They are very interesting reads, to be sure. It was interesting to compare what they said, compared to what I saw in our old mismatched wet cell setup.
They were looking at a very specific use, it appears in the telecom backup stuff, and very carefully picked out what they considered OK or not so OK in results, I think, but that doesn't change what they saw. It certainly does look like for that use, matching is a lot less critical.
A couple of thing stood out a bit. They did see the expected differential discharge and recharge from strings based and capacity, chemistry and style, and age. They did not consider this to be a big issue. They did not really test anything that would simulate how RV batteries are used in relation to partial discharges, either fast or slow, then not recharged, except to note you wouldn't want to leave batteries that had differentially discharged connected after use as they would cross charge.
One critical thing that makes what the did way different than RV use is that the recharges were at float voltage, which can change everything when compared to using higher voltages that go above the gassing temperature.
As Marko says, his setup of mixed capacity, and AGM type is looking pretty good, with everything full, and the articles do back that up with what they saw. I think the critical thing is immediate recharging and a long time on float, with lesser time in absorption that I would normally think would be best. It will be interesting to see how Marko's system reacts to being away from continuous float voltage availability for extended times (if that ever happens).
I think the conflict might appear in that in order to get the batteries back up to full charge regularly, he would have to stay in absorption longer than desirable for the first to fill cells to get done in a reasonable amount of time. On a float recharge that is not an issue, but it could take way longer than you usually have while camping. How long they absorption mismatch would be, I have no idea, but will be interesting to know if it is noticeable. I think that it might be a good idea for a mismatched system, if you are using return amps to determine when to go to float, to try to determine a good compromise between leaving some cells on absorption after they are full and leaving others slightly less than full. That would probably minimize any risk to either groups as far as you could, and it would hopefully even back out once you can get longer float times.
As to what we saw with our mismatched wet cells. They were two 6 volt T145, 260ah GC2 case Trojan wet cells parallel with one 12v Trojan SCS200, 115ah group 27. They were four and six years old at the end of our use this year, respectively.
We did not see them share the discharge or recharge evenly as the articles said they saw. They said the shared the % of the load (amps) the same as the % of their capacities. We could not see the individual discharge amps while on the road, but we could see the recharge amps by battery, and we could check specific gravity by battery. The 12v battery discharged faster and recharged faster than the two 6 volts. When on the recharge side it was finished much sooner, even though it was further discharged when the charge was started. This is probably because of the slightly different style of the battery although both supposedly have the same chemistry from the same manufacturer. It was of significant issue to us because our charger would only drop the voltage .5 volts when the 12v got full so it was still above gassing until the 6 volts got done. We also saw the 6 volt side charging the 12v side if a recharge wasn't done right away, based on specific gravity.
I think what we saw would indicate that what the authors concluded as being OK for stationery battery mismatch would not be as good in the RV world. The recharge side and voltage is probably the biggest part of it, and secondarily the cross charging after a discharge if the batteries do discharge at a different % of load than their capacities would predict.
From what we saw with our system, different capacity batteries of the same style and manufacturer don't match nearly as well as the UPS batteries they tested. I think as soon as you get away from the even % of load sharing based on capacity, your problems will multiply quickly in the RV world, based on our experience.
|
Linear Mode
