New 20s Li-ion battery pack expected behavior

[MJSfoto1956]

Quick question for those who know the answer:

If a typical, brand-new, 20s17p, Li-ion battery pack with built-in "smart" BMS is fully charged to 84v and is left to sit unattended at 68°F (20°C) room temperature, how much time should pass before it "drops" to 83V?

• 1 day?

• 2 days?

• 4 days?

• 8 days?

• never?

Curious minds want to know.
M

 

[jonescg]

I wouldn't expect it to drop appreciably. If it does drop by a volt it will be the quiescent current of the BMS doing it.
Maybe a few days? If at all?

 

[amberwolf]

If the BMS has a quiescent current high enough to take a whole volt of a pack that size in a few days, it's either defective or wasting a lot of power. :/

It can also be imbalanced cells. If a group is low enough relative to the others, then when the BMS stops charging to allow the high cells to drain, they could have enough total voltage on them to drop a volt.

That should happen within hours or less, depending on the overvoltage of each group, the capacity of the group, and the balancing shunt current the BMS is capable of.



Imbalanced cells themselves have a number of causes, but in a brand new pack the most likely is that not all the cells are well-matched to start with.

 

[999zip999]

Cycle it a few light times and leave it on the charger for hours after full. 17p is a good size pack to balance.

 

[fechter]

It sort of depends on the charger and at what point it cuts off. I’d expect it to drop a little right away after changing but then stabilize for a long time (more than 8 days).

 

[MJSfoto1956]

It sort of depends on the charger and at what point it cuts off. I’d expect it to drop a little right away after changing but then stabilize for a long time (more than 8 days).

Well the supplied 5A charger stopped charging precisely @ 83.7V. Twelve hours later the battery was @ 83.5V. Twenty four hours later it was still @ 83.5V. So I'll let it sit for a few more days to see if it stays there.

Thanks!
M

 

[999zip999]

This a home made pack where did you get the cells ?

 

[MJSfoto1956]

This a home made pack where did you get the cells ?

Not home made.
See: https://endless-sphere.com/forums/viewtopic.php?f=14&t=98446

M

 

[Jan-Erik-86]

Dropping to 83.5V would mean 4.175V per cell, which appears to match one of your BMS' listed balancing voltages precisely (4.175±0.02V).

I don't have much experience with BMS in general, but as your BMS looks like it have a rated current draw of only 30µA, I wouldn't expect your 57Ah battery to be affected by this load, at least not in a very very long time. Personally, I can't remember to have ever seen my 17Ah 13s battery with a cheap BMS (50µA rated current draw) even drop half a volt in over two weeks.

 

[MJSfoto1956]

... 83.5V would mean 4.175V per cell, which appears to match your BMS' listed balancing voltages precisely (4.175±0.02V) ... as your BMS looks like it have a rated current draw of only 30µA, I wouldn't expect your 57Ah battery to be affected by this load...

Yeah, the pack seems reasonably happy after its first full charge/balance, even with the initial drop of 0.2V a few hours later.
48 hours on and it now reads @ 83.4V. So I now have a 0.2V loss in the first 24 hrs, and a 0.1V loss in the second 24hrs. Will keep monitoring it for a few days more to see how it behaves (or not)...

M

 

[fechter]

Most cheap BMS units balance at 40-100 ma or so. If there were any high cells it would take a day or so to drain it down to the balance point.

It would be a good idea to measure all the individual cell voltages after it stabilizes to see how well they are balanced.

 

[MJSfoto1956]

so the observed behavior appears to be:

• Initial full charge/balance = 83.7V

• 12 hrs later = 83.5V

• 24hrs later = 83.5V

• 48hrs later = 83.4V

• 72hrs later = 83.3V

Thoughts?
M

 

[Jan-Erik-86]

Could it simply be normal self discharge rate?

I started reading a little on self-discharge rates, and according to batteryuniversity, "Li-ion self-discharges about 5 percent in the first 24 hours and then loses 1–2 percent per month; the protection circuit adds another 3 percent per month.".
This is when fully charged and stored at 0c. At 25c it's supposedly as high as 20% loss in one month.
According to this, ~48 hours after balancing is complete you should be down at around 82,8V or so. This doesn't appear to be too far off from your measurements?
I haven't seen this quick voltage drop myself, but my storage temp so far been very low and battery usually not charged to more then ~90%. Would be interesting to see what others are experiencing.

 

[amberwolf]

I started reading a little on self-discharge rates, and according to batteryuniversity, "Li-ion self-discharges about 5 percent in the first 24 hours and then loses 1–2 percent per month; the protection circuit adds another 3 percent per month.".
This is when fully charged and stored at 0c. At 25c it's supposedly as high as 20% loss in one month.

I dunno what kind of batteries they were using for those tests, but I havent' seen that in any lithium battery I've owned, except where the BMS is an old high-drain type. I suspect the problem they are talking about is just a badly designed or manufactured BMS, not the Li-ion cells themselves (unless they're using poor-quality cells).

I have some EIG NMC packs out in the shed, which while partly insulated, is not climate controlled, and this is Phoenix, AZ, where it gets well over 100F in the summer (even at night for the worst parts). Recently pulled out one of them to use, and it was still just about at the same point I'd left it (full, within a few hundredths of a volt per cell), and I haven't used it or charged it since earlier last year. No BMS to drain it, so essentially no self-discharge.


I have seen high self-discharge only on defective cells, or cells that are nearing end-of-life (or past it) and already exhibit other problems.