Troubleshooting SLA's Wired in Series

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Last seen: 15 years 6 months ago
Joined: Friday, October 19, 2007 - 00:22
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Troubleshooting SLA's Wired in Series

I have 3 12V SLAs wired in series to drive my Electric Bike. It is an EV Global, so I am using the built in charger that came with that. Due to my abuse of the batteries (not plugging them in at times, etc), my ride has been reduced to a couple miles before I "go red".

My hypothesis is that I probably fried one of the SLAs worse than the others, and if I were to replace that one then I might be able to purchase just one replacement rather than all three. Is that true, or is there some rule that they are going to all go bad equally if the same voltage and capacity?

Given that, what is the best way to identify the problematic battery? Should I measure voltage when charged? Voltage when run down? How much different would one expect to see them.

I only have an analog multimeter, and when I measures the three after running them down it appeared that one was just very slightly lower voltage than the other two.

Many thanks for tips,

Last seen: 8 years 6 months ago
Joined: Saturday, December 22, 2007 - 08:04
Points: 211
Re: Troubleshooting SLA's Wired in Series

Since you only have a voltmeter, run the batteries down further and check the voltages again.
If one is bad, it will drop much further then the others. I have a load tester from Sears to
test the 5 batteries in the Zapino. I had one recently that one battery voltage dropped like
a rock, the other 4 were OK.

Robert Dudley
E-Scoot Tech

dogman's picture
Last seen: 14 years 6 months ago
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Re: Troubleshooting SLA's Wired in Series

It really is advisable to replace the batteries as a set. Especially if the others at full charge are not over 13v. If any of em are under that they are ready to go and replacing just one won't increase your range much.

Be the pack leader.
36 volt sla schwinn beach cruiser
36 volt lifepo4 mongoose mtb
24 volt sla + nicad EV Global

Brock's picture
Last seen: 12 years 7 months ago
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Re: Troubleshooting SLA's Wired in Series

I would second replacing all three. Even if one is "bad" the other two might be ok. If you put a new one in it will rapidly fall to the same condition as the other batteries, and may actually cause the other two to fail sooner. It really depends on the age and abuse of the battery as to how well or bad they will work together in the end. In some cases it makes since to change just one, but again it's taking a chance.

Another thing you can do is charge each battery one at a time with an external charger. It could be one is undercharged causing it to never fully charge, which has a cascading effect causing the battery to fail sooner.

Last seen: 6 years 4 months ago
Joined: Friday, December 1, 2006 - 09:01
Points: 712
Re: Troubleshooting SLA's Wired in Series

Balance is a BIG DEAL with SLA's. This is true both on charge (especially if you use a serial charger) and discharge.

Think of the pack as three gas tanks in series rather than a single larger one.

On charge, with an unbalanced pack,the lower capacity battery will fill up sooner than it's mates. When full, the internal resistance goes up, and the voltage across the cell goes up. The over-driven battery then can generate gas beyond the re-catalytic capacities of the battery, using up the electrolyte and cooking the battery.

On discharge, the lower capacity battery will empty faster, and some cells can be driven negative by the other two batteries.

You can see this with a voltmeter. After a ride, put a moderate load on the pack by stalling the motor, and check the voltage of each battery. You'll easily see the bad one!

Replacing just the bad cell WILL work if you: 1) use a parallel charging system

where each battery sips from the same nominal 12V source until all are satisfied, and

2) You don't ride so far that the lowest capacity battery is driven below 10.5V UNDER LOAD. Lee Heart has a circuit to flag a weak battery in a pack, but it's only for an even number of batteries in a pack, AFIK.


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