I can't recall ever hearing about the system failing yet, just these strange, just below what they should be, readings.

It's a lot easier to do this in front of the airplane. When I ask repeatedly if a specific fact is a specific fact and I am told yes repeatedly, I take that at face value that it's a fact and work from there. If however the battery test at the terminals shows 13.2, then it's time to find the resistance in the buss and meter circuits. Mechanics is no different than medicine, you look for specific indicators from specific places and keep ruling things out until you have your fit.

 

[Tom-D]

Your evidence of that is?

The lack of her complaining of a mess in her battery compartment.

I've never broken one, it's gotten me home more times than not, there is nothing to lose as "destroying" a junk battery is like stabbing a corpse. If here battery is reading 12.3v at the terminals with a charge on it, it's junk and needs replacing anyway. I only suggest it as a way for her to do a quick verification test that may (or may not) work. If I wasn't stranded and I tested my battery at 12.3 volts on a full charge, I'd just go buy a new battery cause this one's junk.

Until you have complied with the correct trouble shooting procedures, you can't be sure of any thing.

 

[azure]

Okay, I measured the battery terminal voltage directly with the Fluke. Open circuit is 12.6. With the master on but bare minimum load, 12.3. Drops to 12.15-12.2 with the beacon. The JPI agrees within 0.1 volt. I didn't have the time to go shopping for safety glasses so I didn't test the electrolyte. It's a Gill G-35. I couldn't find the spec for fully charged cell voltage on Gill's site, but I gather that's low for any lead acid battery. So the battery is sulfated up. My question is why.

Folks seem to be making a lot of assumptions here about my flying habits! FYI the plane doesn't sit around very much, at least not normally. Since I bought it, I flew over 130 tach hours between April 2010 and end of March this year. OF COURSE it hasn't flown since then -- it was down for annual, then the corroded spar problem kept it grounded. But during that time my mechanic kept the battery on charge, and it was fully charged by the Benz crew. The ONLY time the battery may have sat in the plane for a long time discharging was immediately after it was installed new in 12/09. The airframe log shows tach time 4.5 hours at that point, and when the new engine was installed and signed off at the end of March it was 4.9. Would a new battery sulfate up that quickly? The only other reason I can think of is a charging system problem.

I also started the engine. I rely so much on the JPI that I never really paid attention before, but the ammeter definitely shows a charge for a couple minutes after start. So the charging circuit seems to work, at least at first. Then the bus voltage drops to 13.3-13.4 and who knows whether the battery ever gets fully charged.

Bruce, thanks for the battery minder suggestion, that sounds like a really good idea for my NEXT battery and maybe something to try now. The one you linked to is obviously non-aviation... is that the one you use? Did you have to make up a connector for your battery or is it plug and play?

 

[Tom-D]

I also started the engine. I rely so much on the JPI that I never really paid attention before, but the ammeter definitely shows a charge for a couple minutes after start. So the charging circuit seems to work, at least at first. Then the bus voltage drops to 13.3-13.4 and who knows whether the battery ever gets fully charged.

That system is working as it should. the regulator is set a little low but it should be no problem.

 

[Henning]

Okay, I measured the battery terminal voltage directly with the Fluke. Open circuit is 12.6. With the master on but bare minimum load, 12.3. Drops to 12.15-12.2 with the beacon. The JPI agrees within 0.1 volt. I didn't have the time to go shopping for safety glasses so I didn't test the electrolyte. It's a Gill G-35. I couldn't find the spec for fully charged cell voltage on Gill's site, but I gather that's low for any lead acid battery. So the battery is sulfated up. My question is why.

Which is the same as it read after having the full charge right? Most batteries without an anti sulfation unit are only good for 2-3 years. Occasionally you get and get more, some times you lose and get less.

You can try putting an anti sulfation unit on it and bringing it back, nothing to lose for trying.

 

[gismo]

Most likely that is a "cold" rating, they lose significant capability as they heat up which is ok since they only need to keep up with the max load after they recharge the starting loss. As long as the system doesn't go below 13.2 volts and increases as you shed load, you're doing alright. You really don't want any more voltage than 13.2-13.4 if you don't have any charging needs as it just is extra wear on the battery. Wet Lead Acid battery cells (the kind you put water in) are 2.2 volts so at a full charge rests at 13.2. If these indications aren't something that is new and different, I would not be inclined to be too concerned just yet. These systems are designed to be as light and cheap as they could get away with. It's a Cessna, not a Rolls Royce....

