2 batteries in series giving different voltages

lazza

Hi

We recently had someone who'd installed their 2 x 250Ah 12V solar batteries in series for a 24V system. Apparently it was all working fine for months, however the guy called me last week to say that the system wasnt working properly. In fact, when he measure the individual voltages for each battery one read 12V and the other 15V (charge controller at 27V for float). At night the bad battery went down to 10V and the other around 12.7V. All this with the 2 batteries still connectd!

I've always seen the voltages split evenly between 2 batteries whilst connected in series, and only notising such stark differences once the batteries are disconnected, so I was somewhat suprised.

-Has anyone else ever seen this?
-Any explanations as to why this might be?

Thanks
Larry

Cariboocoot

Re: 2 batteries in series giving different voltages

Yes, I've seen it.
It's called "failed cell". That would be one of them in that "10 Volt" battery dropping dead, taking its "2 Volts worth" out of the equation and causing the charging process to push the remaining good batteries higher than normal in Voltage.

If you don't replace the defective battery ASAP you will have four defective batteries instead of only one as the other three will fry.

lazza

Re: 2 batteries in series giving different voltages

Ok that adds up- what could have caused a cell to fail in such a young battery?

Cariboocoot

Re: 2 batteries in series giving different voltages

lazza wrote: »

Ok that adds up- what could have caused a cell to fail in such a young battery?

Good question. I had the same thing happen with a brand new battery within one month of purchase about four years ago. Cell just suddenly shorted. Sometimes that's what happens.

inetdog

Re: 2 batteries in series giving different voltages

Cariboocoot wrote: »

Yes, I've seen it.
It's called "failed cell". That would be one of them in that "10 Volt" battery dropping dead, taking its "2 Volts worth" out of the equation and causing the charging process to push the remaining good batteries higher than normal in Voltage.

If you don't replace the defective battery ASAP you will have four defective batteries instead of only one as the other three will fry.

+1.
The battery with the failed cell is definitely the one which measures 10V when idle. Some types of failure in a cell (high internal resistance) could cause that same battery to be the one with 15 volts on it during charging, or the one with 15 volts might be the good one being fried (if the failed cell is shorted).

BB.

Re: 2 batteries in series giving different voltages

If these are flooded cell batteries--Time to buy a decent quality hydrometer and start logging temperature corrected specific gravity for each cell...

Bet if they have been doing this at least once per month, they would have had a multi-month warning of the oncoming cell failure.

It could also be a wiring problem that left a battery string without its fair share of charging current....

And the other suggestion, a DC current clamp DMM (like this one from Sears) so they can measure charging/discharging current per string.

Looking for differences a this point... If major differences are found--then need to figure out if the "oddball reading" is a failure itself (failed cell) or the result of another failure (poor wiring connections, improper charging/over discharging, letting cells boil dry, etc.).

-Bill

lazza

Re: 2 batteries in series giving different voltages

Yes that's exactly what we'll be doing. We're taking a trip to replace the battery, but also to find out why it failed! Very odd because this is the first time it has happened with one of our batteries. I was wondering if a short (dropping a spanner between the terminals) at some time, may have caused the failure.. or simply very dodgy wiring

Cariboocoot

Re: 2 batteries in series giving different voltages

More likely a manufacturing defect. But they probably wouldn't admit it even if they knew.

BB.

Re: 2 batteries in series giving different voltages

If a spanner dropped on the terminals--You should see some sine of spot welding (melted lead spot on post, pits in copper terminals, burned or fresh metal, etc.).

When debugging, I try to have the tech (or customer) leave everything alone until I get there. I.e., you don't want the customer removing cables & cleaning connections, tightening bolts, filling cells, etc. at this point. It will destroy any evidence of what may have caused the failures in the first place.

-Bill

lazza

Re: 2 batteries in series giving different voltages

Ok- Just came across exactly the same problem with an older system, that had suffered insufficient panels for about 5 months. The owner only had one panel 195W to go with 250Ah at 24V (2 x 12V batteries in series). One battery reading 13V and the other 10.5V (no sunshine)

I still dont understand what could cause such a failure in just a few months- Could it be excessive discharge?

Cariboocoot

Re: 2 batteries in series giving different voltages

Other than manufacturing flaw my second guess would be some condition causing excessive heat in the battery which led to premature internal structure failure. This could be either too high Voltage charging (not likely with limited panels) or too heavy a discharge at one time.

nsaspook

Re: 2 batteries in series giving different voltages

lazza wrote: »

Ok- Just came across exactly the same problem with an older system, that had suffered insufficient panels for about 5 months. The owner only had one panel 195W to go with 250Ah at 24V (2 x 12V batteries in series). One battery reading 13V and the other 10.5V (no sunshine)

I still dont understand what could cause such a failure in just a few months- Could it be excessive discharge?

All you need is to have one cell that's a little less efficient than the others during cycles of heavy discharge and partial recharge. If the one weak cell can't be fully charged on each cycle it might eventually end up shorted from cell reversal.

http://en.wikipedia.org/wiki/Rechargeable_battery#Damage_from_cell_reversal