Battery early discharge cuttoff.

[jimbo341]

I'm having some issues with a 36v 12ah battery I purchased around 3 months ago. The issue is the battery discharge cuttoff voltage is occuring earlier than expected, most commonly around 34v when its advertised to be as low as 27.5v (30v seem more reasonable though). The bike low power is set at 31v but the power completely switches off so I guess it the BCU thats shutting things off.

I went with the cheaper option and purchased a battery made up with generic chinese cells but even so, I'm losing around 1/3 of the rated battery capacity due to power shuttoff. Should this be expected for these type of cells in a relatively new battery? Its much worse than I expected.

I tried to balance the cells by leaving it charging overnight a few times using the charger that came with the battery. This normally charges the battery to 41.8v which once unplugged drops to 41.5v after around 5 minutes with no use. Once peddling, battery sag isn't too excessive, maybe around 1v using medium power settings. Should I try to balance it more or for longer?

Also, how can I trouble shoot this? Could one of the cells be bad or is it just a poor quality battery that won't balance? Could it even be a poor connection in the wiring somewhere? I have access to a multimeter so I'd like to try to diagnose it myself before attempting to use the warranty due to high shipping costs from where I live.

I'm running a bafang BB01B 350w which is down tuned to 250w which I'm using for commuting so I don't think I'm putting unreasonable stress on the battery.

 

[99t4]

Just to clarify, the controller LVC is set for 31VDC but the BMS is cutting battery output at 34VDC when its LVC is supposed to be 27.5VDC?

Seems like a BMS problem, or otherwise your battery sag could be greater than you think.

 

[Chalo]

If you regularly run the battery down until it cuts off, you're hammering the pack more and more out of balance. The more out of balance it is, the longer it must sit on the charger to even out the cells' voltages. Overnight won't cut it once you've squashed some cells amp-hours out of sync with the others. Cheap BMSes balance at between 20 and 60 mA bleed current. That means for each amp-hour of imbalance between the highest and lowest cell groups, you'll need as much as 50 hours on the charger (after it's fully charged) to restore balance.

The problem will continue if you continue to run the pack all the way down. So don't do that.

 

[jimbo341]

Thanks guys, this is really useful information. @99t4, the controller LVC is indeed set for 31VDC but the BMS is cutting battery output at 34VDC.

I have been running the battery to cuttoff but thats only because the cuttoff is early and I'm never quite sure when its going to occur. Still my commute is quite short (7km) so its never been a problem to peddle home under normal leg power.

It looks like over time the battery has become more and more imbalanced and I haven't taken into account how slow rebalancing is.

My plan is to open the battey up, check connections/welds are ok, check the cells voltage then leave it on charge when not in use to see if it balances. My back of envelope calculations indicates there's 2ah missing so it could need 100 hours to balance.

Still, I learnt a valuable lesson,buy cheap buy twice is definately the case with bike batteries.

 

[john61ct]

Note that voltage sags very quickly as peak current is demanded

especially with worn cells or poor quality in the first place..

BMS LVC may be opening at its measurement of 31V, while bounceback recovery and/or other slower-adjusting voltmeters are still showing 34V

Going to better power-draw performing cells is one path, getting a higher Ah pack is another, also gets you more cycle lifespan.

Ideally do both, so your LVC can remain higher rather than lower.

 

[jimbo341]

Thanks to everyone for your help on this. I left the battery on charge for a few days and measured the cells. All were around the 4.25v mark so fully charged and balanced allowing for some small error margin in the multimeter. I then ran the battery to shutdown which was slightly better measuring around 31v on the bike display at cuttoff, when in use. This however was dipping from around 33v when the bike was stationary. I tested the battery with the multimeter, all cells were in balance but around the 3.4v mark.

So to summarise, the battery needed to be balanced which took days rather than a few hours which helped with the reduced capacity. However, the culprit seems to be the battery sag which although not to noticable when fully charged is increased at low SOC (2-3v at least). This sag of 25% and the uncertainty that goes with it is what you get for buying the cheaper chinese cells.

Still, this is my 1st build and I am happy for the learning experience. This battery is fine for my daily commute for the next 12 months or so but next time I'll pay the extra bucks and have peace of mind plus the ability to go further without peddling home unassisted.

