Second opinion on LifePo4 cells

[fox87]

Hello folks,

I purchased 4 - 300Ah winston LifePo4 cells for a 12V system in a campervan. I recently got to work with them and am looking for an opinion of some more experienced people.

At the start the voltages were as follows:

Cell 1 - 3.298V
Cell 2 - 3.298V
Cell 3 - 3.299V
Cell 4 - 3.288V

As you can see cell 4 seems a bit off. I did an initial charge on all 4 cells using a 3.65V LifePo4 charger. Cell 4 took about ~16 hours to complete (possible a bit less). All the other cells took about 12 hours.

I found it a bit odd that Cell 4 took quite a bit longer to charge. After talking with the supplier I connected all cells in parallel and did another charge. I let them sit for a day before disconnecting all the cells and checking the individual voltages again with these results:

Cell 1 - 3.551V
Cell 2 - 3.545V
Cell 3 - 3.566V
Cell 4 - 3.495V

Cell 4 still seems quite a bit off compared to the rest. The manufacturer says it should be ok but also says to keep an eye on it. And to contact them if the differences get worse over time.

The cells will be connected in series and I will connect the "123SmartBMS" for safety and balancing.

However I'm looking for some more insight and opinions of more experienced folks. Should I be worried of the voltages differences? Or should it be ok like the supplier says?

It's my first time setting something like this up so I honestly can't tell. Any insight or help will be much appreciated!


Many thanks in advance!

[Ampster]

I dont understand why the resting voltage is so high. When I had Winston's they rested at 3.32.
The next thing you should do is put them in series with the BMS and discharge them. Keep an eye on cell number 4 as they discharge. Log the differences as they discharge. Also log the discharge rate. They thing to look out for as you get past 50% SOC (based on Ahrs out) is if cell number 4 is closer to the bottom or the difference has grown. If it is a weak cell with less capacity it will drop in voltage faster than the others. The only way to truly know is to do a charge and discharge with something that can count Amphours.

The charging times you mentioned are hard to correlate unless you tracked the Amphours because of the taper off of Amps as the cells become full.

[PNjunction]

Ampster is right about being concerned at that high of a resting voltage. Should be nearer 3.38v or so.

What this indicates is that after a 12 hour rest, you have OVERcharged them, and you need to immediately start to discharge them with something ASAP. Right now, the cells are being plated by virtue of overcharge.

So one has to be careful - voltage isn't everything. I suspect when you placed them in parallel and gave them that boost, this is when the overcharge occurred.

But don't wait. Get some sort of load on those things NOW.