Charging sealed lead-acid batteries

[Mark Hammer]

I have a pair of 6V Eagle-Picher lead acid batteries (https://www.eaglepicher.com/sites/de...cf6v_12v45.pdf) that I'd like to be able to use for a portable amplifier. I have them in series and have been attempting to charge them up with a 1A/12V charger. I can leave them charging the whole day, and have never managed to get the pair above 4.5V. I don't know if this signifies the batteries are past their best-before, or whether I'm recharging them wrong.

Any insights?

[Enzo]

Can you put a 6v supply on one and check its current draw?

My battery work has been minimal, but I note the data sheet wants about 7v for charging. Like maybe it needs some "headroom". Limited to 1.6A.

[nhbassguitar]

1. What is the batteries' manufacture date? If they're more than five years old, it's a crap shoot whether they'll be able to come up to full charge, or if they can, how long they'll stay there. Even a battery left constantly on a maintenance (trickle) charger and never put into service will approach EOL around the five-year mark.

2. I agree with Enzo to treat each battery separately. Charging in series can be an issue if one of the pair has high internal resistance due to aging. As a quick sanity test, disconnect the charger at one end, then quickly check the voltage at each battery. You can leave the + to - jumper in place. Observe whether each battery self-discharges over the next few minutes. You will have to move your meter leads back and forth (only one lead if you have a digital meter and you leave one connected to the + to - jumper). Move the lead (or leads) every ten seconds or so, so you get a feel for the relative discharge rates.

In theory, charging in series should work, but in my book it doesn't qualify as "best practices." Buy a six volt charger that restricts its max current to 1.5 amps, and charge each battery separately. That way, you're guaranteed a pristine charging scheme, i.e. you've eliminated all unknowns.

[shortcircuit]

If you shake the battery and you hear internal noise like sand, they are sulfated and are not any good. Your charging method should have given you adequate results .

[Mark Hammer]

Thanks for the quick replies. Couldn't tell you what the date-of-manufacture was, but I bought them easily 6-8 years ago, and they have sat unused since that time. Silly me, I though they were both Eagle-Picher, because they look the same. But now I pick up the pair and see that only one is an Eagle-Picher, and the other is a Panasonic LC-R065P, not that that probably makes a difference.

Although they don't charge up fully, the batteries do, in fact "charge up" to different voltages, suggesting something different, internally, with one of them. I'll poke around the garage and see if I have a 6V charger.

[g1]

I had a flashlight with a 6V SLA batt. The charger was a 9V wall wart.

[nhbassguitar]

Thanks for the quick replies. Couldn't tell you what the date-of-manufacture was, but I bought them easily 6-8 years ago, and they have sat unused since that time. Silly me, I though they were both Eagle-Picher, because they look the same. But now I pick up the pair and see that only one is an Eagle-Picher, and the other is a Panasonic LC-R065P, not that that probably makes a difference.

Although they don't charge up fully, the batteries do, in fact "charge up" to different voltages, suggesting something different, internally, with one of them. I'll poke around the garage and see if I have a 6V charger.

Meh... those things are shot. You're wasting your time. Get yourself a new 12 volt battery and be done with the problem. I've used batteryclerk in the past and have never gotten a dud from them. What you want is probably on this page: https://batteryclerk.com/collections...acid-batteries

[Mick Bailey]

The differing internal resistance of the batteries causes one to charge more than the other. With SLA they're not to critical and they eventually self-equalize and come up to the same terminal voltage. It does mean though that one battery will eventually fail sooner than the other. Some dedicated in-situ chargers use methods to balance the charge (similar to parallel resistors with series PSU caps) That's with pair of good batteries. The internal chemistry of lead-acid means that to prevent deterioration they need a maintenance charge. Deep-discharge over a prolonged period results in one or more cells not being able to accept a charge.

Interestingly, series charging has become really important with Lithium battery packs. One cell can hog the charge and get hot or vent. For those you need a distributed charge controller to make sure each cell has the correct voltage and current.

I made up a simple charger from a published schematic that really works well for a pair of SLAs. No equalized charge, but regulated current and voltage for a portable busking amp. I'll dig it out and post it.

[Mick Bailey]

Here's the schematic. The input power supply is 1.2A and needs a high enough voltage to account for regulator dropout. I used a 15V supply and it's all been working fine.