AGM/SLA charging question

[JCG]

Hi all, I have a fast and possibly easy (and at the moment hypothetical) question regarding charging sealed lead-acid (or AGM) batteries.

Suppose that you have a system that requires 96-108 VDC or so, and you use eight (identical) 12 V AGM batteries in series to provide it. Now, also suppose that you have on hand a DC power supply that provides exactly 108 V. Would you feel safe charging the AGMs (still connected in series) with this power supply over a long time period? The 108 V would correspond to 13.5 V per battery, or 2.25 V per internal cell (2.25 V is the "standby charge voltage" or "float charge" per cell that I've seen on the AGMs I own). I've read that you can apply a float charge indefinitely.

I know it's best to just charge them all in parallel at 13.5 V instead, but if you had a high voltage DC power supply, could you use it on a series-connected bank without ruining something? You would never try this with any other chemistry, I'd bet; but maybe with AGM it's possible. Thanks in advance for any advice.

 

[dogman dan]

Depends on the amperage of the power supply. A smart charger will keep voltage constant while lowering the amps when it goes into float mode. So your power supply , continuing to supply high amps will cook off the batteries if not turned off at the right time. It can be done, but requires monitoring to avoid overcharging.

 

[JCG]

Thanks DM - I believe that when charging at the constant-voltage condition with a low applied voltage (float charge potential), the current will drop to a low value automatically as the battery charges, even if there is no current regulation by the supply. In the way that an automotive alternator would work, for example, with only voltage regulation. I was considering this after reading an item on the Battery University web page:

Battery as a buffer
While dwelling on float-charge, an external load can be connected to a lead-acid battery. In such a case, the battery acts as a buffer. Micro-towers on cell sites work this way. During off-peak periods, the batteries get fully charged. On peak traffic times, the load exceeds the net supply provided by the rectifier (charger) and the battery supplies the extra energy. A car battery works in a similar way.

So I wouldn't worry about pumping too much current into the battery stack at the proper voltage, the battery will resist that current input more as it charges until the low float charge current is achieved. But, I'm really worried about cell balance. If I charge a series of twenty identical-capacity batteries using exactly 270 VDC, can I feel safe that some won't be charged to 15.5 V, and others to 12? Again, I wouldn't even consider it with Li-ion, but maybe PbA... they are supposed to be more "forgiving" and aren't often used with sophisticated balancing circuits. Has anyone tried this?

 

[grindz145]

I built a honda CRX EV as part of my senior project in college. We used 14 or so in parallel and we just charged the whole thing in series. If they are identical its not going to be a big deal. You may want to keep an eye on them to make sure they one doesn't get really out of balance from the rest, and periodically tap in (with alligator leads or whatever) and another low volt supply to help out some of the higher impedance Batteries.

 

[fechter]

I think a lot of car-sized EV's just charge them in series. If the cells are well matched, they should stay reasonably well balanced for a while. At the equivalent of 13.5v per 12v battery, the charge current should drop to near zero and you can float indefinitely.

Eventually, one cell will get weaker than the others and tend to cause an imbalance.

To be really safe, you could use shunt regulators on each cell. If the shunts were set at ~15v, they should normally stay off even when fast charging. Check out Rudman regulators http://www.evsource.com/tls_MK2x.php
It may be worthwhile to just make a simple version of something like that.

 

[JCG]

Thanks guys. I should have thought of it in terms of the approach that lead-acid EVs have used - for example the 96 V systems that people have used in DC motor car conversions.

At the equivalent of 13.5v per 12v battery, the charge current should drop to near zero and you can float indefinitely.

Yes, this is what I'm seeing in a simple experiment that I'm running during the day (left it in the lab while I do other things today): exactly 56 VDC with no current regulation to charge four small (4 Ah) SLAs in series. The current went to near zero quite quickly. I'm going to leave them "floating in series" during the day and will check each cell separately when I get ready to leave. I'll hope for the best!