Charging LiFePO4 while in series (24V bank)...Opinions requested.

[Goodoz]

I recently ordered and installed a 24V LiFePO4 battery bank on my garage power wall for short term/grid down backup. I connected 2-12V in series and have a 2K inverter and a 24V charger connected. All seems to be working ok right now however after I built my system I discovered through additional research of the conflict of the 24V charge and independent 100A BMS on each battery...ie, if one fills up faster it shuts off the charging circuit for the other. Equipment is as follows...2 x Ampertime 12V 200A batteries with a Progressive Dynamics PD9140-24ALV converter. The inverter is a Samelex 2KWh but really is not part of the dilemma. I'd like for the system to require little interaction from me...just be there when needed. Should I sell the 24V converter and install 2 12V converters to charge the batteries independently? Is there some device that can sit in between the 24V converter and batteries to distribute 12V charging independently? (these will also be charged with up to 1200W of solar, charge controller in hand, but I'm now holding off on finishing until I figure the charging scenario)

 

[Bob B]

Were these manufactured 12V batteries or did you DIY yourself?

 

[Goodoz]

Were these manufactured 12V batteries or did you DIY yourself?

Hello, Bob...manufactured. These : https://www.amazon.com/stores/page/...&visitId=830af90a-072f-44a3-8f9b-2da71e6d9454

 

[Bob B]

HMMM .... The only experience I have with this situation is in a boat for a trolling motor battery. In those cases, 2 batteries in series would be charged with multi-bank charger with each battery having an independent connection.

But ... that is a lower current application than for a power wall.

Since you already have the 24V charger, and the batteries are the same age .... maybe monitor it for a while and see how it goes. It will be critical that the connections are all good.

 

[Goodoz]

HMMM .... The only experience I have with this situation is in a boat for a trolling motor battery. In those cases, 2 batteries in series would be charged with multi-bank charger with each battery having an independent connection.

But ... that is a lower current application than for a power wall.

Since you already have the 24V charger, and the batteries are the same age .... maybe monitor it for a while and see how it goes. It will be critical that the connections are all good.

That's what I have been doing for a few days now...discharging a little then coming back to see where they sit. And I was ok with that...The battery with the + lead to charger is 13.3 and the battery with the - lead sits at 13.2. Pretty close. I became untrusting when I saw this person post this on FB a few days ago...when his first battery is full and the BMS shuts off, he's showing 15-16V across the posts of the battery that finished. That can't be good...

 

[Bob B]

His BMS isn't doing the job if it lets on battery get to that high a charge voltage ..... But, there is definitely a possibility of the batteries getting out of "sync."
When they are in series, if one BMS cuts off charge .... both batteries will quit charging.

 

[bobkart]

I have a UPS I built out of two BattleBorn 12V 100Ah LiFePO4s and a 24V inverter/charger. I ran into this same situation. I resorted to separate 12V LiFePO4 chargers per battery, turning off the charger built in to the inverter.

I only rarely need those chargers though, now that I have a couple of solar panels in the back yard, each one keeping one of the two 12V batteries topped up (via small 14.6V charge controller).

As mentioned above, one battery will fill up before the other, and when that happens, the other won't get any more charge. Only by having one BMS across the entire 24V bank can that be prevented (or no BMS at all, but this risks individual cell overcharge if/when the cells get far enough out of balance to cause that).