Charging lead acid batteries

Blarg King · Jan 23, 2015

Hello all.

So I've got two small sealed lead acid batteries that I've been meaning to use for a small solar power setup I'm working on. Problem is, they're both 6 volt cells instead of the regular 12 volts, and they aren't equally charged anymore.

I have a 12 volt charger that I considered using by hooking the batteries up in parallel, but since they aren't equally charged I imagine that isn't a good idea.

If I was able to get a 6 volt charger and then charge them both fully, would it then be safe to wire them in series and use them as a 12 volt system? Or would it be better to get two 6 volt chargers that can take the 12 volts from my solar panel and charge them interdependently but draw power off them in series? Is that even a possibility?

Thanks!

DashApple · Jan 23, 2015

From what I know as the 6V cells are just 3 2V cells in series , you can put both batteries in series on the 12V charger and charge then , they should all settle to their charged voltage as it would be 6 2V cells

Blarg King said:
I have a 12 volt charger that I considered using by hooking the batteries up in parallel, but since they aren't equally charged I imagine that isn't a good idea.

you cant put two 6 Volt cells in parallel on a 12V charger , that would damage the cells

Hiemal · Jan 23, 2015

Parallel batteries = double the capacity Same voltage

Series Batteries = add the voltages keep the capacity to whatever the lowest battery is

Now as far as charging these goes; You could technically series them and just use a 12 volt charger on them but I don't know how well the batteries would balance.

The safest option would be to get a 6 volt charger and then series them back into the system. That way both batteries are charged at nearly the same voltage (a little difference is okay)

Blarg King · Jan 23, 2015

ionlaser555 said:
you cant put two 6 Volt cells in parallel on a 12V charger , that would damage the cells

Oops. I meant series.

So if I use a 6 volt charger to bring both batteries to full charge then it should be safe to hook them up in series to my solar panel?

Hiemal · Jan 23, 2015

Blarg King said:
Oops. I meant series.

So if I use a 6 volt charger to bring both batteries to full charge then it should be safe to hook them up in series to my solar panel?

Should be safe then, yes.

Blarg King · Jan 23, 2015

Okay good. I read that sealed lead acid batteries are more picky about charging since they can't loose heat as easily and can't just vent electrolyte if something bad happens.

Cyparagon · Jan 24, 2015

You can put them in series without balancing them first, but ideally you want them balanced. After all, your car battery is 6 lead-acid cells in series, and they run for years without individual re-balancing. The balancing happens when it's charged - the current eventually gets converted to heat in the full cells, and continues charging the others.

Le Quack said:
Parallel batteries = double the capacity 1/2 the voltage.

Uhhh... I think you mean same voltage.

Hiemal · Jan 24, 2015

Cyparagon said:
You can put them in series without balancing them first, but ideally you want them balanced. After all, your car battery is 6 lead-acid cells in series, and they run for years without individual re-balancing. The balancing happens when it's charged - the current eventually gets converted to heat in the full cells, and continues charging the others.

Uhhh... I think you mean same voltage.

Ah woops! Yeah I messed that up, sorry.

Blarg King · Jan 24, 2015

Cyparagon said:
You can put them in series without balancing them first, but ideally you want them balanced. After all, your car battery is 6 lead-acid cells in series, and they run for years without individual re-balancing. The balancing happens when it's charged - the current eventually gets converted to heat in the full cells, and continues charging the others.

Hmm. I should get my multimeter out and see if theres a big difference between the two batteries. If its not that big I'll see how many amps the solar charger puts out. If its not too strong a current then I imagine the charged cells would be able to dissipate the heat well enough.

Also its winter so the panel probably won't be at peak efficiency and its cold out so the batteries will have extra cooling.

djQUAN · Jan 24, 2015

You can probably parallel the batteries and leave it alone for a while so that the higher voltage battery will drain and recharge the lower voltage battery so they settle at nearly identical state of charge.

Then you charge them in series with the 12V charger.

Blarg King · Jan 24, 2015

Ah good idea!

Anyone know what the fully changed and discharged voltages should be around?

Your not supposed to bring lead acid batteries below 50% charge right?

ped · Jan 24, 2015

I had a 12v lead acid cell out of a alarm panel ,that was at 0.0v.

I put it on charge and it didn't draw any current. So I broke out the lab power supply, limited the current to 200mA , connected the battery and wound the voltage up to 35v , it started drawing about 10mA , so I left it on, about an hour later, the current limit kicked in, so I used a small 12v bulb to discharge the battery, then I put it back on charge, went through the same process a few times and the battery was rescued, and is in service to this day.

Just thought I'd let that little story out

Gadget · Jan 24, 2015

Blarg King said:
Ah good idea!

Anyone know what the fully changed and discharged voltages should be around?

Your not supposed to bring lead acid batteries below 50% charge right?

edit: Correct...you can take them below 50%, but what happens is you lose part of the total lifetime of the battery every time you take them too low. Better lifetime can be had by keeping the charge state above 70%. And here's a very nice charger, I have their 12v only model. \edit

I've got 3 that I use for camping. One is 200Ah AGM, two are 35 Ah Gels, but they are 12v. I get ~12.9-13v at full charge, and around ~12.3-12.4v at ~70% charge....

Here's a little table I found for ya...

Hope that helps!
-G

USAbro · Jan 24, 2015

I've never seen a charger that can't switch between 6v and 12v.

Gadget · Jan 24, 2015

Well now ya have.

It's just a matter of having 2 transformers to choose from in the same package.

-G

Blarg King · Jan 24, 2015

Very handy chart. Now if I could just find my second battery. Apparently my brother took one to try an make a coil gun and now he doesnt know where it is.

Blah. I'm thinking I'll probably switch to lithium iron phosphate batteries soon anyways.

Charging lead acid batteries

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