Henning, while 13.2 might be the correct voltage for a long term float charge, a lead acid battery needs more than that to get "topped off" so in virtually every GA aircraft with a lead acid battery the charging system is set to produce 13.8-14.2 volts at all times. If the bus voltage is always below that the regulator is mis-set but if it reaches that when lightly loaded but drops off with less than rated output current (assuming the RPM is sufficient) something is still amiss.

 

[Tom-D]

Henning, while 13.2 might be the correct voltage for a long term float charge, a lead acid battery needs more than that to get "topped off" so in virtually every GA aircraft with a lead acid battery the charging system is set to produce 13.8-14.2 volts at all times. If the bus voltage is always below that the regulator is mis-set but if it reaches that when lightly loaded but drops off with less than rated output current (assuming the RPM is sufficient) something is still amiss.

The battery is being checked with the master on and the engine not running, 13.2-13.8 is normal.

remember the battery is holding the master solenoid closed and that plus other circuits will put the voltage down.

 

[azure]

The battery is being checked with the master on and the engine not running, 13.2-13.8 is normal.

With the master on and the engine off, I was getting 12.1-12.6. AFTER starting the engine, I saw a gradual drop from 13.8 to 13.3. Some of that was certainly the avionics displays warming up. But in cruise, 13.2-13.4 is now common.

Richmor Aviation remembers seeing 13.8-14.0. I'm not sure, though, whether that was with the avionics master on. I see 13.8 too, if I leave the avionics off.

remember the battery is holding the master solenoid closed and that plus other circuits will put the voltage down.

 

[Tom-D]

With the master on and the engine off, I was getting 12.1-12.6. AFTER starting the engine, I saw a gradual drop from 13.8 to 13.3. Some of that was certainly the avionics displays warming up. But in cruise, 13.2-13.4 is now common.

Richmor Aviation remembers seeing 13.8-14.0. I'm not sure, though, whether that was with the avionics master on. I see 13.8 too, if I leave the avionics off.

I'd fly that. How old is the battery?

 

[bbchien]

Liz, they are plug and play. You put the ring terminals of the fused harness under the bolts that an aircraft battery uses, and plug it in whenever it's in the hangar. Actually my bus is set to 13.9 so we don't boil the electrolyte in the air, and it tops to 14.2 at 40 mah every hour it sits in the hangar- on peizo pulses.

7 years is pretty good. That battery still passes the mfr's ICAW test, each spring.

 

[Ted]

Liz, overall I'd say this whole deal worked out pretty well for you since the insurance covered almost all of the cost. The downtime is inevitable with aircraft.

When my Aztec's right engine mount broke a few months back, I had to pay for all the labor (40+ hours worth), mailing the unit out to the rebuilding shop (which was not cheap), and then pay the $2000 for the rebuild. I was down for close to a month.

Major maintenance issues are never fun. Unfortunately, they're inevitable. Especially with aircraft that are several decades old or more.

 

[Dan Thomas]

Getting the mice dead as soon as they show up is the only solution to keeping them out of your airplane. I've seen so many methods to try to keep them out, but nothing works for me like bar bait. A few chunks of the stuff placed near the wheels and elsewhere around the hangar will catch them and they'll eat a bit and go somewhere else to die. I very rarely find a dead one in the hangar, and even then the poison dehydrates them so much they don't stink.

Keep the stuff away from kids. It looks too tasty. If the airplane was outside you'd want a sturdy, locked box sitting under the airplane with small holes for the mice and the poison inside. The bait has to be kept dry, too, or it'll rot and won't attract them.

http://www.plumbersurplus.com/images/prod/1/Farnam3005449.jpg

I, too, have seen plenty of corrosion caused by both mice and birds. Nasty. Aluminum is one of the most abundant metals on this planet, but wasn't commercially available until 1920 or so and was pretty expensive. It takes a lot of electricity to separate it from the ore, and it can't wait to get back to where it came from. It reacts too easily with almost anything and starts heading home to the rocks again.

Dan

 

[azure]

My mechanic is finally back from wherever he was... said his Battery Tender works (I misspoke before, it's a Tender not a Minder) and gave the green light to hook the battery up to life support. He also admitted his load tester is not very good, so I don't put any stock in that result. He recommended just checking the voltage after the charger stops, and see how quickly it goes down by itself.

