Adding a second battery?

[jlhog]

I currently have one battery that is rated at 81 amp hours. I assume that is what maters when running from the battery. And I hate to ass u me anything. Anyway, I am considering adding a second identical battery hooked in parallel to increase my run time when boon docking. Nice county rustic campground near me is $8.50 a night with senior discount. Current battery is just 2 years old so I don't really want to spend the money on a lithium yet. I have a small solar panel on the roof and can run my generator during the day to recharge both batteries. Total cost would be under $200 which is not bad if it will double my run time. Would there be any downside to this plan?

 

[Mark_K5LXP]

An 80Ah (assuming marine) battery is under $100 all day long. They are not remarkable batteries either for power delivery or longevity but for 'light duty' they can work OK. When you mix old with new you can expect less than optimum life out of the set but again, in some applications it's a workable solution. Ideally you'd know your power requirements and would select batteries that met those but all things being equal, adding a second battery will give you 'more' available storage. Whether it's enough without having actual usage data is a separate question but given the nominal cost, you'll find out with a nominal risk.

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

Battery gurus will tell you it's not good to mix new & old batteries, even if the same type. That's true, but not a terrible thing as long as the old battery is still in good condition. The problem may be that many RV owners simply aren't good judges of that. Even so, the downside is only that the new battery loses some of its life potential.

So the short answer is, YES, your plan works. You might want to load test the older battery to get an idea of just how good it is. As Marks, says , you can replace it for another $100 .

 

[jlhog]

Battery gurus will tell you it's not good to mix new & old batteries, even if the same type. That's true, but not a terrible thing as long as the old battery is still in good condition. The problem may be that many RV owners simply aren't good judges of that. Even so, the downside is only that the new battery loses some of its life potential.

So the short answer is, YES, your plan works. You might want to load test the older battery to get an idea of just how good it is. As Marks, says , you can replace it for another $100 .

Load test I will. I have a tester in my shed. That will give me a starting point. Thanks

 

[Lou Schneider]

When you connect the second battery, put the (+) lead going to the trailer on one battery and move the (-) lead going to the trailer to the second battery. This equalizes the overall wire lengths and balances the load better than having both leads on the same battery.

 

[Kirk]

And if you have not done so, make sure the electrolyte levels are proper in the older battery before you install the new one.

 

[John From Detroit]

Battery myths.
A lot of folks tell you that multiple battery systems all batteries must be identical as possible. Ideally sequential serial number identical and this is true.. IN SOME CASES.
They say thigns like "Do not mix a high capacity with a low capacity" or Do not mix old and new"

Oh.. now I will tell you why that is True: If you take two GC-2 6 volt batteries and put them in series you get. effectively one 12 volt (Roughly a 4D or 8D) So why not just get the big 12.. Well you lift ONE GC2 Then the other. and go to the battery store to exchange.. You lift one 4D or 8D you may well go to the hospital with a hernia (Ok I exagarrate a bit I can lift one of those.. but i won't like it)

When you put two batteries in series they MUST be as close to identical as possible
I mean would you want to buy a 12 volt battery where half was brand new and half was 2 years old?
Where half was 200AH and half was 230? (in fact you can't buy one like that.

Now let's talk about batteries in PARALLEL
This is the ultimate commune.

Let's say you put a 100 AH and a 200 AH battery in parallel What happens when you charge
Well after the initial charge/discharge cycle the state of charge on BOTH batteries will be identical.. So let say you pump 30 amps charge into them the smaller battery gets 10. the larger one 20 and the state of charge thus remains in perfect lock step
You fire up the microwave and start drawing 100 amps off them to run the inverter They split the load 33.3333 66.666 same way. And remain in lock step.
They simply work it out.

now the one legit danger
Batteries have basically 3 failure modes
HIGH resistance/open circuit (The battery basically vanishes) The charger may overcharge the remaining battery causing damage.

SHORT. (Shorted cell) Generally one won't cause damage but if two cells short the good battery will try to charge the 4 remaining and may cause damage.

Low capacity... Well this is least likely to cause an issue and is the most common failure mode.

I ran a group 27 29 and 31 in parallel for years.. Still do in fact. all different ages as well. Now today they mostly sit on float but when the lights go out in my apartment, as they did recently for 13 hours.. The Fridge stayed cold... So did the aux freezer but that's a different backup system (30 AH of LiFePO4 and a small plug in inverter. that sucker only draws 30-40 watts runnng)