3 yr old Navitas /Topband 25Ah cells. 3.285 to 3.297V - Should I parallel charge them?

[petee_c]

I've got 14 cells leftover from an order back in March 2021.

Saw Wil's video on LiFePO4 cell battery life. Concerned a bit about self discharge overtime as these have been sitting in my 67F basement for the past 40 months.

I was thinking about parralel charging them back up. But the cell voltages are good with my multimeter. I hooked the battery to the leads for about 5 seconds and wrote down the cell voltages when they stabilized.


Sitting in a box for 3.5 yrs.

There is only a 12mV difference between the highest cell and lowest.


I've got:
2 @3.297V
5 @3.296
2@3.295
1@3.293
1@3.292
1@3.291
2@3.285

Looking at this chart, they are between 65-70%SOC



Should I top them back up and put them back in the box? I have no idea what useful thing I would make with these cells.

I remember when I made a couple of 12V50Ah battery for trolling motors , I connected a bunch of them in parrallel and top balanced them with my hobby charger. I used bare solid 14g wire (ground wire from some scrap romex) and hooked them up that way to connect them to my charger.

My search of the internet (and this forum suggests a 20mV delta is safe to hook up in parralel) Is this what other people found?

I might hook the bottom 5 cells together in parralel since there's only a 0.008V difference between their resting voltages and charge them back up to 3.7V. I could probably charge them at 15A safely through the aligator clips to 14g wire. Repeat with the top 9cells.

The bottom 5 cells might need 35% SOC added to them, should take me about 4-5hrs at 15A.
The top 9 cells should take 5-6hrs to bring them back up to 100% SOC @15A

My hobby chargers have a LiFe profile at 3.7V.

 

[sunshine_eggo]

That chart isn't very accurate. I get cells at 3.28-3.29V, but they're at about 40% SoC determined by test.

Pick a line and recharge when they cross it. Recharge on a time/capacity basis. Simply add 5Ah/cell to ensure the cells gain about 20%.

Hooking individual cells up in parallel isn't that big a deal. I wouldn't hesitate to connect individual cells in parallel at up to 0.1V difference. The cells have internal resistance as well as the connection, and as current flows, the voltage delta rapidly diminishes. At 0.1V difference, you'll get about 50A briefly with those specific cells.

I have 4 of the green Navitas and 4 of the blue topband. The little Navitas cells are pretty amazing. I haven't done a lot of testing/monitoring on the blue topband.

 

[BatteryBuff]

Self discharge of LFP batteries is almost entirely from the BMS which you don't have. I recommend that you leave then in the box and recheck them every 6 months.

Alternative, you can put them in parallel, charge them up fully and the Discharge them to get to about 40% SOC.

Either way, just get them back into the box, kiss them goodnight and then check them again in 6 months.

 

[petee_c]

thx for the info.... do u have a better chart to use.... i understand voltage is not the best way to keep track of SOC for LiFePO4

 

[sunshine_eggo]

thx for the info.... do u have a better chart to use.... i understand voltage is not the best way to keep track of SOC for LiFePO4

No. There's some variability between manufacturers and with extended storage periods.

I would draw a hard line at 3.20V and then add 5Ah/cell for a controlled increase in SoC.

 

[sunshine_eggo]

And check every 6 months like @BatteryBuff suggested... maybe the first 90 days and THEN every six months... I'm a little paranoid...