Absolutely Ted, it all worked out better than I could have imagined in fact. It just should have been caught at pre-buy, that's all -- or at the 2010 annual. I can only think that the previous mechanic got complacent because he thought he knew everything that was wrong with the plane. Meanwhile there was a time bomb sitting there -- and Tom got lucky that it didn't go off while he owned the plane.

 

[wabower]

Even though 180's aren't known for problems in this area, I found myself thinking about your headaches and had the guy give mine a good lookover during a windshield change last week. No problems found, but figured it would be the best time to look.

 

[azure]

You are confusing "voltage" being an indicator of "current". Current is amperage, call it the density of electrons, and voltage is the pressure it is being put through with. The alternator can only produce so much pressure differential. When the base value of the differential equation drops, so does the upper value.

Golly, I sure hope I'm not confusing voltage and current, after all I've taught a few students the difference.

No I'm saying that the only problem I can think of that would lead to reduced voltage under load only involves IR losses somewhere. How does a sulfated battery cause IR losses upstream from the bus? Or do you mean something else?

What differential equation is affected by putting a sulfated battery into the circuit?

 

[Henning]

Golly, I sure hope I'm not confusing voltage and current, after all I've taught a few students the difference.

No I'm saying that the only problem I can think of that would lead to reduced voltage under load only involves IR losses somewhere. How does a sulfated battery cause IR losses upstream from the bus? Or do you mean something else?

What differential equation is affected by putting a sulfated battery into the circuit?

Well, you did call voltage current...

The battery feeds the buss. Just get a good battery in it before you do anything else to your charging system and then see what the numbers are.

 

[gismo]

The battery is being checked with the master on and the engine not running, 13.2-13.8 is normal.

Maybe if the battery was under charge just before taking the measurement but even with no load other than the master contactor coil the voltage on a fully charged battery will drop to 12.6 or less very quickly.

remember the battery is holding the master solenoid closed and that plus other circuits will put the voltage down.

I though Liz said she sees the bus voltage drop to the mid 13v range with the engine running. At that time the voltage should be closer to 14v unless the load exceeds the alternator output.

 

[azure]

Well, you did call voltage current...

Where?

The battery feeds the buss.

With the alternator running and holding a higher voltage across it than the cells can produce?

Just get a good battery in it before you do anything else to your charging system and then see what the numbers are.

If the battery confirms as bad, that's certainly what I'll do. But if not... I'm not sure. My mechanic confirms that he never did replace the alternator wire, so that's pretty high on the to-do list too.

I took it off charge at 1300 this afternoon and measured 13.32 V. I'll take another measurement this evening.

 

[azure]

Maybe if the battery was under charge just before taking the measurement but even with no load other than the master contactor coil the voltage on a fully charged battery will drop to 12.6 or less very quickly.

I haven't tried this yet with the battery fully charged. At Ionia we ran the flaps up and down several times trying to set the partial extension angles right, and after that it was reading 11.5. Back home after letting it sit for a couple of days, I was seeing just what you say though -- 12.6 with minimal load, 12.3 dropping even lower with the beacon.

I though Liz said she sees the bus voltage drop to the mid 13v range with the engine running. At that time the voltage should be closer to 14v unless the load exceeds the alternator output.

Yes -- 13.4 typically with only nav + strobes, 13.2 more usually now with beacon + nav + strobes or even just beacon + strobes. The only time I see 14.0 is after engine start with the avionics off. And in flight, with beacon + strobes, it goes back up to 13.8 if I turn off the avionics master.

FWIW, this is not a Kelly rebuild alternator but a brand new Plane Power -- but the Kelly acted the same way last summer when it was still (apparently) healthy.

 

[Dan Thomas]

Take a voltage reading on the field terminal of the alternator or regulator. Start the engine. If the voltage goes up when you add load by turning on more lights, the regulator is working. If all is OK there, check the bus voltage; if it drops while adding such loads, the alternator isn't keeping up, most likely because there are problems with slip rings and brushes. Very common. The makers sometimes use too much grease in the rear bearing of the alternator and it ends up fouling the slip rings and brushes and coats the rings with a marginally conductive goop that limits field current. Low field current equals low rotor magnetism, which equals low stator output.

Remove the field wire from the alternator's terminal and measure the resistance between it and the alternator's case; it should be around 4 or 5 ohms for a typical 12V alternator. Turn the rotor and see if this value remains relatively steady; if it jumps around, you have problems.

