14/28 VDC Conversion - Basic Theory Question

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Toobuilder

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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?
 

TFF

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Charging of the batteries will be uneven because the draw is uneven. How much is going to be about the installation. After a couple of flights you could charge the batteries individually. Starts are going to be hard on the batteries and uneven charge will kill them quickly. I have worked on planes that series the batteries for start, and then split them to charge and run electrical.
 

Toobuilder

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Ok. So the avionics are pulling off one side of the pack, but the power draw is nothing compared to the output of the generator. I have a hard time understanding how a 10 amp draw off one of the batteries is going to unbalance the charge. Isnt it an average?
 

Pops

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TFF is right on as usual. You will have a very low battery life. My solar system is 24 volts. On a solar system you can't put older batteries with new batteries for a long battery life. My batteries are about 8 + years old now and doing good, but when the first battery go's bad, I'll replace all of them.
 

rv7charlie

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+3; don't do it, for the reasons listed. The GNX375 will be perfectly happy on the 28V bus (see the specs), and the 430 *likely* will be as well, but you need to look at your particular part number & check Garmin's specs to be sure. The older 28V models (I have one) require 28V for the comm transmitter; everything else can be run on 14V or 28V. I haven't looked, but I *suspect* that the later 12V versions will work on 28V, as well. Again, check the manual for your particular part number.

The only reason to use more than one converter box is for redundancy among critical 12V instruments. If some critical instruments demand 12V, you could reduce your converter count to two, and feed both to the same 12V bus via isolation diodes. (You'd need some method of verifying both for proper operation prior to each flight, of course; separate switches for each converter could serve that purpose.

Low draw items with consistent load, like a 12V LED, could be on the 28V bus by using simple dropping resistors, if you want to go that route.

Charlie
 

bmcj

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I have worked on planes that series the batteries for start, and then split them to charge and run electrical.
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.
 

Hephaestus

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I wonder if you could straight up swap to 29.6v lipo if the issue is mostly cost/availability. You'd need a battery management system, charge controller etc - but we've seen that done enough on the 12/24v side to know much of the process.
 

Hot Wings

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Had to think about this a bit, because my first thought was "Sure no problem. I did that way back with my 6V to 12V converted VWs and never had a problem with the 6 volt battery life"
But.............
Putting a load in the middle of what is otherwise a single distributed 24 volt battery is, as far as the battery sees it, a partially dead cell in the middle with high resistance. This downstream battery will not see the same charge volt delta the first one does. If the charge voltage each sees is within the range specified for that particular battery chemistry then it may all work. If not then you may see reduced battery life as the others have mentioned.

I'd personally probably simplify the system and accept the cost of poor battery life..........but I've found my risk tolerance/ignorance is a little greater than the average pilot.
 

Del

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We are using dual buss 12v and 24 v on our Jurca MJ77 P51 Mustang project. Research Vanner Voltmaster Battery Equalizer, ours is a Model 65-60. These units are designed to equalize the charging of each battery and used on recreational vehicles. They work, extensive testing has shown after operating under load and after shutdown the voltage of each battery was within 0.05 vdc of each other. We downloaded a user manual that explains all the connections required.
 

Pops

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The computer in my Solar controler runs a battery equalizer and a desulfate program every so often. Dependent on the battery specs by the manufacture.
 

TiPi

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Without a battery equaliser like the Vanner, you will have the following problem:
taking charge out of 1 battery only (12V), then recharging both batteries (24V as they are in series), will cause the battery used for the 12V supply to be under charged and the 12V battery not used to be overcharged, as the charge from the alternator flows equally through both batteries (as they are in series). The regulator will only read the voltage across the 24V setup.
It is like having 2 water tanks, you take water out of one only but refill both at the same time. One will not be full unless the other spilled everyting overboard.
The battery equaliser spreads the 12V load across both batteries so that each one supplies the same amount of energy. That way the recharge through the 24V alternator is recharging both batteries as if they have been used as a 24V supply.
bmjc's solution would require the charging system (and everything else except the starter) to be 12V. Verner did that on the JVC-360 for some unknown (current draw for the starter motor?) reason, making a simple engine overly complicated (and causing a lot of problems for the owners as these bits are no longer available).
 

Toobuilder

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Ok, I'm convinced. Looks like all the stuff I want to run is good to 30 volts anyway, so the problem is "not" a problem. I appreciate the education.
 

Hot Wings

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I appreciate the education.
As do I!
One of the great things about this forum is that we all get to learn a little from others questions. I'd never taken the time to think about this particular 'problem' before. It is was nice to see that there are solutions - if I ever have need to do similar in the future.
 

pfarber

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TFF is right on as usual. You will have a very low battery life. My solar system is 24 volts. On a solar system you can't put older batteries with new batteries for a long battery life. My batteries are about 8 + years old now and doing good, but when the first battery go's bad, I'll replace all of them.
lol wut? Lead acid batteries are the most forgiving and in the Navy we have hundreds of cells and they would pull them individually once the resistance was out of spec. Even LiPo (like Prius battery packs) you can certainly replace one cell without throwing out the other 100.

Your battery salesman must love you.
 

Pops

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When you try putting a new battery in with a lot of old batteries you are wasting money with shorter battery life. Gov has lots of our money to waste so that is no surprise.
 

pfarber

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When you try putting a new battery in with a lot of old batteries you are wasting money with shorter battery life. Gov has lots of our money to waste so that is no surprise.
Not at all. You will never get voltages and resistances to match. The heat from the electrons are enough to heat the electrolyte and alter resistance/voltage. You definitely don't want 13.6 on 10 cells and 8 on another. But you are not saving the batteries by chucking all the GOOD cells for one BAD one. That's just crazy talk.

You batteries will nominal range of a volt or two, if you get one out of spec, try and condition it, otherwise replace it.

But again, throw out perfectly good cells? Why?
 

wsimpso1

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I would post this question on AeroElectric connection, and see how the folks there think you should solve it.
 

Mad MAC

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2nd the battery equalizer.

Some civilian hueys run 4 truck batterys (batterys in series & parallel), that gets a bit complicated.
 
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