Initial charge for lithium iron phosphate batteries.

[dmitch]

I have set up a new solar panel system for my RV, with 4- 200ah lithium iron phosphate batteries. I don't think they are charging properly. When I purchased the batteries, I did the initial full charge on each battery separately with a lithium iron phosphate specific battery charger to 100%. Now that they're connected to my panels and charge controller. I seem to be getting a very uneven use of the batteries all four seem to be sitting at different states of charge. Some of its rather drastic by 15 to 20% or more. The batteries are in parallel connection and I staggered the connections and the charging point from the charge controller to try and equalize the charge between the batteries. I attached a photo of the way I connected the batteries. I guess my long-winded question is, should I have done something different for the initial charge of the batteries before putting them into the system? Or could something else be causing my inconsistent charge rates between batteries?

 

[LakeHouse]

This is likely not really a problem. Your connection scheme is ok. Not as good as bus bars, but totally fine.
Because LFP batteries have a such a flat charge/discharge curve, minor differences (in battery characteristics, cable lengths, connection resistance, etc.) can cause batteries to charge and discharge slightly differently. But since the voltage is the same across a broad SoC range, they don’t ‘balance’ on SoC unless they are near full charge or near empty charge. And this is ok! Functionally there’s no difference between four batteries at 50% vs. two at 40% and two at 60%. When you charge them they balance at 90-95% all by themselves, or if you discharge they’ll balance at about 10% all by themselves because at those SoC’s the voltages actually change.
So for example on the low end, the more discharged batteries will start to decrease in voltage and then stop discharging until the others ‘catch up’ to that low voltage and SoC.

 

[dmitch]

Okay, thank you. I had seen somewhere that the initial charge of the batteries should be done and then fully discharge and then charge again. I can't remember where I saw this but I also don't see anyone else recommending this procedure so I didn't do that. I just made sure to fully charge each one separately to 100% and then installed them in my system. I'm learning as I go and am trying to limit mistakes or FUBAR ?

 

[Pierre]

Are your grid and solar chargers capable of charging each battery at 0,2C ? You need a total charge current of around 160 amps for optimal use.

 

[dmitch]

I am working on adjusting my solar grid to a 6s2p configuration. Right now I am averaging around 700w, 50v, and around 36A. I'm hoping to improve that with the new configuration... My controller is capable of up to 1500w, 150v and 100amps. Not sure if that answers your question, I'm a serious newbie at this solar, inverter, battery bank stuff. Please forgive my ineptitude.

 

[sunshine_eggo]

Are your grid and solar chargers capable of charging each battery at 0,2C ? You need a total charge current of around 160 amps for optimal use.

HUH?

These are LFP batteries.

 

[Pierre]

HUH?

These are LFP batteries.

So what is your suggested charge rate then ?

 

[sunshine_eggo]

So what is your suggested charge rate then ?

Anything less than rated.

 

[Pierre]

Anything less than rated.

Yes , 0,2C is less than rated. So sorry , I bow to your superior knowledge.

 

[sunshine_eggo]

Yes , 0,2C is less than rated. So sorry , I bow to your superior knowledge.

Cool sarcasm. What is your basis for the following:

You need a total charge current of around 160 amps for optimal use.

The above is nonsensical. How do you identify 0.2C or 160A for "optimal use". You're setting some arbitrary goal with no substantiation.

What is "optimal use?"

The reality:

Any charge up to rated current is fine.
Lower charge currents to lower voltages may increase cycle life.