Dan

 

[azure]

Battery Minder is on order and in the meantime I'm keeping the battery fully charged with a borrowed Battery Tender.

But last weekend I discovered something interesting. After being pulled off the charger, my battery reads 13.3V and then drops within hours to 12.6. However it doesn't drop any further than that in the next 3 days. Also, I've taken to measuring the battery terminal voltage immediately after flight and based on last Sunday, it seems that even with the 13.1-13.4 bus voltages I routinely see in cruise, terminal voltage reads 13.2 just after shutdown. Considering that I had a somewhat difficult start at home base, that's hard for me to understand if the battery is only getting enough from the alternator to float.

I'm going out to the practice area today to brush up and will see if this holds consistently.

 

[azure]

I'm going out to the practice area today to brush up and will see if this holds consistently.

It does. And we made some more measurements that completely change the picture (as well as make me wonder whether I'm going crazy or totally dreamed up what I thought I remembered from last winter).

With the engine running, and 13.2V bus voltage as displayed on the JPI, the battery terminal voltage is more like 13.8. With everything on including the pitot heat, bus voltage drops to 12.8 but the battery still reads 13.5. These readings were taken by my mechanic, but immediately after shutting the engine down we compared his meter to mine with the Fluke using the no-load, engine-off terminal voltage and mine reads only 0.05V lower.

So all indications now are that my battery is fine, my charging system is fine, and the only problem is that there is resistive loss somewhere leading to an abnormally low bus voltage. No point in changing out the alternator wire obviously. The only question now is whether it is worth chasing. It seems to be getting lower gradually so at some point my Sandel is going to crap out (I think that happens around 12.4V). Might have to really dig into the rats nest and try to trace it. Even my mechanic is afraid to do that because it would be very easy to short something out.

I still haven't found that circuit diagram...

 

[Henning]

It does. And we made some more measurements that completely change the picture (as well as make me wonder whether I'm going crazy or totally dreamed up what I thought I remembered from last winter).

With the engine running, and 13.2V bus voltage as displayed on the JPI, the battery terminal voltage is more like 13.8. With everything on including the pitot heat, bus voltage drops to 12.8 but the battery still reads 13.5. These readings were taken by my mechanic, but immediately after shutting the engine down we compared his meter to mine with the Fluke using the no-load, engine-off terminal voltage and mine reads only 0.05V lower.

So all indications now are that my battery is fine, my charging system is fine, and the only problem is that there is resistive loss somewhere leading to an abnormally low bus voltage. No point in changing out the alternator wire obviously. The only question now is whether it is worth chasing. It seems to be getting lower gradually so at some point my Sandel is going to crap out (I think that happens around 12.4V). Might have to really dig into the rats nest and try to trace it. Even my mechanic is afraid to do that because it would be very easy to short something out.

I still haven't found that circuit diagram...

Hmmm.. well that sheds a different light, welcome to the insane.

As for the highlighted, why are you so quick to dismiss any component of the circuit if you are looking for a resistive loss?

As long as you pull the ground strap off the battery, you won't cause any major failures (and injuries) due to shorting. Can you put it under full load and feel around for warmth?

 

[gismo]

Hmmm.. well that sheds a different light, welcome to the insane.

As for the highlighted, why are you so quick to dismiss any component of the circuit if you are looking for a resistive loss?

As long as you pull the ground strap off the battery, you won't cause any major failures (and injuries) due to shorting. Can you put it under full load and feel around for warmth?

The most practical way to find a voltage drop is to measure the voltage between points along the current path. FWIW, it sounds to me that the problem is between the main bus and the avionics bus (assuming the JPI is on the latter). The fact that the battery voltage is reaching 13.8 in flight suggests that the main bus is also at that voltage since the path from alternator to battery is via the main bus and master contactor. It's also possible that the problem is in the ground (negative) side of the power to the JPI. IIRC Cessnas typically have a relay which opens the path between main and avionics busses when the starter is operating. That relay and it's connections would be my chief suspect but tracing the voltage drops is the way to locate the problem.

 

[azure]

@Henning: If the loss was in the alternator wire, wouldn't that mean the loss happens before the battery? How then could the voltage at the battery not be affected?

@Lance: The JPI is NOT on the avionics bus. I thought it was odd that they wired it to come on as soon as the master is switched on, but that way you can easily check the bus voltage with all the avionics off.

