PIGGYBACK BATTERIES

[chris]

HELLO B/ERS LOL I AM GOING TO PUT IN A SECOND HOUSE BATTERY .SHOULD I PIGGYBACK THEM /CONNECT POS TO POS AND NEG TO NEG OR KEEP THEM SEPARATE AND WHEN I RUN OUT OF POWER IN ONE ,THEN PUT THE CABLES ONTO THE FULL BATTERY.ALSO IF THEY ARE PIGGY BACK .AND I CHARGE THEM ,DO I JUST CHARGE THE ONE AND THEM BEING CONNECTED IT WILL SIMULTANEOUSLY CHARGE THE TWO. ANY INPUT OUT THERE IN B LAND?

[Mike]

I'm no expert, nor am I an electrician.
I would parallel them (pos with pos, neg with neg) as I believe they'll drain just as fast together,
as separately. Same holds true for charging. There's only so much juice they'll hold,
so charging/discharging them together should be fine. They'll probably last longer, too.
Just my opinion.

(I'm not much of a speller either. Corrected, I hope )

[g1g]

I questioned the same thing only because if the house battery startes the gennerator and I kill it how would I start the genny. I hate to think I would have to start the engine to charge the house battery to start the genny.
But on your idea don't just disconnect it get a dual battery switch. I know may boats have them. That will allow you to charge both when charging is available and then you can run off just one or the other and maybe both depending on the switch.
While on the subject does anyone know if the house or chassie battery starts the genny on a 95 Coachman Saratoga?

[Davydd]

If you are going to use two batteries it makes more sense to connect them up (12V batteries) in parallel (+ to +, - to -). That way they discharge about equally at the same time. Thus the two, for example, would discharge each 25% instead of one alone 50% in a given usage. The idea is to increase your amp hours to prevent draining your batteries too much. If you treat as second one as a back up then the first one most likely will eventually get abused because you most likely will not get up in the middle of the night to switch them over until it is too late.

[visionquest]

One thing that is important to remember when hooking batteries in parallel is that they should be the same age, capacity and condition. Adding a new battery to an older weaker one will shorten it's life and usefulness significantly.

-Mike & Heidi
97 Roadtrek 170P "Taj Ma Trek"
HTTP://WWW.VanTramps.Com

[Mike]

Just curious, but why?
I've seen this mentioned on other websites, and can't quite get my head around the
physics of it. Unless the physics is a battery store owner, trying to sell more batteries.
As I said, if anyone has a layman's explanation, or something a little more complex?

update: I've googled the heck out of this and can't find any explanation that makes sense
to me in any kind of entropic flow of energy between 2 batteries with different capacities sense.
I could understand if one were fully charged, and you added a half charged battery to the mix, but
when you add a battery to a parallel pairing, you can't avoid that initially unless you charge both
batteries fully prior to combining them, can you? Once the parallel pair is created, the charging
should be uniform across both batteries and when the total "fully charged" status is reached, I
don't see how there would be any flow one way or the other. Once a load is put on the pair,
or directly on either battery, and the charge state begins to drop on one or the other, I can
understand how there would be flow between them then, as the power draw continues.
As I said, I'm not an electrician, electrical engineer, or a physics professor. This is high school
science classes stuff that I'm talking (and admittedly, struggling to recall ).

[visionquest]

I can give you the basics of why and know it from experience, if you want the exact science behind it, that might take a bit of research. I don't mind getting the specifics for you but it will take me a day or two.

What I do know from experience in both one of my first RVs and customer rigs in my old shop is this. When adding a second battery, typically it will be newer and stronger, any fault in the first battery will cause the second battery to quickly discharge. The stronger will keep attempting to charge the weaker one which will reduce it's life pretty quickly. That is why this time when I went to two batteries, I went ahead and bought 2 new identical AGMs and gave the old AGM less than a year old to a friend. I won't kill a good battery again.

Hope that helps for now, I will look up the exact science behind it for you when I am done with my current project though. I have to get my new floor and seats in so we can get back on the road. Very much in the middle of it now.

-Mike & Heidi
97 Roadtrek 170P "Taj Ma Trek"
HTTP://WWW.VanTramps.Com

[Mike]

No problem, Mike. I'm not questioning your experience, just that I found some other blogs
where they didn't have the same results. I googled parallel/different capacity/old/new/
charging/deep cycle/etc./etc., and came up with several discussion threads with some
(may have been marine applications) different results and opinions, that's all.

I've been running a hypothetical scenario in my head, and still can't figure out why electrical
energy would try to travel from a fully charged battery at rest, to another fully charged battery
at rest, no matter what their respective ages or conditions are. Fully charged is fully charged.

Like, Battery A is new, and can hold a finite amount of charge, let's say 160 amps, and Battery B is
3 years old, and now can only hold 100 amps. If both batteries are at full charge capacity,
there's no place for energy to go between them, because they're already both full.
If there was a draw (or phantom) on the pair, I would expect the discharge rate would be
balanced over both batteries with the lower capacity one draining first, because it holds less.
Do you see where I'm coming from with this?

I've got a technical question in to a local battery specialty store, to see what their thoughts are.
I thought I'd ask because in all the times people have purchased batteries (me too) I've never
been warned not to use a new and an old deep cycle battery together. I would think if it were
an issue, it would be a standard warning, especially for the really big rigs that have more than 2
deep cycle house batteries.
I'll post up the answer, when they get around to me. I asked for a techy explanation in
simple terms, if they can explain it.

Peace.

[Davydd]

Here are a couple of RV battery references that have been on the internet for some time and might be useful reading.

The 12volt Side of Life (Part 1)

The 12volt Side of Life (Part 2)