RV Alternator Load question

M

MauleSkinner

Touchdown! Greaser!
Joined
Oct 25, 2005
Messages
21,670
Location
Wichita, KS
Display Name
MauleSkinner
Short version: can I pull 60 amps off of a 130-amp alternator all day long without dramatically affecting the life of the alternator?

Long version: Our motorhome has (or should have-I haven’t verified it yet) a 130-amp alternator. It has the normal vehicular electrical loads—headlights, taillights, marker lights, etc, and chassis battery charging. In addition, it charges the coach battery, which is currently a 100-amp deep cycle, and the “house” loads on that are just a few amps while driving.

The refrigerator is either 120V AC power or Propane, and we don’t want to run the propane while driving, so our only other option to keep the fridge cold is to run the generator. Not a big deal, and in the summer we’re often running at least one of the air conditioner units while driving anyway. But for cooler-weather trips, it seems like kind of a waste to run a 5500 Watt generator for less than 4 amps draw on the refrigerator. I’m thinking about adding a dedicated inverter connected to the coach battery to run the refrigerator when driving. The 4 amps (3.5, actually) at 120V would be just under 40 amps load on the 12V system.

Ultimately that would mean adding a constant 40-amp load to the alternator. I haven’t put a meter on it, but I’d guess that even with all the lights, etc., on while driving it would be less than 20 amps for the rest of the vehicle once the battery is recharged after start. So once the chassis battery is recharged, I’d be pulling less than 60 amps off of my 130-amp alternator.

Is that reasonable and/or are there major holes in my knowledge here?

The secondary question is that the 120V load for the refrigerator is just heating elements. The electronics are directly wired to the 12V system. So a “modified sine wave” inverter advertised as suitable for resistive loads (like https://www.etrailer.com/RV-Power-Inverter/Go-Power/34280175.html ) would be appropriate?

Thanks!
 
From what you're saying, it sounds like you have no inverter for 120V and so your only sources are therefore the generator or hook-ups. I'm also not sure what sort of motorhome you have, but from the description, I'm guessing a Class C gasser of some sort? With that, addressing your questions:

I wouldn't have any worries about burning up the alternator at 60 amps continuous. My last RV I had the alternators running at pretty well full bore and they didn't have issues burning up. I had physical build quality issues, but those were separate. So you'd be fine there. You'll be pulling more than that since the inverters don't run 100% efficient, but with a 130A alternator you should be fine.

There are lots of inverter options out there. I would suggest going for something greater than 800W just because 800W won't run a whole lot. But yes, something like what you linked should be fine for that purpose.

If you have an RV with minimal electric on it and the propane fridge works for you, then my thoughts on extras might not be for you. We converted our previous RV to highly electric and our Prevost started highly electric, and we've only made it more electric from there. We went so far as to delete the propane heater from our previous RV (as well as the propane fridge) and replaced it with a residential fridge. We were much happier with that setup. The all electric heat (heat pumps and resistive electric loads) had its downsides, but even in well sub-freezing temps we were happy with it. I also put solar on the last RV, which I loved both for the silent power but also for how much the shade it produced cooled off the inside of the RV. The Prevost has a diesel fired heater, I avoid using it.

My channel (especially the videos on our previous RV - the Holiday Rambler Endeavor) have a lot of ideas that may not be useful to you, but may give you some ideas:

www.youtube.com

Ted's Garage

Welcome to Ted’s Garage! Here you’ll find videos about my cars, motorcycles, RVs, and everything else that burns fuel. I’ve had a life long love affair with cars, and do (almost) all my own wrenching on them, having started off as a Jaguar mechanic and now just wrenching for fun, building a...
www.youtube.com www.youtube.com
 
Thanks, Ted…good to know.

Ted said:
From what you're saying, it sounds like you have no inverter for 120V and so your only sources are therefore the generator or hook-ups. I'm also not sure what sort of motorhome you have, but from the description, I'm guessing a Class C gasser of some sort? With that, addressing your questions:
Click to expand...

It’s a Class A gasser, and you’re correct—no inverter.

It is pretty minimal electrically…if we’re not running air conditioners, we can go at least a couple of days on the coach battery. My wife has talked about getting a residential fridge, but I haven’t looked into it enough to have an idea for electrical capacity. Part of the problem is we’re only a 30-amp shore connection, so I can’t get too wild with adding loads. I had to add a switch-out to run one air conditioner on a dedicated shore power circuit because running both is just a bit too much for the 30 amp connection.

Now that I’m semi-retired we’ll be using it more, and that might get us looking at mods/upgrades. We’ve got a big trip coming up, which is what got me thinking about the alternator question again. I did sort of follow your thread here, and delved into a couple of your videos, but I’ll add your YouTube page to my favorites for future reference.
 
  • Like
Reactions: Ted
The fridge is designed to run LP while driving. In extreme cases of wind it may go out, if it does the thermostat on the burner will shut off the LP. There should be a sight glass inside the flange of the fridge to check the burning running while driving. Ive driven with LP on for decades no trouble. A 40 amp draw added to the normal load on the alt may cause trouble during idle on hot days with the dash AC running. LP is safe and more efficient.
 
MauleSkinner said:
It’s a Class A gasser, and you’re correct—no inverter.

It is pretty minimal electrically…if we’re not running air conditioners, we can go at least a couple of days on the coach battery. My wife has talked about getting a residential fridge, but I haven’t looked into it enough to have an idea for electrical capacity. Part of the problem is we’re only a 30-amp shore connection, so I can’t get too wild with adding loads. I had to add a switch-out to run one air conditioner on a dedicated shore power circuit because running both is just a bit too much for the 30 amp connection.

Now that I’m semi-retired we’ll be using it more, and that might get us looking at mods/upgrades. We’ve got a big trip coming up, which is what got me thinking about the alternator question again. I did sort of follow your thread here, and delved into a couple of your videos, but I’ll add your YouTube page to my favorites for future reference.
Click to expand...

A residential fridge is going to use a lot more power. If you've only got a 100 Ah battery (which I'm guessing is at 12V, so by comparison at 12V my battery bank would be 2120Ah, plus I'm running lithium so the effective Ah capacity is a lot higher) it's not going to last long. The propane fridges are so nice on smaller batteries because they're basically only consuming enough current to keep the propane valve open, something like 5mA as I recall reading. Our last bus had something around 400Ah @ 12V (lead acid) and before we added the solar, 2 days was about as long as the batteries would last to run the fridge. That got worse as the batteries degraded, and of course got better when we added the solar.

I don't post much new content these days (I think it's been 6 months since the last video), but a lot of my older stuff would likely be relevant to your rig.

Which reminds me, I have a bunch of work to do on the Prevost to get it ready for this season...
 
May not burn up your alternator, but the belt that drives it will take a beating. Be prepared with a belt on board.
 
I did a lot of charging systems engineering in Marine applications which is similar.... I would offer this advice ....max alternator ratings mean little and have different basi, but running full time 50% load should be no problem ..suggest keeping alternator cool. most have own fan but check is you can duct or deflect some cooler outside air to it....a little goes a long way. if a V belt, should have at least 2 belts which it likely has if OE install..if wide ribbed belt type, and a tension indicator installed, I use green and red fingernail polish so limits easily seen. if you find you are getting up close to amp limit, and/or not a lot of cool air flow.

you may consider a load limiter which will decrease output if approaching limits and are cheap and good piece of mind knowing you won't over temp the unit on a trip ..although many smart voltage regulators have this feature . I always look for a good deal on a spare alternator or at least voltage regulator if external before you need it, and good to have on the road .
 
You must log in or register to reply here.
Back
Top