Batteries Not Holding a Charge

[oldcatsailor]

Our Rolls 2HHG325 wet cell batteries have given us over 8 years of reliable service on our live aboard catamaran. Now, however, they are failing to hold a charge overnight, even after several equalization procedures.
These batteries are composed of 6 individual 2.1V cells bolted together in series, with Hydro Caps; we have two for a total of 650 AH.
Prior to equalization, the SG of several cells appeared low: 1.160, 1.200, and 1.210. After equalization and rest, these cells came up to 1.210, 1.230, and 1.260 respectively. The other cells are all in the 1.250 to 1.280 range after equalization.
Since the batteries are this old, and since we are located in a place where we cannot get new batteries until we return to Cartagena, Colombia this spring/summer, I am considering taking all the cells apart from each other, measuring the SG of each along with the open circuit voltage, and then combining the best 6 into just one 325 AH battery that hopefully will hold a charge better than the original arrangement of two in parallel. My thinking is that perhaps two or more of the cells are so bad as to be dragging down the whole bank. Granted, we will halve our capacity, but we don't have huge DC loads; our charging system consists of 300W of solar panels and a KISS wind generator...plenty of sun and wind here in the San Blas most of the time.
Does this sound reasonable; is it worth the effort to try it? I know one should not mix "old" with "new", but in theory this would not be doing that since all the cells are the same age and have had the same maintenance over the years.
Or, should we leave it as is and limp through this season, running the engine more on those cloudy days? We would keep the worst of the cells not being used, just in case this "new" combination of cells proves to be no better.
BTW, the price of new Rolls cells (I dont' need the Hydro Caps and Blue Sea boxes again) is $143 USD each, (times 12!) plus shipping 360 lbs at $1.65/lb...do the math; not cheap! Thinking of replacing with new Trojan T-145's when back in Colombia, but that's not cheap either at over $300 each!
Thanks for your help with this.

[Pblais]

The trouble with bad cells is they trick the charge controller into over charging the rest of the cells. The result is a bad cell cooks the others. You may have suffered some of that damage already. Unless you can find a better magic equalization process to get back a little more life I'm not sure what else is possible. Given you are already using flood batteries you could set up a temporary system using some other batteries that might be locally available with the idea you can replace them when you get back. Setting up the minimal system mean having a good idea of how little you can get away with.

If you don't have one now you might consider adding a battery monitor. It provides a lot more information about what is going on in a battery bank that help you spot problems earlier and thus prolong your next bank. There are quite a few number of threads here about the subject.

[btrayfors]

I think your idea of working with half the number of cells, picking just the best ones, is worth a try PROVIDED that you have some reliable way to identify the good ones. While SG is an indicator, it's not really definitive in terms of reading the health of the battery.

A good way, but probably not do-able there in San Blas, is to use a sophisticated battery tester which measures internal conductance....like one of the Midtronics testers. Just maybe somebody's got one...they cost several hundred dollars.

A cheap load tester might help as well; every auto shop has one.

The best way to evaluate the capacity of a battery is to put a resistive load on it, calculated at about the 20-hour rate (i.e., enough to drain a battery completely to 10.5VDC for a 12-volt battery or 5.25VDC for a 6-volt battery in a 20-hour period based on the rated capacity). Then, you just see how long it takes to reach the terminal voltage. This would be a big chore, though, in your case, so the tester would be better.

Another indicator would be resting voltage after a full charge. Let the batteries sit overnite or longer with no charge and no drain on them, then measure the voltage. This tells you how well they "hold a charge" but, unfortunately, can't tell you their capacity.

Bill

[Viking Sailor]

Having tried to charge bad batteries I have noticed that more often then not only one of the cells had a lower SG. This implies that the charging process is in fact charging the good cells in an otherwise bad battery. Without your having access to test equipment that can test each of the cells under load, I suggest that you combine the cells with the highest SG and charge them until the charging voltage reaches about 13.6 volts (but not more then 4 hours). You will need to note the SG of each cell before and after charging. Place a small load (40 to 100 watts) on the battery for about 15 minutes. Remove the load and measure the battery voltage and the SG of each cell. If the voltage is above 12.5v and the SG on the initially lowest reading cells have come up, then do a normal charge and recheck the battery voltage and the cell SG's. Else replace the cell with the lowest SG and try again. If the battery looks charged try equalizing the battery and recheck the cell SG's.

Best of luck,

Paul

[Cheechako]

I have had this occur a few times over the years, and had it happen recently. Usually it means that one battery is shorted. Charge the system, then turn off all charging devices. disconnect the links between batteries, wait an hour or two and check the voltage on each battery. Usually you will find one that is way out.... like 10+ volts and the others will be 12V or so. Also, when you first stop charging, feel each battery for heat.... usually the shorted one will be very warm....

[oldcatsailor]

Thanks for all the suggestions so far. Seeing as how we cruisers have nothing to do all day except sip umbrella drinks and watch the sun go down, I think I will give all of this a try.
Thought about buying the locally made "truck batteries"; almost as expensive, poor quality, and only good for about a year...so I think I'll spend my hard earned money just once on the Trojans, which are available in Panama City.
I have labeled the cells, so will be able to keep it all straight. Will have to wait on the overnight resting test as the fridge is full and no way to keep the food cold while we have no electricity.
All in all, not a major catastrophe, just kind of inconvenient, location-wise.

[Lost Horizons]

Originally Posted by oldcatsailor

I am considering taking all the cells apart from each other, measuring the SG of each along with the open circuit voltage, and then combining the best 6 into just one 325 AH battery that hopefully will hold a charge better than the original arrangement of two in parallel.

You should definitely disconnect the banks from the parallel connection. With age, the internal resistance of individual cells varied and now you have current constantly flowing through your system (hence, high self discharge rate).

Residual voltage should be a better indicator than the gravity to tell shorted cells from healthy once, as was already suggested.

After disconnecting the banks from each other you may be able to use them both in turns, unless some cells are shorted. If they are, you may still be able to assemble one healthy bank and have a couple of good spare cells.