why 24V battery?

olasek

olasek

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olasek
What was the main reason they switched from 12V to 24V in smaller GA aircraft some time ago?
I am looking for some sound argument to counter some silly arguments on another forum.
 
My wild guess is that less amps are then needed to get the same power.

Watts = Amps x Volts
 
Disclaimer: I wasn't there.

24V vs. 12V makes a big difference when conductors are "iffy". As your switches and contacts get a little arcing, corrosion, etc., the higher voltage makes a big difference in how well things work.

Also, as Ed said, double the voltage, half the amperage which changes wire gauges, contacts, everything. For controls (18-20 gauge) no big deal. For battery cables, starter cables, etc. big deal in both weight and cost.

John
 
EdFred said:
My wild guess is that less amps are then needed to get the same power.

Watts = Amps x Volts
Click to expand...

As an example, I used to need 24 amps in the cherokee for the lights alone. Add in the radios, transponder, etc and I'm up to 40 amps (transmit mode on radios). So using Ed's algebra, I've got 480 watts of power. With a 24v system, 960 watts.
 
If you want to be really pedantic, it's a 28 volt system having a 24 volt battery in aviation terms. The battery full charge voltage is about 27 volts or a bit more.

It was designed that the higher voltage would allow smaller cables for high current applications, and fewer issues with connector resistance and heat effects.
 
murphey said:
As an example, I used to need 24 amps in the cherokee for the lights alone. Add in the radios, transponder, etc and I'm up to 40 amps (transmit mode on radios). So using Ed's algebra, I've got 480 watts of power. With a 24v system, 960 watts.
Click to expand...

Actually, you would only then need 20A to make your 480W of power. That's the advantage.
 
The reason GA and commercial aviation lean towards 24V is the same reason Apollo capsules used even higher voltages… And the same reason that airport runway lights use hundreds (or thousands) of volts…

You get equivalent wattages (same power) with less amps (less current)… the lower the current the lower the losses to resistance/conductor heating… especially over a distance of a long wire run (which in big planes can be hundreds of feet, and in runway lights can be miles long)…
 
In GA, its mainly to sell batteries for twice as much money that are twice as likely to fail.
 
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olasek said:
What was the main reason they switched from 12V to 24V in smaller GA aircraft some time ago?
I am looking for some sound argument to counter some silly arguments on another forum.
Click to expand...

More volts means fewer amps. Less voltage drop in wiring. Theoretically able to use smaller, lighter wiring. Same reason power companies boost voltage to tens of thousands of volts for long distance transmission.
 
Concorde RG batteries

12V start at $192.95

24V start at $429.50
 
Not only that, but there is guy on here, flying a Tampico I think, and the airplane maintenance manual says to replace the main ship battery every 36 months.

I have no idea if his airplane is 24 volt but his mechanic is dropping a new battery in his airplane every 36 months. :yikes:
 
It's trading dollars for pounds. The larger the aircraft, the better that tradeoff appears, as you have a lot more wiring that can get lighter. For simple small GA I would say it doesn't make much difference at about the C-172 level, smaller than that and the weight savings is minimal and won't outweigh the dollars spent.

On my RV9A I went with a 12-volt system, I calculated once that I could save about 9 ounces of weight with a 24-volt system but my overall cost would go up just over $2,000 due to changes in the battery, starter, lighting, and instruments. Not worth it to me for 9 ounces of weight savings.
 
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Pure speculation, but I suspect the switch was more of a move to increase alternator output capacity than it was to reduce weight. In a typical light airplane like a 172 the electrical systems are minimal enough that any weight savings by going to smaller wires would be negligible. Look at airguy's example.

The 12V and 24V charging systems all seem to have alternators of similar capacity so you'd be able to support more equipment off of the 24V system, which would be an obvious advantage. If you also consider the era in which these planes were built the cars of the same era were often limited in the alternator capacity department. This was possibly the aviation world's work around to a capacity limitation.
 
mondtster said:
Pure speculation, but I suspect the switch was more of a move to increase alternator output capacity than it was to reduce weight. In a typical light airplane like a 172 the electrical systems are minimal enough that any weight savings by going to smaller wires would be negligible. Look at airguy's example.