 

[E-HP]

Thanks to everyone for your help on this. I left the battery on charge for a few days and measured the cells. All were around the 4.25v mark so fully charged and balanced allowing for some small error margin in the multimeter.

Your charger is overcharging your battery. I bit dangerous.

 

[amberwolf]

All were around the 4.25v mark so fully charged and balanced allowing for some small error margin in the multimeter.

I'd recommend checking the multimeter battery, or just replacing it with a known good one, and retesting that, since a low meter battery results in high/incorrect voltage readings. If it still reads the same, check the meter against some other known voltage sources, such as a 5v USB port on a computer, etc.

If that is a correct reading, the cells are being overcharged, which will shorten their lifespan, and the BMS is not set correctly if it is programmable, or not designed correctly if it is not programmable, and will not protect the cells against this overcharge.

If it's not a correct reading, the new battery in the meter should tell you the real reading, you can then know if the BMS is correclty shutting off before overcharge.

 

[john61ct]

Also, if you are drawing down anywhere near 3Vpc isolated at rest you will not get good lifespan even out of the best li-ion cells.

Shorten your trips or expand your Ah capacity, or you will need to replace your packs in a third the time, maybe even every few months.

 

[m3vuv]

just a warning about elcheapo multimeters,i found if the dmm battery is low the readings are well low,by upto 10x low can be lethal,i use an old avo8 mk9 or my fluke 83v,at least the readings are reliable!.

 

[Comrade]

Also, if you are drawing down anywhere near 3Vpc isolated at rest you will not get good lifespan even out of the best li-ion cells.

Should a battery be used around it's nominal voltage?

Say it's 3.6V nominal, and the rider uses 0.8v per cell. Of these 3 options, which is the most optimal regiment for battery longevity?

a) 4.0v to 3.2v
b) 4.1v to 3.3v
c) 3.9v to 3.1v

 

[Comrade]

As far away as from upper charge limit and lower discharge limit as possible.

So if nominal voltage is considered to be the middle of the charging range, where the charge level is by definition 50%, then (A) from the multiple choice? Is there a community consensus?

 

[john61ct]

Should a battery be used around it's nominal voltage?

100% yes

> Say it's 3.6V nominal, and the rider uses 0.8v per cell.

Note the proper unit is SoC, % of capacity or in Ah.

So if you are using 90% of capacity each cycle, your cell lifecycles may be 5x compared to 100%

80% much longer, maybe triple the above.

Always positioning the top charge termination point well below spec'd max voltage, and otherwise staying as far as possible away from the spec'd minimum voltage.

If using 20-30% of capacity, and that usage centered around the midpoint, the cell may last thousands of times longer.

If doing so with only 5% of capacity, you may be able to leave the pack to your grandkids.


======
Given all that above, C

------
Of these 3 options, which is the most optimal regiment for battery longevity?

a) 4.0v to 3.2v
b) 4.1v to 3.3v
c) 3.9v to 3.1v

-------

Note that the Ah capacity stored between 3.1 and 3.5V will be a tiny fraction of that stored between 3.6V and 4.0V

Finally, high C-rates and high ambient temps can wipe out most of the longevity gains from otherwise coddling

The above factors discussed only dominate in a solar / mobile **storage** scenario not propulsion use cases.

Where carrying around heavy weights of unused battery capacity would usually be silly.

 

[Comrade]

Given all that above, C

Why C if you've said midpoint is best, but C is shifted -0.1v from midpoint?

 

[john61ct]

OK a is fine too

None of the above is the real answer, do not use voltage to guesstimate SoC / Ah

unless you have thoroughly calibrated for your cells and use case

 

[E-HP]

unless you have thoroughly calibrated for your cells and use case

I'd say my brain cells are pretty calibrated to my battery, but not with much precision. I look at the voltage, and my cells say "ya, got plenty left, keep riding" or "hmm time to turn around. my butt hurts anyway". Haven't been wrong yet.

 

[Comrade]

do not use voltage to guesstimate SoC

How else do you tell SoC if not for settled voltage?

 

[john61ct]

https://endless-sphere.com/forums/viewtopic.php?p=1732212#p1732212