Considering Adding a Second 12-V Battery in Parallel

[LaMancha]

Do what you want.

[Rwriley]

I have a novice question. If you hook 2 12V batteries in parallel, do they need to be of equal ability. Said another way - do they need to be the same make and model (so to speak) ? Would something bad happen if 1 battery was significantly stronger than the other ?

And actually - I have the same question about hooking 2 6 volts in series.

[Najataagihe]

Yes.

What happens is the weaker or lesser-capacity battery drains the stronger or higher-capacity battery until they both have the same state of charge.


Think of batteries hooked together as behaving like buckets of water connected by a hose at the bottom of the bucket.

They are going to "level out" until they reach equilibrium.


Both parallel and series connections work this way, the only difference being the output voltage.


You don't HAVE to have the exact same make and model (it is better if you do and you are going to replace them all at the same time, anyway), but more the same age and size (capacity in amp-hours and cranking hours), as well as matching types.

You don't want to mix a lead-acid with a lithium battery, for instance.


You always want to replace EVERY BATTERY IN THE BANK, when one goes dead.

If you don't, the older batteries are going to draw on the newer batteries, shortening their life span.

[justdede]

I always get a second battery. We have had 2 5th wheels a popup and a bumper pull. We put two 12 volt batteries in all of them. There is just not enough amps to boondock comfortably with one Size 27 Marine battery. With our Reflection we have two 120 watt solar panels that will help keep the batteries charged and a Honda 2000 watt generator to boost the battery in the morning and make my husband's coffee. On our bumper pull we used the generator and a 120 watt portable solar panel too do the same thing (keep our batteries charged and boost the battery in the morning, etc.). On our first 5th wheel we had an inverter installed and wired to the TV and one or two plugs so we could run 120 V appliances but we never used it so we didn't bother installing an inverter in our new 5th Wheel. We have a small portable inverter we plugged into a 12V socket the we had installed in our trailer to run 12 volt vacuum, etc. With the inverter we could plug in the TV if we wanted to. We rarely used it. Where we boondock, there is rarely TV reception so we just keep our computers charged and use a pico projector to watch movies or TV shows from our MP4 movie library.

When we replace the batteries on our Reflection, we will probably covert to 6volt golf cart batteries (if we don't move to lithium). We did that on the 5th wheel and we able to get more amp hours from the two 6 volt batteries than we got from the 2 size 27 batteries.

In any case, what you do will really depend on how you want to use your RV. If you want to use all your 120V appliances, etc. and boondock you will need a larger battery array, an inverter, and a bigger generator and/or a solar setup. If you only want to use your lights, water, refrigerator, heater, charge your phones and devices etc. two batteries and your generator will do fine. So make sure you know what you want to have while you boondock and build your system around it.

[rbolam]

The bottom sketch is the better way to go regardless, the trick is to keep a balanced load on the battery bank, both in pulling a load out of the bank and charging back. The more balanced the battery bank is, the longer the battery life will be.

[TheRealRoland]

Very cool discussion, people. Thanks for the added insights. Apologies for reviving an old thread, but i feel that this would be the icing on the cake for newbies. So, couple of questions:
- would you wrap some insulation tape around the connectors that connect the 'battery cables' to the 'load' cable?
- and, just to be absolutely safe - the grouping of the 'red' cables, connect to the 'red' cable of the trailer, and the grouping of black connects to the 'black' cable of the trailer, right?

Then, where would i need to connect the cables coming from the solar charge controller onto? that same 'grouping' bolt/nut?

[RickLight]

Glad your digging into this vast pool of info.

Some insulation on the terminals is smart. There are terminal covers that are far easier than tape. Many rely on the battery box, if your using vented units.

In theory, no 2 cables will follow the same path at least for very far. Grouping is more of a neatness thing than technically needed. Cable insulation is rated at many times the 12V your managing.