Charging Sealed Lead Acid Batteries (BP8-12)

[jimnms]

I'll try to make a long story as short as possible. I had a UPS on my computer that one day just shut off and wouldn't turn on. I contacted the manufacturer only to find out that it was a few months past its warranty. They said that the symptoms sounded like dead batteries.

I suspected the UPS went bad and quit charging the batteries, so I didn't want to buy new batteries and find out it was the UPS all along. It uses two BP8-12 SLA (12v 8Ah) batteries. I removed the batteries and my multimeter was showing them at 8v. I have a portable air compressor that uses a smaller 12v SLA battery, so I opened it up and connected a BP8-12 battery to it and let it charger overnight. I checked its voltage and it was holding at 13v, so I put the 2nd battery on and charged it overnight. Then I let them sit for a couple of days and checked them, and they were both holding their charge.

I put them back in the UPS, plugged it in and it came on. I simulated a power failure by unplugging it from the wall and it shut off. It didn't even attempt to run from the batteries. I connected my multimeter to the UPS to test the current and found that with it plugged in it was drawing a small current from the batteries rather than charging them.

So now I'm stuck with a dead UPS and two good batteries. It's not economical to try and sell them because shipping would cost too much, so I though I'd try to make up some lights out of them to light up the house when the power goes out. They're the same batteries used in some of those emergency lights you see mounted on the walls of a lot of buildings.

My only obstacle is how will I charge the batteries. I didn't know if they needed any special charger, which is why I used my air compressor to charge them, but I can't keep taking apart the portable air compressor to charge them. I searched for 12v SLA chargers and the only ones I could find were designed for car, motorcycle and marine batteries, and they costs over $100.

I have a couple of portable jump starter/air compressors that use a larger 12v SLA battery. One of them died, and I still have the wall plug that was used to charge it. It puts out 12v @ 500 mAh. Can I just cut the wires and hook it to the + and - terminals of the batteries to charge it, or does it require a special charger with electronics to monitor the charging?

 

[LEDAdd1ct]

I am *not* a battery/charger expert, but perhaps this would work for $16? Make sure someone else here can vouch for it first.

Link

 

[Light Sabre]

Batteries Plus has some SLA chargers, you might wanna check there.

Sealed Lead Acid Battery Charger - 12V 6V Chargers For AGM or Gel Batteries - Page 1

I just use my 12v power supply to charge my old UPS batteries. I use a 3 amp or so diode between the power supply and battery in case the AC power goes out. I use my DVM's ammeter setting and when the charging current goes way down, then I disconnect the battery.

 

[zipplet]

Hi,

Any lead acid charger should do a fair job if it puts out enough current. I have a 1.5A lead acid charger that is permanently connected to a large 40Ah 12V absorbed-glass-mat battery in here to provide backup lighting and inverter power during long power outages.

As long as the charger floats the battery at the right voltage it can be left connected indefinately. For an 8Ah pack I would not recommend a charger of more than 1-2 amps, but others may have different opinions. Also if you are permanently connecting the charger please put a fuse between the battery and the charger just incase the charger gets killed by a power surge or some other event.

 

[Anders]

Hello jimnms.

I think your lead acid batterys got damaged by the very low Volt, 8 Volt is way too low for any lead acid battery.

I have a motocykle with a Gel lead acid battery, the motorcykle sometimes didnt start, I charged it and after that i let it rest a couple of ours.
The volt was thereabout 12,7 volts and i could start the MC a week maybe, then, after a week it wouldnt start again.

I charged it up and discharged it with my CBAII, it only hold 2,5 A.
The battery was a yuasa 12A.

So, even if I got a high volt reading from the battery it was not capable to work right.

When a lead acid battery is at 10,5 Volt its completely dead, if you leave it there, empty, it would be very hard for the battery. It is very important to always have more than 12 Volt in a lead acid battery.


Anders

 

[J_C]

Come fall I hook up a float charger to our lawn tractor battery. It's just a 12VDC, 500mA AC-DC unregulated adapter with a series resistor to limit current.

If you don't need it to recharge fast and don't want the expense and time to come up with a charge controller, that's what I'd do. Pick a resistor that limits it to about C/100, 80mA. Because it's an unregulated adapter it will keep the battery at about the right float voltage when the current is limited.

Downside is it takes over 4 days to fully recharge, but if your power were out often enough and long enough that this is too long a charge time I expect you'd want higher capacity batteries than 8A in the first place.

 

[VidPro]

Hello jimnms.

I think your lead acid batterys got damaged by the very low Volt, 8 Volt is way too low for any lead acid battery.

Anders

that is the First thing i thought when i started reading the post.
one of my UPSes Croaked when the Battery Itself died, but in that case there was certannly bad cells, and the UPS charger got overworked and gave out.

if the batteries are a few years old, they might not be worth bothering with, depending on the number of cycles, and the time spent down.

any of the good motorcycle battery chargers , or "maintance" chargers (low amperage float) would charge it just fine. motorcycle "maintance" chargers are just cheap and slow.

 

[Capo_au]

If the cells do turn out to be ok and you only plan on using the cells once a week or so then this may be in your price range.

Just note that for an 8Ah cell it would take around 30 hours charge time, hence the once a week use.

-Capo

 

[London Lad]

Hi Jimnms,

I suspect your batteries are dead not your UPS.

There is quite a bit of incorrect information posted here I'm afraid.

SLA batteries should be constant voltage charged not constant current charged. Connecting them to car battery chargers or other batteries is no good.

A 12 volt SLA battery should be charged at 13.5 volts if it is to be float charged (i.e. left on charge all the time) or 14.1 for cyclic charging.

A flat battery will initially draw quite a lot of current but after a while will settle as the terminal voltage rises. However it's the voltage you need to control not the current.

SLAs are not really suited to cyclic use but much prefer float situations such as UPS or emergency lighting use.

 

[jimnms]

Thanks for the replies, I would have responded sooner but I had to go out of town and just now got around to messing with it some more.

It looks like the batteries are the problem, or part of it. They're only just over 3 years old too. I was trying to think of a way to put a load on the batteries since the UPS refused to use them. Then I remembered the air pump I used to charge them up. I connected each one to the air pump and aired up a 110psi tire. Each battery managed to air up the tire, but by the time it was full, the low battery light on the pump had come on. When the tire got to about 60 psi, you could hear it starting to struggle a bit.

The smaller 12v battery in the air pump is good to air up a at least a dozen 110 psi tires before the low battery light comes on.

I still think the UPS is probably bad too because it died so suddenly and since it doesn't even attempt to charge the batteries. The last UPS I had before this one, lasted over 10 years, and it died slowly. The run time gradually reduced to the point where there wasn't enough time to shut down the system before it ran down.

I would usually shut down my system or hibernate it if the power went out during a storm and I knew it wouldn't be back on for a while. If it went out on a normal day, I'd run it for 5-10 minutes then shut down if it didn't come back on.

Only once did it run until the UPS shut off. The last time the power went out (over 2 months ago) it ran just fine for about 15 minutes before it started beeping and I shut down. It doesn't start beeping at you until the battery is down below 30% or 25%.

The screwed up thing is that this model UPS is still being sold, but it now sells for $150 and I only paid $90 for it a little over 3 years ago. A set of replacement batteries would cost $40 ($20 each), but I'd hate to buy the batteries and find out that the UPS is dead too.

 

[London Lad]

Put one good battery from your air pump in the ups in place of one of the bad ones and switch it on. Measure the voltage across the good battery within a minute of switching on and post it here.