Charging 12V Batteries in Parallel

[thirtydaZe]

We are set to pick up our new TT monday. For the past 3-4 years the wife and i have been pretty recreational campers, camp sights, lakes, etc.

I'd like to get a bit more off the grid, and have been doing some study on parallel 12v batteries via, rv.net on google.

I've read both parts of the 12v side of life, and some other reference, so my question is now, how do you charge the 12v system when in parallel?

I've concluded that having both 12v batteries wired parallel, and then run the pos. camper wire to pos. post from battery A, and the neg. camper wire, to neg. post of battery B is the best solution.

So when charging, via a battery charger, do you use the pos. post from batter A, and the neg. post from battery B, or does it matter? Can they even be charged while in the parallel configuration?

Also, I'd like to add a disconnect so i'm not using any juice when stored. can i just add that to the pos. cable into the camper and be done with it?

Other than having two like batteries, proper gauge wiring, it seems to straight forward, what else am i missing?

[hershey]

OK, Pianotuna your off the hook. By comparison, your response is hardly worth mentioning now that wa8yxm took the response to a whole new universe. 🙂 🙂

[wa8yxm]

In the electronics dictionary a battery is a collection of cells.

Now, when you go shopping for replacements you get two 12 volt batteries (Why I don't know) and pay a bunch of money.

Each battery has six 2 volt cells, wired in series to make 12 volts

When you install them however MAGIC happens... You now have a 12 volt battery with 12 2 volt cells wired in series/parallel to make 12 volts.

But the magic, is the two, have become one.

So you do not charge "Them" (The two in parallel) you charge IT. the one they magically became.

Why the "Why I do not know)

Most 12 volt RV batteries are Marine/Deep cycle, these are really starting batteries and like to be kept fairly well fed, say 75% or more.

The 20 hour rate method of determining amp hours gives about the following ratings.

Group 24 batteries are about 75 amp hours each
Group 27, around 95
Group 29 105
Group 31 130 *NOTE*

GC-2 (Six volt golf car batteries, which may well be the single most common lead acid rechargable battery made, and thus lower in cost than other styles) are 230 amp hours

NOTE; that 2 of anything other than Group 31 adds up to less than 230


GC-2's are also DEEP CYCLE, they can go down to half full (50% state of charge) safely and even an OPPS, is often recoverable (What's an OPPS, say 10 Percent SOC).

They also cost less.

And when properly hooked up (in series) magically become one BIG (Think 8D) 12 volt battery.


That said: Since you just got new 12 volts, I'd not be getting replacements till you need 'em.

[hershey]

pianotuna wrote:
Hi hershey,

Yup--and I'll continue to post the BEST possible way to do things.

Go ahead and recommend less than optimal solutions that will damage a battery bank over time. I'd define the original suggestion as poor answer--not "absolutely correct" to quote you. It is safe--but it is not best practice.

This is best known as a "Gotcha'" 🙂 🙂

[deleted-2]

Hi there,

I've tried two batteries paralleled that weren't matched.
That didn't work very well.
I knew better working with electrical on aircraft for many years.

The best option for us on longer stays was the 2 battery switch.

If I may recommend:
Bought a MAXX deep cycle from WalMart ($109.00 USD).
Last time out it lasted a solid 3 days without topping up.
The MAXX really hung in there, we were hard on it too.

Like to get second one, yet we just bought a new trailer and it has a Exide 27F deep cycle in it.
We'll have to try and cycle it correctly to get good service (been lazy about cycling batteries sadly)

[Matt_Colie]

If you put 12V batteries in parallel, you will be hard pressed to get capacity X2 from them. Lead acid batteries just do not play well together. If the two are not of the same line the same day. They will never both come to full density (charge) at the same time. And, if they are different ages - just forget it....

If you are stuck with it that way, separate the batteries for charging, but the weaker battery will still drag the better down while they are connected.

This is true of all banks of paralleled LA batteries.

IF this is just for a season, do what ever, but if you are going to replace either or both then go get a pair of golfcart batteries and put them in series for 12V.

I made a living do DC electric on expensive boats for many years.

Matt

[pianotuna]

Hi hershey,

Yup--and I'll continue to post the BEST possible way to do things.

Go ahead and recommend less than optimal solutions that will damage a battery bank over time. I'd define the original suggestion as poor answer--not "absolutely correct" to quote you. It is safe--but it is not best practice.

[hershey]

I told ya' so. 🙂 🙂 🙂

[pianotuna]

Hi,


This is what is balanced and best for twin twelve volt batteries.



As it often doesn't cost a dime more to do this, I think it is worth the trouble.

If you wish to understand the "why" surf here:

correctly interconnecting multiple twelve volt batteries

Others may say it doesn't matter--but unless there is a compelling reason to not optimize charging and discharging why not do it the best possible way?

[hershey]

Yours is a simple query but something that indeed needs the correct answer. The answer you received is absolutely correct.
There are some that will try to make it a convoluted project out of something very simple. Just put the positive cable from the charger to any positive post and the negative to any negative post and you'll be fine.

[thirtydaZe]

Always hate to assume, thanks for confirming.

[bldrbuck]

You can use either psoitive poat and either negative p9st. Thet are already tied to gether.