14/28 VDC Conversion - Basic Theory Question

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tallank

Active Member
Joined
Sep 22, 2020
Messages
42
Ok, my electrical engineering accumen ends at basic ohms law, so please bear with me for some very basic questions.

My L-39 is a 28 VDC system. I'm using two 12VDC AGM batteries in series to replace the insanely expensive 28 V Mil spec Concorde. The airplane is wired like a car in that the negative ground is local, to the airframe, and the hefty positive cable runs back into the bowels of the airplane and ultimately connects to an APU stater motor and a giant Soviet Bloc generator that appears to be as large as a Volkswagen Beetle engine. This particular airplane has a few 28-14 converters to run most of the American avionics, but I'm seeing an opportunity to "pick off" a 14V avionics buss between the two 14 V cells and eliminate the converter boxes. In my electronic ignorance, this seems like an elegant way to eliminate some converter boxes and complication. Admitedly, some of the systems I plan to run off 14 volts are cheap - like a $30 LED warning light, but some, like a Garmin 430 and GNX375 represent a significant investment.

If things are working fine, I "think" pulling off one cell gets me 14 VDC, what happens when things go sideways?
Most avionics can handle 14 to 33 volts.
 

tallank

Active Member
Joined
Sep 22, 2020
Messages
42
That’s how my Starduster is setup. The selector temporarily bridges them in series for the 28V starter, then back to parallel for normal operation and charging. There is now engine driven charging system... it has an external jack for plugging in a ground charger.
Batteries do not like to be run in parallel. They are not balanced exactly the same so one will take all the charge an the other one goes along for the ride and one will take most of the load and the other is not doing much. Well shorten the life of the batteries.
 

tallank

Active Member
Joined
Sep 22, 2020
Messages
42
That’s how my Starduster is setup. The selector temporarily bridges them in series for the 28V starter, then back to parallel for normal operation and charging. There is now engine driven charging system... it has an external jack for plugging in a ground charger.
Why did you do this? I switch to a LiFe battery in my high compression IO360 RV-6 and it really spins up the engine and saved a bunch of weight.
 
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