I found the service manual but not the big circuit diagram so I'm still not sure how the alternator, main and avionics buses, and the battery are connected, but the only way I can picture it is the battery and the main bus being in parallel with each other, with the loss happening somewhere on the main bus leg. Lance, are you saying that the main bus and the battery are arranged in series when the alternator is running? My mechanic said the same thing, but if the alternator only puts out 14 volts, how could it hold 13. something volts across both the battery AND the main bus?

Oh, as I was doing a night currency run tonight, the ALT breaker tripped on me. I thought I had an alternator failure at first. I landed, pushed it in, and then very cautiously tried running the flaps and switching the beacon on and off (which I had been doing in the pattern). When I couldn't reproduce the problem on the ground (and didn't see any other sign of a high electrical load) I did one more run around the patch. No dice, the breaker stayed in. Another gremlin to try to figure out... yes Henning, the world of the insane.

 

[wabower]

Have you asked the Cardinal user group or one of their guru's for a wiring diagram?

@Henning: If the loss was in the alternator wire, wouldn't that mean the loss happens before the battery? How then could the voltage at the battery not be affected?

@Lance: The JPI is NOT on the avionics bus. I thought it was odd that they wired it to come on as soon as the master is switched on, but that way you can easily check the bus voltage with all the avionics off.

I found the service manual but not the big circuit diagram so I'm still not sure how the alternator, main and avionics buses, and the battery are connected, but the only way I can picture it is the battery and the main bus being in parallel with each other, with the loss happening somewhere on the main bus leg. Lance, are you saying that the main bus and the battery are arranged in series when the alternator is running? My mechanic said the same thing, but if the alternator only puts out 14 volts, how could it hold 13. something volts across both the battery AND the main bus?

Oh, as I was doing a night currency run tonight, the ALT breaker tripped on me. I thought I had an alternator failure at first. I landed, pushed it in, and then very cautiously tried running the flaps and switching the beacon on and off (which I had been doing in the pattern). When I couldn't reproduce the problem on the ground (and didn't see any other sign of a high electrical load) I did one more run around the patch. No dice, the breaker stayed in. Another gremlin to try to figure out... yes Henning, the world of the insane.

 

[azure]

Have you asked the Cardinal user group or one of their guru's for a wiring diagram?

I haven't really, Wayne, because there have been so many add-ons and mods to this airplane that I'm not sure how accurate a generic diagram would be. Also I'm pretty sure that my circuit diagram is serial number specific. Strangely, in the back of my service manual is a series of diagrams showing different parts of the electrical system, i.e. gear, beacon, avionics bus, etc. They are labeled "CHANGE 2", whatever that means, but the problem is they are labeled as being specific to two serial numbers that are much older than my plane. I'm going to look them over more carefully today.

 

[wabower]

FWIW, specific diagrams for individual light airplanes are seemingly few and far between, other than those that some avionics shop include as documentation for their aftermarket installations. In large part I think it's attributable to the issue you mentioned. No two of them are alike, and the electric/electronic briar patch in some of them makes Hogan's goat look like a well-oiled machine.

I haven't really, Wayne, because there have been so many add-ons and mods to this airplane that I'm not sure how accurate a generic diagram would be. Also I'm pretty sure that my circuit diagram is serial number specific. Strangely, in the back of my service manual is a series of diagrams showing different parts of the electrical system, i.e. gear, beacon, avionics bus, etc. They are labeled "CHANGE 2", whatever that means, but the problem is they are labeled as being specific to two serial numbers that are much older than my plane. I'm going to look them over more carefully today.

 

[gismo]

@Henning: If the loss was in the alternator wire, wouldn't that mean the loss happens before the battery? How then could the voltage at the battery not be affected?

@Lance: The JPI is NOT on the avionics bus. I thought it was odd that they wired it to come on as soon as the master is switched on, but that way you can easily check the bus voltage with all the avionics off.

JPI insists that their monitors should be connected to the avionics bus but many (including mine) are powered by the main bus so you can monitor and record engine parameters from before startup. I was just guessing that yours was on the avionics bus since that's pretty much the only way you could see 13.5 at the battery and 12 something on the monitor without something really weird going on (like grounding issues) assuming those "busses" are physically in the form of a busbar (thick copper bar). It's quite possible that your "busses" are really just a collection of wires connected together which would open up lots of potential high resistance points. Typically the output of the alternator goes through a high current CB to the main bus. The alternator current passes through the main bus to the master contactor (aka relay or solenoid). When the master switch is on the alternator current passed through the master contactor to the battery. The return path includes the negative battery cable (attached to the airframe), the engine grounding wire, the engine, and the alternator case. The avionics bus is connected to the main bus through a relay or CB/switch (pretty sure it's a relay in your plane).

