DIY Electric Car Forums banner
1 - 4 of 4 Posts

· Registered
Joined
·
2,318 Posts
Do you have an LFP pack with no BMS?
No. I have a minibms with no shunts.

Do you have any data from the history of your cells, particularly how well they stay in balance?
Yes, I've posted data several times on my build thread SwiftE. Generally, 32 of the 36 have remained balanced to within +/-4mV or less for almost 1 year and around 5k miles. That is both after full charge, or at around 35% SOC. Four cells were replaced, and these tend to slowly drift lower in voltage compared to the others, around 0.2V (edit, that should be 0.02V!) in over about 2-3 months, as measured at 35% SOC. I usually charge them individually with a power supply at that point to re-balance, so don't know if they would continue to drift or reach some stable voltage.

Did you initially balance them?
No.

Top or Bottom balance?
Sort of bottom, about 3.1V if I recall correctly, and that is only the 4 replacement cells.

What discharge rate are you typically drawing from your pack?
Around 0.1C to 0.3C at lower speeds, 0.4 to 0.8C or so on the highway, about 0.7C to 1.4C up an about 4.5% average grade (varies quite a bit) for 19 miles, I drive maybe once per week. Up to about 2.2C fleetingly when I accelerate more aggressively, usually more like 1.5C.

Do you balance your pack every so often to bring them back in line?
Just the 4 replacement cells. The other 32 have never been balanced.

Do you use cellogs or other device to log all or some of the cells?
I did log around 4 cells at a time a number of times but it is pretty boring as they are always in balance and all show about the same voltage changes under load. Some of the plots are posted on the thread.

What is your max charging current?
~0.2C. Usually charge at 0.14C to 0.17C depending on temperature in the garage (charger overheats, Manzanita PFC30)

What is your max regen current?
It has been as high as a bit over 1C fleetingly in a quick stop on a highway. Usually is 0.8C or less.

I think the larger fraction of C current you draw, the more you will have to balance your cells, and I expect their life will also be shorter. I expect racers have real problems with keeping cells balanced, and that is why they think you are nuts if you don't use a balancing bms.
 

· Registered
Joined
·
2,318 Posts
I just checked my cell voltages since it had been several weeks, and I can't use the car (charger died). Voltages ranged from 3.298 to 3.300V, including the 4 replacement cells. SOC is 55%. I think I last individually charged the 4 replacement cells to balance them back in June.
 

· Registered
Joined
·
2,318 Posts
Dan, note the soc was 55%. The change in voltage with Ah is very small at 55% soc. At 20% soc I expect there would be greater imbalance. But that doesn't matter to me, as I never go below 30%, rarely below 35% soc, and there the balance is typically almost as good as I reported at 55% soc. I find life is very simple if you just operate on the flatter part of the V versus Ah curve, 30% soc to about 95% soc - where the exponential voltage rise starts. But like I said above, I think the higher C currents you regularly drive through cells, the more frequently you will need to balance. No data, just a guess that more stress accentuates cell differences. A comparison of JRP3's imbalance data and mine seems as if it might support this, but scant data so hard to say. I consider a 0.01V difference big, and usually re-balance the cell at that point. His cells are 100Ah, mine are 180Ah, both using the same controller and similar motors, so he likely pulls higher fraction C currents than me. For several months I measured all cell voltages before and after each charge, and I swear they became better balanced over time, without me doing anything. I quit doing it because the voltages were always almost the same. I still do it about once every few weeks or so, with the same result.
 

· Registered
Joined
·
2,318 Posts
please clarify what the minibms does if it does not do any shunting at end of charge.... does it just shut down the charger as soon as the first cell hits the target high voltage?
You can get all the info you want on the minibms in the thread on it in this forum. You have a choice of ordering the minibms with or without shunts. I ordered it without them. Its other functions are HVC and LVC, similar to other bms. So yes, it shuts down the charger when a cell hits a high voltage of 3.6V, and it has a relay on the main board which can be used to trigger an alarm and/or cut the throttle if a cell hits a low voltage of I think 2.5V - but not sure on the actual voltage level. The low level has some temperature compensation to account for more voltage sag at lower cell temperatures.
 
1 - 4 of 4 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