Adding House Battery

My uneducated guess is that batteries should be of the same capacity and cell condition whether two 6-volts in series or two 12 volts in parallel for best charging and performance. I would even try to use the same brand batteries.

With RC model electric power system batteries, we use sophisticated balancing chargers that try to equalize the charge levels/voltage of all separate cells. We consider a battery pack no longer unusable when one or more cells stay at much lower voltage despite the balancing charger's efforts to bring all cells up to the same voltage/charge level. I use two 3-cell 2200mah li-poly battery packs in series to power the electric motor in my 6-foot wingspan J-3 Cub model, same as using a single 3-cell 4400mah battery pack.

Would it be correct to say batteries in series are more sensitive to being of equal quality than batteries in parallel? I'm thinking unequal batteries would have different internal resistances, so during charging the one with higher resistance would have higher voltage across it - causing the other one to be undercharged. Batteries in parallel of course have the same voltage across them and should both charge normally.

Kind of late now, but before buying, did you consider going with two 6 volt batteries? It is my understanding that 12 volt batteries are not really deep cycle, or at least, are not as adept at charging and discharging as the golf cart batteries.

Thanks everyone.

After doing some research and reading lots (and also that my house batteries were 5 years old) I went to walmart and got two new deep cycles.

It is not best practise, but it will work and give you more capacity.

I wonder how well-matched three new batteries of the same brand and capacity would be. If the two older batteries are in good shape and have been kept charged properly (not discharged too deeply) they may work fine with the new battery. I would make sure that the older batteries were fully charged and test each cell with my hydrometer to verify the condition of each cell. If two older batteries check out "good as new" and the new battery is fully charged, and it's cells check out good per the hydrometer, they should work well together.
I bought two 6-volt USA brand batteries equivalent to T-105's for 99 dollars apiece. I believe they also make 12 volt RV batteries.

I think the two old batteries -in decent shape- with the new battery (generally) will provide more amp hours of capacity than just the old batteries alone. Of course three new batteries is better.

We dry camp for weeks at a time with two 12 volt batteries and solar I added myself (bought two, 100 watt Renogy panels from Amazon). Like upstech76 said above, the batteries will tend to equalize to the lowest charge of the three when you add a new one to the two older ones.

All that being said, if I didn't dry camp all the time (we seldom use hookups) I would go buy the battery and stick it in for this trip and see how it worked.

A lot of guys get real specific about batteries. I just go to Walmart or one of the discount clubs and buy from them because I am cheap and refuse to spend a fortune for something like a battery. Learning the tricks of living on 12v and battery management and discharge/charge is essential to long term boondocking and you can get all that on these forum archives.

Buy yourself a third 12v deep cycle, stick it in there and see how it works. Just make sure they are the same TYPE of batteries (deep cycle, lead acid, etc.). Don't mix types.

Or, best yet, like was said above buy three new ones that match and REALLY have great power for your trip.....and no concerns.

Enjoy the beach!!!

Paul

It will work but not recommended because your old battery will pull down the new one slightly. Basically your new battery will only be as good as the old ones. Do you have a generator that u can use to charge batteries during the day? Batteries should really be replaced all at the same time. If this is only temporary then Maybe add the extra battery. If your looking for a long term solution. Buy 3 new batteries.