What would be the best way to charge individual 2v cells?

[justin]

Hello, been a long time reader/lurker but I'm now building my little off-grid house and have a question about charging individual cells.

I have fourteen 2v 110Ah cells which I'll be using in 2 paralleled strings of 6 series cells. This uses 12 cells which will be charged by my solar panels (2x85w) via a charge controller. I have 2 spare cells but I'm not sure how best to charge them to keep them topped up for whenever they might be needed in the future, either swap them into the strings every so often and rotate a different cell out each time, or is there a 2v charger I can find (or build) to keep them topped off as necessary.

I've not found any answers to this elsewhere. Is it possible to modify a regular 12v charger? I've looked at wallwart type power supplies and found some that output 3v but at about 0.5amp which I thought might be quite close to what was needed to at least trickle charge it.

I'd appreciate any suggestions you might have.

Many thanks,

Justin.

Moved to the proper section. TW

[wpowokal]

Justin, provided you have enough incoming to keep the main banks full I would using jumper cables connect them across other cells. I would rotate which ones I connected them to, I have done this with no adverse affect, it is not necessary to leave them connected, maybe every two months when you have surplus incoming power.

allan down under

[Flux]

To the purist this may seem a crazy solution but I am inclined to go along with it.

If you were using the spare cells it wouldn't work but if they are in good condition then they will take very little charging and as long as the main string is being well enough charged things will equalise out fairly quickly. I agree that it is best to alternate the main cell that you parallel with. Only do it when there is plenty of charge and remove the spares when topped up.

Any other method will need some form of charge controller or manual intervention that will be far more labour intensive than Allan's suggestion.

Flux

[justin]

Many thanks, that sounds like a reasonable solution, good to know it's not overly complicated to keep the spares up to scratch.

Cheers,

Justin.

[Ungrounded Lightning Rod]

I'd put them in series and hook them to the main bank through a small lamp - like a dashboard indicator lamp.  This would provide enough power to offset leakage and keep them topped off without risking overcharging them and boiling off too much of the electrolyte or being a major drain on the main bank.  (It also doubles as a handy nightlight if you have to work on the electronics at oh-dark-ridiculous some night.  B-) )  Or use a resistor to reduce risk of burnout (and maybe an LED for a current indicator).  Within a reasonable range of trickle currents the batteries will self-regulate (though they'll outgass a bit - these ARE flooded cells, right?)

Hang a cheap digital multimeter across them and check now and then to be sure the voltage is not drifting.  Adjust the trickle current up or down (by changing lamp size or paralleling/seriesing additional lamps) so they stay at full charge rather than creeping up to equalization voltage or down to partial discharge and sulfation.  Also check water level.  You'll need to check it every few days at first but once it's balanced you can cut back to doing it when you do your other maintenance.

Another approach would be to build a small current-limited voltage regulator (say a switcher for efficiency) and set it to the desired float voltage.  This would keep them topped off correctly with minimum power consumption.

You will want to cycle your spares (down to a safe-discharge level) a handful of times.  Deep cycle batteries don't have full capacity until they've been cycled a bit and you'll want them roughly matched the the rest of the bank if you have to swap out a bad cell in the main bank.