Multiple Battery Setup

This came from a separate thread "Typo in Battery Article in Soaring"

On Feb 2, 10:11 am, Karl Kunz wrote:
John, also enjoyed your article and will try your technique on a couple of batteries I am unsure of. Also, since we have an EE on the line I wanted to ask about battery setups. My partner and I have installed some new toys in our ASW20 (transponder, etc) and are wondering what is the best way add more power. We are currently running a single 12v 9amp battery. Would it be better to add another 9amp battery or to go to a single higher capacity battery. If using two batteries, what is the best way to tie them together.

-karl

I suggest multiple batteries for a couple of reasons;

Redundancy - you can have a failure either in the air or on the ground
so with two batteries you have a spare.
Expense - I replace one of my batteries (I own 4 and fly with 2) every
year so over the course of four years I replace them all (the older
units are given to the club). While two 9ah batteries are more
expensive than a single 12ah battery, my yearly replacement cost is
less of a single time bite to the budget.
Size - You are probably set up for the 9ah size and a larger 12ah may
not fit. However, someone mentioned a 12ah to me that has the same
footprint as the 9ah only taller. Anyone seen this?
Weight - Is 2x9ah heavier than 1x12ah? Is that important? Hmmmm.
Power - 2x9ah has more energy capability than 1x12ah

Now, how to connect them? What I recomment is three runs of 14-16
gauge Tefzel wire between the batteries and the panel. This gauge is
overkill but I don't want any voltage drop with a smaller gauge. 3
runs = a common ground, battery 1 and battery 2. I use red (battery)
and black (ground) heat shrink to mark each run. I use PowerPole
connectors - very robust and secure.

At the panel I use three switches to run the batteries in parallel.
Battery 1 on/off, battery 2 on/off and then the output of both to a
master switch and then to the avionics. The thinking of separate
battery switches is that you can disconnect a "bad" battery. I always
have all three switches turned on during flight - the need to be able
to turn off a bad battery hasn't arisen for me and I am unsure how I
would even know (everything dies? smoke?). I do, however, have a
voltmeter, so I turn on/off each battery to see how they are doing (on
the ground typically).

Some people wonder that with both batteries connected to one another
in parallel, is there an issue with one battery charging the other?
Certainly cross charging happens (no two batteries are identical) but
unless one battery is significantly discharged from another, this
cross charging would be slight. I always swap out both batteries
right off the chargers at the same time to minimize this. Some
solutions suggest installing diodes in series to prevent this cross
charging but diodes "steal" 0.5v-0.8v of your voltage and every volt
counts in a glider. So I don't use diodes. Does anyone else?

Finally - ABSOLUTELY CRITICAL - install a fuse on each battery right
at the terminals. Not a breaker, a fuse. Fuses are faster than
breakers. This fuse is more important than fuses/breakers in the
panel.

My $0.02. Let me know how your testing turns out.

- John DeRosa
http://aviation.derosaweb.net