Normally the alternator output current does not pass through the alternator switch, that usually just controls the voltage feed to the regulator and alternator field. Another variable is the way the regulator senses voltage. Sometimes that's just the voltage on the terminal supplying power to the regulator, sometimes it's a separate connection to the main bus (never the battery side of the master contactor), and on some the voltage is sensed at the AUX terminal on the alternator. It's important to understand where the regulator senses the system voltage because that's the place the regulator is trying to maintain at 13.8-14.2 volts. It's also important to measure the alternator field voltage (preferably relative to the alternator case and/or ground terminal) when the bus voltage is sub normal. If the regulator is doing it's job the field voltage will be very close to (within a volt) the voltage on the power input terminal of the regulator whenever the voltage at the sense point is below the set point (e.g. 13.8v). If the bus voltage is low but the sense point is normal, you would look for resistance between those two. If the sense point voltage goes down with the bus voltage under load and the field voltage is low either the regulator isn't working right or there's a voltage drop between the regulator and the alternator field terminal (or in the ground path). If the bus is low when the field voltage is high, either the alternator is overloaded, the alternator is bad, or the alternator output connection to the main bus (including the alternator CB) has high resistance. If you suspect an overloaded alternator check to see if the bus voltage rises with RPM, and check the current out of the alternator at or above 2000 RPM. If that current is below the alternator rating the alternator has a problem (assuming full voltage on the field). Don't forget to consider the possibility of a grounding issue, those can be very confusing if you aren't careful about using local grounds (e.g. the alternator case when checking field and output voltage at the alternator, airframe for measurements at the main bus, battery minus for voltage at the battery). If you suspect a weak ground connection you should be able to see more than 100-200 mv between different ground points when the low bus voltage occurs.

BTW, the alternator output CB is actually there to protect the alternator and the wire to it from excessive current out of the battery.n It's not supposed to trip if you overload the alternator because the alternator is magnetically self limiting WRT output current and therefore shouldn't be able to produce enough current to trip the CB. This assumes the CB rating is equal to or higher than the alternator output rating (e.g. 50A, 70A, etc). The fact that your alternator CB tripped (I'm assuming it was the output CB not the field/regulator CB) suggests that you either have an intermittent short to ground on the alternator output wire (fairly unlikely) or the CB has a problem. A bad CB could also cause a voltage drop from the alternator to the main bus. Checking the voltage across that CB with enough loads on to drop the main bus voltage would confirm/disprove that cause.

I found the service manual but not the big circuit diagram so I'm still not sure how the alternator, main and avionics buses, and the battery are connected, but the only way I can picture it is the battery and the main bus being in parallel with each other, with the loss happening somewhere on the main bus leg. Lance, are you saying that the main bus and the battery are arranged in series when the alternator is running? My mechanic said the same thing, but if the alternator only puts out 14 volts, how could it hold 13. something volts across both the battery AND the main bus?

As I mentioned above the (positive) path from alternator to battery starts at the BAT terminal on the Alternator, goes through a wire to the high current CB, through another wire to the main bus, through yet another wire from the main bus to the master contactor, through that and the positive battery cable to the battery. The connection to/through the main bus is the least obvious. Could be a couple wires on a copper bus bar with a bunch of CBs also attached to the bar, or a bunch of wires connecting all that stuff together, or some combination of a bar and several wires.

 

[azure]

Thanks Lance. Well, I can say that I do NOT suspect an alternator problem, since even with everything on including the pitot heat, the voltage across the battery measures at least 13.5V. So there really doesn't seem to be much of a point, as far as I can tell, to doing the full troubleshooting and probing the VR's response to a load, making sure the field is adequate. The alternator is putting out what it needs to under all operational loads.

It's true that I have reason to think there might be a problem with the battery ground wire -- Matt @ Y70 told me he thinks it looks "funky" -- but I don't see how that could begin to explain anything I've observed and especially the difference in voltage between the battery and the bus. My main issues now are

(1) Bus voltage is 0.5-0.8V lower than the battery voltage with the alternator running. I still don't have a circuit diagram that would show the relationship between the alternator output, the main bus, and the battery while the alternator is running. It's really hard for me to imagine how the alt CB could be responsible unless it and the bus were on a separate leg of the circuit from the battery, but that (I think) would contradict your statement that the alt CB doesn't protect against an excessive current draw from the alternator.

