Hello

I've just bought 2 brand new 12V/39Ah deep cycle batteries (CSB EVH12390) and a mean well 24V charger (ENC-120-24).
Connected the 2 batteries in series
Without charger: both 12.96V
But charging I have one battery with 15.29V the other one 13.64V (Charger has 28.93V)
Tried two hour and and the first one (15.29 goes to 15.32V)
Specification of battery says 15V max. for cycle use.

I tried also 2 other batteries 12V/40Ah (GP12400) almost same result but this batteries changes voltage rapidly to 14.9V and 13.88V after 5min

My question. What is wrong? Both batteries have same characteristics (same internal resistance). Why they don't divide the voltage igual?
Changing order of battery makes no difference i.e same battery has same voltage...

I don't want to burn up batteries.
Does this equalization itself charging for more time?
Should I charge first both full with a 12V charger?

Thanks for any hints!

 

Does this equalization itself charging for more time?

Yes, I suspect it trying to equalize them first.

Let it do its thing for a few hours and see what happens with the two voltages being most new batteries are not fully charged and when first used in series they need to be balanced out.

 

I've connected each one at at 12V charger. They charged full in less than 1 hour.
Now connected to the 24V charger same result...
Both without charger have 13.1V connected charger and charging one battery rise to 15.2V the other 13.6V
Because they are AGM batteries I think this is not the nice way...
I hear blubbering.

Will to tomorrow a test charging over several hours and hope they will survive...

 

You might need to put them under the normal load conditions until they need a full charge again for a few cycles to condition the new cells for similar internal resistance.