How long does a top balance last?

[SilverbackMP]

Tried searching for this. I topped balanced (3.6) my first set of 16 cells about a three weeks ago, another set about a week ago, and my third set today. All were charged several times with contestant voltage to .02 Amps and left for about a day in parallel for voltage to equalize and then repeated several times. And then the bus bars were removed. How long will they stay in balance? Will my first set still be roughly still in balance? Going to check with a clamp meter but I won't get to this until the end of the week and trying to get my game plan together (as I have five more batteries after these three to get up and running).

Planning on bringing any low cells up via individually charging if needed. I don't think it's absoluely necessarily as I have REC BMS, but I'd rather get them as close as possible.

Might take the Andy approach as shown on his last few videos. Thoughts?

 

[sunshine_eggo]

Unanswerable.

Your balance will be maintained based on the difference between the self-discharge rates of the cells. Personally, I do not expect a balance to last for more than 2 weeks if allowed to sit unused. Will they still be close? Yes.

 

[yabert]

Yeap, from few minutes to few years.
I've seen excellent quality cells (LG chem) stay balance for years, but I also seen LFP cells start to drift after few weeks.

 

[timselectric]

Until any discharging occurs.

 

[yabert]

Until any discharging occurs.

Nop! I have some LFP cells at home who drift without any discharging.
But I discharge Chevrolet Volt (LG chem) battery module 90Ah at 700-800A peak and 70-150A continues for 2 years (20k+ km) in my electric Smart Fortwo before I need to balance another time 3 cells over 48 cells in series (read here 45 cells was still perfectly balanced).

Good matched cells stay balance. It's why I like to play with high quality used EV cells instead of low cost LFP from china.

 

[timselectric]

Nop!
Discharging Chevrolet Volt (LG chem) battery module 90Ah at 700-800A peak and 70-150A continues for 2 years (20k+ km) in my electric Smart Fortwo before I need to balance another time 3 cells over 48 cells in series (read here 45 cells was still perfectly balanced).

Those aren't LFP.
Which is what the OP is asking about.

 

[yabert]

Any cells chemistry can drift/self-discharge.
My comment was about high quality cells VS low quality cells. So OP can have an idea of how long does a top balance could last based on what is the quality of his cells.

 

[SilverbackMP]

Those aren't LFP.
Which is what the OP is asking about.

Mine are 230 AH CALB cells sourced from Current Connected who warrants them for 10 years…so possibly “EV grade” or at least “A+.”

 

[timselectric]

Mine are 230 AH CALB cells sourced from Current Connected who warrants them for 10 years…so possibly “EV grade” or at least “A+.”

Yes, those are LFP. (Lithium Iron Phosphate)

 

[SilverbackMP]

Yes, those are LFP. (Lithium Iron Phosphate)

Yep, just saying I think the quality is pretty good over some others that might be sold by Shinhau Xi Limited on Alibaba.

 

[RCinFLA]

Self-discharge rate ranges from about 0.5% and 2.5% of total cell capacity per month depending on temperature and cell condition.

If you assume a 1%/month worse case delta self-discharge rate between different cells, and less than about 1% delta in SoC can start to give you issues with cell overvoltage shutdown on BMS when you attempt a full absorb level charge, then that is less than a month.

Just passively paralleling cells will not get them balanced enough. Overpotential delta at low cell current is 5 to 15 mV. This yields a net 10-30 mV delta rested OCV difference between cells which can be 10-15% delta in SoC.

 

[SilverbackMP]

Self-discharge rate ranges from about 0.5% and 2.5% of total cell capacity per month depending on temperature and cell condition.

If you assume a 1%/month worse case delta self-discharge rate between different cells, and less than about 1% delta in SoC can start to give you issues with cell overvoltage shutdown on BMS when you attempt a full absorb level charge, then that is less than a month.

So probably a good idea shoot 5 - 10 amps back into each individual cell until each hits 0.2 amp absorb. My (2) power supplies can hit 40 amps each but that’s probably overkill (?).

Gonna be a pain in the ass until I get the system up and going with at least a few of the batteries as I am fiddling around with gas gensets for this.

 

[Ampster]

Nop! I have some LFP cells at home who drift without any discharging.
.......

Good matched cells stay balance. It's why I like to play with high quality used EV cells instead of low cost LFP from china.

View attachment 139627

Ha, I haven't seen that photo since my days on diyelectricar forum. I sold my VW conversion and have been messing with stationary storage since then. It is a whole different experience dealing with Grade B cells from China but the cost benefit is there because I do not stress them like I did then. I first experienced the EV grin in that conversion and I am sure I pulled 300-400 Amps out of that 100 Ahr pack for a few seconds. My zero to sixty feet times were phenomenal and the best part were the looks I got from people when that 40 year old VW shot off the line when the signal turned green.

 

[Chris Down Under]

Gonna be a pain in the ass until I get the system up and going with at least a few of the batteries

Exactly. Until you have regular charge going into the battery bank it'll be hard to determine what their self discharge rates are between all the different cells. It might surprise you though, especially if they've all come from the same source and were manufactured around the same time. I'd be tempted to wait and see if the balance was any good once you hooked them all up to the charging source, I mean they're 230AH cells so it will take a while for them to self discharge anyway. I had some LiFePO4 batteries sitting around for about six months before connecting them up and the balance was fine. Life sometimes gets in the way of the perfect! Cheers. Chris

 

[Ampster]

If you assume a 1%/month worse case delta self-discharge rate between different cells, and less than about 1% delta in SoC can start to give you issues with cell overvoltage shutdown on BMS when you attempt a full absorb level charge, then that is less than a month.

That has been my experience. I use a 2 Amp active balancer on my 42 kWh 3P16S pack. I also charge to 55 volts or 3.4375 volts per cell. My absorb (CV) stage lasts about an hour. If I try to raise the ending voltage, one buddy pair becomes a runner.