(2) Why did my alt CB trip? I agree that the CB may be bad as we've never checked it. BTW from what my mechanic says I do think my "bus" is really not a bus bar but a very messy collection of wires. Which makes tracing electrical problems under the panel very time consuming and difficult -- so much so that he keeps putting off working on it.

 

[Henning]

Thanks Lance. Well, I can say that I do NOT suspect an alternator problem, since even with everything on including the pitot heat, the voltage across the battery measures at least 13.5V. So there really doesn't seem to be much of a point, as far as I can tell, to doing the full troubleshooting and probing the VR's response to a load, making sure the field is adequate. The alternator is putting out what it needs to under all operational loads.

It's true that I have reason to think there might be a problem with the battery ground wire -- Matt @ Y70 told me he thinks it looks "funky" -- but I don't see how that could begin to explain anything I've observed and especially the difference in voltage between the battery and the bus. My main issues now are

(1) Bus voltage is 0.5-0.8V lower than the battery voltage with the alternator running. I still don't have a circuit diagram that would show the relationship between the alternator output, the main bus, and the battery while the alternator is running. It's really hard for me to imagine how the alt CB could be responsible unless it and the bus were on a separate leg of the circuit from the battery, but that (I think) would contradict your statement that the alt CB doesn't protect against an excessive current draw from the alternator.

(2) Why did my alt CB trip? I agree that the CB may be bad as we've never checked it. BTW from what my mechanic says I do think my "bus" is really not a bus bar but a very messy collection of wires. Which makes tracing electrical problems under the panel very time consuming and difficult -- so much so that he keeps putting off working on it.

Sometimes the "bus" is no more than a bunch of wires under one screw... If you have the bare braided type of ground straps, pull them off and soak them in vinegar for a while if they look "funky".

 

[flyingcheesehead]

I know the problem. Through the many years of rewiring one thing at a time, the ground was actually running through the spar carrythrough and its conductivity depended on the concentration of rat pee present on that particular day. When the spar carrythrough was replaced, the electrical problem was thus solved.

 

[azure]

I taxied around the field today with my A&P upside down in the right seat taking voltage readings on the bus, across the ALT breaker, etc., with a normal load of lights & radios running. Normally by the time I'm to the runup area I'm seeing 13.2-13.4 on the JPI, and from there it never gets higher (and sometimes goes lower in flight, 13.1-13.3, unless I turn off the beacon and use the strobes only, in which case it settles at 13.2-13.3). This time it started at 14.0 at engine start (normal) and only barely inched downward when I increased the load. The lowest I saw on the JPI was 13.5, and that was with the pitot heat on. Otherwise even with beacon, navs, and strobes (plus all the usual avionics), it stabilized at 13.6. A&P's voltmeter got bus readings that were consistently 0.1-0.2 higher -- I suspect it's his meter, as my Fluke compares similarly to his. He never measured more than about 25 mV across the CB.

So now I'm totally baffled. This has never been an intermittent, it's 100% consistent. Well, more like 99% now. Until today I have never, NOT ONCE in over a year, seen the JPI reading stabilize at 13.6. The best it's done is oscillate between 13.4 and 13.6 in the dead of winter. Put a 200-lb guy in the right seat with a voltmeter, and the system acts like there was never a problem.

I guess there's a remote chance that one of the connections under there is funky and he rubbed something off, or jostled something, in the process of getting set up. Otherwise, Murphy seems to rule with an iron fist.

 

[Henning]

LOL, I still don't think you have anything wrong. You have a bit of voltage drop in your system, that's the way the laws of physics work.

Here's the thing, "Are you running out of electricity? Do you have breakers popping and the charging system going off line? Is the charging system failing to provide for your electrical needs? Are your radios kicking off, transponder not working? Any real problem at all?

If you don't have a problem, it ain't broke. If it ain't broke, don't fix it. If these are long term accurate and stable readings on your instruments with no problems in operations, there is nothing to fix. At this point you have inspected all the cable ends and ground straps for condition and cleaned any contaminated areas. You've checked over everything andit all seems to be indicating fine. Just keep monitoring for any changes and trends and leave it alone.