Best way to charge two 12v batteries connected in series

[Guna]

When connecting two 12v lifepo4 in series for 24v which is the best way to ac charge them? Using a single 24v charger for both batteries or using two 12v chargers on the batteries individually like they show in the attached diagram?

 

[pollenface]

I vote single 24v charger

 

[rmaddy]

Is this basically for a one-time initial charge of the batteries while setting up your system or is this for regular charging while they are in place and permanently connected in series?

Before ever putting two (or more) batteries in series they should first be charged and brought to the exact same state of charge (SOC). This can be done by first fully charging each 12V battery separately with a 12V charger. Then connect the batteries in parallel (yes, parallel, not series). With the batteries in parallel, reconnect the 12V charger so the + lead is on one battery and the - lead is on the opposite battery. Charge them as a unit. When fully charged disconnect the charger but leave the batteries in parallel overnight. Now the batteries are all at the same SOC and can be connected in series.

Once properly balanced and in series I would use a single 24V charger as shown in your picture. Using two separate 12V chargers will likely lead to the two batteries getting out of balance.

 

[Bob B]

Take a look under the hood of any bass boat and you will find a multi-bank charger for the trolling motor batteries.
I don't know why this is the standard in fresh water fishing boats, but it works very well.

 

[Aussie Collie]

To make sure they don't get out of balance once in series, is it feasible to use something like a Victron Battery Balancer?
I read a very old post on the Victron forum that they are suited to lead acid only.
Anyone know if this is still the case?

 

[rmaddy]

To make sure they don't get out of balance once in series, is it feasible to use something like a Victron Battery Balancer?
I read a very old post on the Victron forum that they are suited to lead acid only.
Anyone know if this is still the case?

I originally planned to use that battery balancer but I too saw it was only for lead acid. So far, after 5 months of full-time use my two batteries in series have stayed at the same voltage.

 

[Aussie Collie]

Thanks rmaddy. I'm glad it's working out for you.
I had the same question in regards to keeping them in check.

 

[Roswell Bob]

Wouldn't a single 24v charger be more likely to get the batteries out of balance? i would use a pair of 12v chargers - no?

 

[Pierre]

I take it the batteries are in service when you want to charge them ? Good question in #3 , 1st paragraph , above.

 

[12VoltInstalls]

Wouldn't a single 24v charger be more likely to get the batteries out of balance? i would use a pair of 12v chargers - no?

No.
In parallel two batteries charged from one end will eventually be unbalanced but two in series? The neg(-) and pos(+) have no choice but to be equal value of opposing polarity.

 

[rmaddy]

Wouldn't a single 24v charger be more likely to get the batteries out of balance? i would use a pair of 12v chargers - no?

The opposite is true. With two 12V chargers you end up charging each battery independently so you can never get them to the same SOC. If the two batteries in series are at the same SOC to begin with (using the steps I described in post #3 above) then using one 24V charger across the two batteries in series will charge the two equally.

 

[Roswell Bob]

The opposite is true. With two 12V chargers you end up charging each battery independently so you can never get them to the same SOC. If the two batteries in series are at the same SOC to begin with (using the steps I described in post #3 above) then using one 24V charger across the two batteries in series will charge the two equally.

I see. I worked for an outfit that did some battery packs for Airlines. They were K2 cells - a little bit bigger than the 18650. I'm fairly certain they were the cobalt technology. The were 8S with around 50p. They used 8 chargers, one for each s. I suppose if you drive them all up into the curve you will be ok?? Does that make any sense?