The 12V and 24V charging systems all seem to have alternators of similar capacity so you'd be able to support more equipment off of the 24V system, which would be an obvious advantage. If you also consider the era in which these planes were built the cars of the same era were often limited in the alternator capacity department. This was possibly the aviation world's work around to a capacity limitation.
Click to expand...

The 24-volt battery weighs quite a bit more than the 12-volt version. Any weight saving in a few feet of battery and bus cables is totally lost in that extra battery weight.

The 12 and 24 volt alternators are the same physical size and weight, but the field coil resistance is higher to get about the same amperage through it at max alternator output. Any such alternator can easily produce 110 volts DC at its usual 60 amps or whatever its capacity is, since its the amps that heat it up and burn it out. You used to be able to buy a doodad to stick in your vehicle to generate 110VDC with the alternator to run lights or tools that had series-wound motors. I had one years ago. It simply disconnected the alternator's output from the battery and put it to a 110V outlet, and ran the field directly to the battery. You raised the engine RPM up until you got 110V and held it there with whatever you could rig up, and went to work. I got the 110V at around 2500 engine RPM.

60 amps at 110V is 6600 watts. The 28-volt system, at 60 amps, can handle 1680 watts, while its 14-volt grandpa can only handle 840 watts, which is most likely why they went to 28 volt systems. More equipment needs more power.

Dan
 
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Would something like G1000 work under 12V?
(I am having a real tough time to get this info from the web).

Excellent point about more equipment ...
 
olasek said:
Would something like G1000 work under 12V?
(I am having a real tough time to get this info from the web).

Excellent point about more equipment ...
Click to expand...

Garmin likely has a 12V version of it. Some older airplanes have been converted to glass.

Dan
 
Dan Thomas said:
The 24-volt battery weighs quite a bit more than the 12-volt version. Any weight saving in a few feet of battery and bus cables is totally lost in that extra battery weight.
Click to expand...

I don't think so.

Compare two Gill models, 35 and 243, which output 12 and 24V respectively. Both have Cessna 182s in their list of applications. Both weigh about 27 pounds. Both require 3 quarts of electrolyte. So they sound comparable, right?

And they are comparable in every respect but cost.

The CCA cold cranking amps are 150 for the 24V model, while the 12V model has almost double the CCA at 250. Multiply that by voltage: they both output about the same power when cranking.

So, pretty much the same weight and same power when cranking. That's why I don't think your claim is right.

As was mentioned, they differ in cost. The 24V version costs twice as much.
 
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bnt83 said:
Not only that, but there is guy on here, flying a Tampico I think, and the airplane maintenance manual says to replace the main ship battery every 36 months.

I have no idea if his airplane is 24 volt but his mechanic is dropping a new battery in his airplane every 36 months. :yikes:
Click to expand...

In the Cessna 310Q I owned it was 2 12 volt batteries in the left wing. In 10 years of owning and operating that airplane I only replaced the batteries once.
 
bnt83 said:
Not only that, but there is guy on here, flying a Tampico I think, and the airplane maintenance manual says to replace the main ship battery every 36 months.

I have no idea if his airplane is 24 volt but his mechanic is dropping a new battery in his airplane every 36 months. :yikes:
Click to expand...

It is 24v
That's what I fly too.

I guess I need to go check the date on my last battery replacement
 
RotorAndWing said:
In the Cessna 310Q I owned it was 2 12 volt batteries in the left wing. In 10 years of owning and operating that airplane I only replaced the batteries once.
Click to expand...


Yes but the constant misconceptions of what is required vs recommended = continued pushing of bogus OEM replacement times of otherwise fine components by young mechanics.
 
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