[SOLVED] Battery mismatch in series

[hassaan1309]

Hi,
I am using a 24V inverter in my solar system. Recently I have changed its batteries. Both batteries are bought together, they are of the same value, same brand and from the same batch. But still, there is a mismatch in their voltages. During charging, when the total voltage of battery bank reaches 27V, the voltage of one battery appears to be 13V while that of other battery appears to be 14V (batteries are ofcourse connected in series). This is a big mismatch. I don't know why is it happening? And how to correct it? If anyone has any idea or similar experience, can you help me resolving the issue please? :|

 

[red_alert]

Try to fully charge them separately then reconnect them in series.

Perhaps they had different state of charge when you bought them. When you put them in series, the most charged one (A) will dictate the rate of charge for both of them (its internal resistance raises up more quickly).

Even the other one (B) needs more bulk charging, the charge current will be limited thus (B) might reach the full state of charge after a much longer period (although there is a risk to overcharge (A) during this time).

 

[KlausST]

Hi,

the keyword is "cell balancing".

Search the internet on this. There are many informations on this.

Klaus

 

[BradtheRad]

A battery hydrometer can help you track down an imbalanced cell.

There is a slim chance that a lead plate has bent or loosened, causing it to touch a plate of the opposite polarity. The cell is short-circuited.

Or, plates can develop bad contacts, etc.

 

[Warpspeed]

Hi,
And how to correct it?

It sounds as though these batteries are fairly new, in which case there is more likely to be a different state of charge than an actual faulty battery.

Try connecting a small constant load across the higher voltage battery, maybe a 12 lamp, and leave it connected until the voltages become equal.
That may be all it needs, but you will not know for sure until you try.

 

[KlausST]

Hi,

Do a test:
When charging, then check voltage of both batteries. stop charging wehn the voltage of one battery is beyond 14.4V.
Mark the battery with the higher voltage.

Now discharge the batteries and check both voltages. Stop discharge when one battery is below 10.5V.

If now the battery with the lower discharge voltage is the same as the one with the higher charge voltage, then it is problematic.
Then you have a stronger and a weaker battery. The problem is, that you can´t use the full power of the stronger battery, only about the same ammount than from the weaker battery.
But because of the higher charge voltage and lower discharge voltage the (yet) weaker battery becomes more stressed than the other. The weaker becomes even weaker...

And lifetime of the overall battery system is determined by the weaker battery.

How to correct this? I think the only way is a relatively complex charge balancing circuit.
Or you select two matching batteries from a lot of batteries...

Klaus

 

[hassaan1309]

Thanks for all the inputs. I charged both the batteries separately to full voltage i.e. 14.4 and then reconnected them in series. Now the batteries are well balanced. Maximum 40-50 mV difference which I think doesn't matter.

Try to fully charge them separately then reconnect them in series.

Perhaps they had different state of charge when you bought them. When you put them in series, the most charged one (A) will dictate the rate of charge for both of them (its internal resistance raises up more quickly).

Even the other one (B) needs more bulk charging, the charge current will be limited thus (B) might reach the full state of charge after a much longer period (although there is a risk to overcharge (A) during this time).

Thanks, this worked