DIY Electric Car Forums banner

LiFePo4 pack balance data from non-BMS Users

5404 17
I am trying to find real world pack balance data from LFP users with no BMS. I want to see how the pack stays in balance over time. There probably isn't much out there, but anything is better than nothing at this point.

Do you have an LFP pack with no BMS?

Do you have any data from the history of your cells, particularly how well they stay in balance?

Did you initially balance them?

Top or Bottom balance?

What discharge rate are you typically drawing from your pack?

Do you balance your pack every so often to bring them back in line?

Do you use cellogs or other device to log all or some of the cells?

Would you share this data via a link or email?

Thanks,
Dave
1 - 3 of 3 Posts

· Registered
Joined
·
1,015 Posts
I am currently running without BMS. After balancing, my cells are within 0.002v. I balance them when they are more than 0.02 out of balance.

I have 99 ThunderSky 100Ah.

I top balance about once a month by charging the low cells (@ +5 amps) to match the voltage of the highest cell (usually 3.420v). Then I shunt the highest cells (@ -1 amp) for 10 minutes.

I have my cells located in 4 groups. The cells which are mounted under the car and under the hood are generally lower in voltage compared to the cells mounted in the trunk. Sometimes I charge an entire group of around 30 cells before individually charging each one.

My Manzanita shifts to CV when the cells are around 3.400v, it holds this for 35 minutes then shuts off.

Resting voltage is around 3.33v.
Pulling 1C they dip to 3.23
Pulling 2C they dip to 3.13

After 25 miles of driving (-25Ah)
Resting voltage is around 3.30
Pulling 1C they dip to 3.19
Pulling 2C they dip to 3.09

At 2.8C, my max controller current, I've never gone below 3.00V ave/cell.
I typically only use about 30% of my capacity, so I'm far away from the extreme ends of the SOC curve. I have about 10k miles on the car.

Temperature seems to be the biggest factor that affects the normally consistent performance. I usually see 10V sag for every 100A of current. Works out to be 1 mOhm per cell.
 

· Registered
Joined
·
1,015 Posts
After reading some comments in other threads about the troubles of balancing mid-soc, I turned my charger up to see what would happen. I increased the voltage from 336 to 337, just one volt, about 1/8" of a turn.

I had always been charging to 3.37, and the cells would finish 3.40, they generally stay within 20mv.

With the charger turned up ever so slightly, I noticed my highest cells started shooting though the roof, about 1mv every few seconds. I let it get to 3.5volts, and while it was doing so, my low cells were starting to drop in voltage due to the current tapering off. I now had over 120mv spread when I almost never had more than 20mv before.

It's pretty easy to have cells within 10mv when I don't charge above 3.37v. I'm starting to realize how out of balance things can be when you get to the knee.
 
1 - 3 of 3 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top