RV Batteries 101

Noah · #1 01-21-2008, 06:39 PM

Greetings,

I haven't worked on my electrical scheme yet (just started reading Nuckols book), but I was thinking of mounting the battery box on the firewall (7A). I have the "standard" 877 APP bracket, all primed and ready to go. I've noticed from this website that most people seem to mount an Odyssey PC680 battery, and Vans even has a dedicated drawing (31A) for the battery box for this battery. Vans also sells this battery box with the firewall forward kit (I think).

So, why does everybody seem to install this battery? What are the pros and cons to this vs. a battery which would fit the bracket that I already have? Is the PC-680 required for negative g's? Is it lighter? More power? Last Longer? Does ANYBODY install the "standard" wet cell battery, or is EVERYBODY installing the PC-680?

What are the factors that go into making a battery decision, and can I be reasonably assured that a PC-680 will do the job? Or do I need to do the math, and design my electrical system prior to deciding which bracket (and therefore which battery) to put in?

Guy Prevost · #2 01-21-2008, 07:16 PM

I'm sure you'll get more technical answers, but here's my thought process. The PC680 is sealed, costs less, weighs much less, cranks at least as well (if not better), and is quite a bit smaller.

I paid an extra $1500 for my WW 200RV prop, mostly to save 20 lbs. The fact that I could save 8.5 lbs by using a PC680 and save money at the same time made it a no brainer.

Guy

Neal@F14 · #3 01-21-2008, 07:27 PM

Everybody installs Odyssey batteries in their RVs because they are light weight, trouble-free, dependable, long-lasting, spill-proof, and provide oodles (technical term) of cranking current even in cold weather.

I wish Odyssey had one PMA'ed to fit my Cherokee, but I'm stuck with a heavy, old-fashioned, FAA-PMA certificated flooded-cell battery that always oozes acid out the vent caps every time I have to put the charger on it to start my plane... which happens far too frequently... and my battery is only 2 years old, it is not overfilled, and my alternator and regulator are charging perfectly. This is just part of life with a battery that's got marginal cranking capacity to begin with, mounted way under the back seat with a really long cable to reach the old boat-anchor of a Prestolite starter.

I think a Walmart sealed riding lawn mower battery would probably more dependable than a Brand "G" or Brand "C" classic flooded cell aircraft battery.

Putting the battery on the firewall would keep your wires to the starter much shorter, lessen the voltage drop, and help the starter motor crank faster. It would also subject the battery to a lot more heat, which may hurt its longevity.

About the only "pro" a flooded cell battery has going for it, is that if your regulator ever shorts out and overcharges... the flooded cell battery is a bit more forgiving. Overvoltage / overcharging will trash a sealed AGM battery like the Odyssey real fast.

AlexPeterson · #4 01-21-2008, 07:37 PM

I've got four years and about 600 hours on a PC680. Amazing battery. I've messed up hot starts a couple times, and had to crank a long time with that battery. I checked it for sulfation recently, and it was fine (used Odysee's battery charger/tester).

Only disadvantage of firewall forward is heat, and I don't know how much that matters. The battery in my car is in the trunk, and is over 7 years old. Never got even close to that in Minnesota before, so heat might play a fair role..

Sam Buchanan · #5 01-21-2008, 07:46 PM

Quote:

Originally Posted by Noah

Greetings,

So, why does everybody seem to install this battery? What are the pros and cons to this vs. a battery which would fit the bracket that I already have? Is the PC-680 required for negative g's?

Yes.

Quote:

Is it lighter?

Yes.

Quote:

More power?

Yes.

Quote:

Last Longer?

Yes.

Quote:

Does ANYBODY install the "standard" wet cell battery, or is EVERYBODY installing the PC-680?

No, yes.

Quote:

can I be reasonably assured that a PC-680 will do the job?

Absolutely.

Noah · #6 01-21-2008, 08:17 PM

Great responses guys, exactly what I needed to know! A couple of you mentioned potential issues with mounting the Odyssey battery firewall forward due to heat - does anybody mount these somewhere else in a 7A, or does everybody pretty much follow the plans? I know that RV-6 builders sometimes mount on the aft side of the firewall. If I follow the -7A plans, are there any known interferences or issues using the 200 hp IO-360 engine? Is a blast tube recommended since its on the hot side of the engine baffles? I noted that Walt Tondu made the front of his odyssey box removable, presumably because something above it made it difficult to remove? (http://rv7-a.com/firewall_finish.htm)

Neal@F14 · #7 01-21-2008, 11:46 PM

I don't think a blast tube to blow on the battery in flight would be of much benefit, since while you're flying, the massive amount of air blowing thru keeps all the heat pushed out the bottom of the cowl. The biggest heat issue is after you're done flying and you shut down the engine on the ground, and all that heat built up inside the engine then radiates out and heats up everything inside the cowling after there's no more air blowing inside. Opening up the oil door on top of the cowl helps the heat escape some, but everything inside the cowling (like the fuel injector spider and lines... and battery) still basically gets cooked

Everybody who mounts their battery on the firewall still gets pretty good enough service life from the battery, it's just one of those things that's part of the of pros and cons, trade-offs and compromises about building an airplane that you gotta live with.

With a 200hp IO-360 on the nose, especially if you use a CS prop, the weight shift of moving the battery to the cockpit side of the firewall probably won't disrupt the W & B any at all, but then it would be up to you where and how you want to attach the battery box there and figure out all the mounting and wire routing issues and whether or not it will affect anything else*.... but then that's what a homebuilt is all about anyway, eh?

If/When I ever get to build a -7 of my own, I'd probably just mount the battery on the firewall under the cowl and not worry too much about it.

* (e.g. it's fun to watch a whisky compass spin when you crank the starter if the battery cables are routed too close to the compass

)

Jekyll · #8 01-22-2008, 12:11 AM

Sam:

Spartan yet eloquent . A masterful use of just 3 words and 15 letters to transfer significant knowledge.

Jekyll

Camillo · #9 01-22-2008, 07:09 AM

I can guess that a firewall "rearward" makes difficult, once upper skin is riveted, to access to battery. In my car (VW POLO), battery is wrapped with something like goretex outside and tissue inside. It lasted at least 5 years and I now replaced it when it was still cranking the engine. Maybe this kind of protection may help the Odissey.

airguy · #10 01-22-2008, 07:34 AM

That's a good thought, actually - it's a lot less work to put a very lightweight insulating layer over the battery than it is to fabricate a new installation location. If the main heat worry is on shutdown, an insulating blanket really only needs to work for about 15 minutes before cowl temps are down pretty good - especially if you open the oil door on shutdown.