Almost Ready...

[ObiRich]

Ok, I need some more battery advice. If you've followed my earlier post, I needed to add more capacity. I already had 2 golf cart batteries (RoadHawk GC15, 225AH). Well, I've now done away with the single boxes, and have them in a double box. The other double box is waiting for batteries.

Now here's the questions.

1. When I took my current ones out, I separated them and tested them as 6 V batteries, and they both tested good under a load. Is there any reason that I shouldn't keep them and just buy 2 new ones?

2. I'd kind of decided on getting the Duracells that Sam's Club sells. They're $90 each, but I'm hoping that they put them on sale (usually $20 off) before I actually need them. But I've read a whole lot of good things about Trojan T-105s, and I finally found a local source for them. But they are pricey, $150. So question, if you have the experience with them, are they really worth the extra $60 each? I mean if the only thing is longevity, then to make it worth it, if I get 3 years out of the Duracells, then the Trojans would have to go 5 years to justify the cost.

[JRTJH]

I'm not sure you can "pair" two GC2 Duracell or two T-105 Trojan batteries (215 amp/hr) with your existing GC15 batteries.

While I can't say "it won't work" I will say that it's similar to pairing a GP24 battery with a GP27 battery (both 12 volts, different amp/hr). If you do, the two batteries will either:
A) burn up the smaller battery while continuing to charge the larger battery, or
B) never fully charge the larger battery if the charger turns off when the smaller battery is fully charged. That will eventually damage the larger battery, making it incapable of accepting a full charge.

IMHO (and as I said in the first and second sentences, I'm not sure) you can't "pair" 4 batteries in a "series/parallel bank" if they are two different sizes, two different charge capacities and two different amp/hr ratings.

I don't think you'll get maximum benefit from the larger batteries and may overcharge/damage the smaller batteries at the same time.

[flybouy]

To John's point I think (dangerous proposition) the only way to charge 2 differing ampacity battery banks would be with a multi battery charger and an isolation circuit in between the battery banks.

Can you do this with a simple A/B manual battery switch? Yes you can IF and this is a big IF, you have an effective battery monitoring system to accurately monitor the state of charge AND commit to monitoring and then changing the switch to charge the other bank. This would be a lot of time committed if using solar to charge, providing you have enough excess solar capacity in the first place. To set it up automatically in my opinion would not be cost effective.

Everything I've ever read on the subject states to replace all the batteries in a storage bank with the same age and capacity. This is an oversimplified example but if you have a 4 cell flashlight you don't replace 1 battery at a time.