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I know with multiple lead-acid batteries you should really charge them separately so that one battery doesn't try and charge another and waste power.

But that holds true for the main charging process.

What about the "float" portion of the charge?

I have a number of SLA batteries that I'd like to keep charged up. The initial charge of each battery will be done separately as and when they get removed from the bank and used, but between uses, when they have been fully charged, I'd like to have a single float charger to keep them topped up.

Is it safe to just whack them all in parallel with the one float charger, or would I need to have some form of separation (e.g., a diode per battery), or even an individual float charge circuit per battery? I'd like to keep it as simple and cheap as possible.

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    \$\begingroup\$ In actual practice, people put lead acid batteries in parallel and cycle them that way frequently. Just look at RV's and boats and off-grid installations. A fuse for each battery would not be a bad idea. \$\endgroup\$ – mkeith Jan 11 '18 at 1:42
  • \$\begingroup\$ If you are charging them all anyway then what does it matter if one discharges into another? Apart from possible sparks when connecting them, of course. \$\endgroup\$ – immibis Jan 11 '18 at 2:15
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In theory it is OK to connect them in parallel with two conditions:

  1. Each battery must be in a state where it can be voltage charged. This is fine for lead acid batteries unless they are very run down. Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged.

  2. The power supply is capable of maintaining the fixed float voltage.

In practise, I think it's a good idea to put at least a diode in series with each battery just because stuff happens. Of course the power supply voltage needs to be adjusted to get the float charge voltage after the diodes.

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  • \$\begingroup\$ If the batteries stay connected all the time and have the same capacity (and age), can one assume that the state of charge is very similar? \$\endgroup\$ – 0x6d64 Nov 30 '11 at 17:12
  • \$\begingroup\$ @ox6d64: Probably mostly sortof. \$\endgroup\$ – Olin Lathrop Nov 30 '11 at 17:24
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Float charging in parallel should work well enough as long as you charge them to this state separately, as you say you intend to do.

This probably violates the most proper method of long term maintenance where a "topping charge" is occasionally applied to floated batteries BUT this is usually only once per 6 months so probably an issue.

As ever with batteries Battery University is an excellent place to look. Their Pages on lead acid charging are here .

"Fully charged" is notionally 1.2V /cell or 13.2V/battery but do read battery university advice.

This discusses quite a number of aspects that may be of value in this application. A crucial level not to go below is 2.1 V/cell or 12.6V for a "12V" battery. This is the voltage below which sulphation can occur - which you really do not want. At the other extreme, go to high and you get grid etching which can permanently reduce capacity.

As Olin noted, diodes can be used with a suitable increase in target voltage to compensate. Not essential but helps prevent magic smoke release.

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  • \$\begingroup\$ I have read that desulfation on SLA batteries (I guess because of their gel/fibreglass pads) is pointless and ineffective. Do they suffer from sulfation in the same way as non sealed ones then? \$\endgroup\$ – Majenko Nov 30 '11 at 14:52
  • \$\begingroup\$ @Majenko - All lead + sulphuric acid batteries have potential sulphation problems. Sulphation only occurs below 2.1V/cell so a cell kept above that voltage will not "sulphate up". | Whether you can remove sulphation is the subject of extensive and intensive debate. Some claim that some pulse charging schemes can do this and others hotly deny it. You'd think that testing would be easy enough (if not quck). Some people sell proprietary chemical solutions to add to battery acid which claims to allow you to recover heavily sulphated batteries. SOME such sound very plausible. Some don't. \$\endgroup\$ – Russell McMahon Dec 1 '11 at 8:14

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