BMS Included for Recycled Laptop Battery Builds?

[DrkAngel]

I've been sadly disappointed with any BMS\PCB\PCM that I have tried, $40-$60 range.
"Balance" is typically a trickle bleed-down of the over voltage cells.

I am presently investigating a recent observation.
On a recent purchase of old, setting for years, HP notebook packs, I noticed that all cells in each pack were precisely at the same voltage.
All packs had different voltages, but every cell in each pack was precisely balanced to the 1 hundredth of a volt.
Logical and most likely explanation is that the built in bms type board was maintaining balance ... without any external power!

Theoretically, overlapping 3 of these across the 7s of my 25.9V builds should maintain, or attain, a voltage balance between all cells.

I would not charge or discharge through these controllers, merely allow them to do their thing ... as they have been for these past many years.
This should simplify bulk charging and keep it much safer!

 

[dnmun]

have you examined the little BMS they put on the strip inside the package? from what i have seen they use little mosfets on each channel so when the battery pack is removed then the mosfet is turned off and no current can leak out of the cell so that would account for why you see identical voltages long after the pack was taken outa service. jmho.

 

[DrkAngel]

have you examined the little BMS they put on the strip inside the package? from what i have seen they use little mosfets on each channel so when the battery pack is removed then the mosfet is turned off and no current can leak out of the cell so that would account for why you see identical voltages long after the pack was taken outa service. jmho.

New packs! - old stock! - customer returns?

All cells self-discharge.
Various cells at various rates.
To imagine that all these cells self discharged at an identical rate while sitting several years, (identical to the hundredth volt), would require a quality control beyond anything imaginable.

Logically, the more rational explanation is that the oem "BMS" actively maintained balance.

 

[dnmun]

i have seen the same thing in cells sitting unused for years. all right at 4.01V. that is the only good thing about these little lipo cans. they cannot get the charge out fast enuff to be useful but they can save it from self discharge much better than lifepo4. imo.

 

[DrkAngel]

I decided to test my observation that the laptop BMS seemed to be balancing the cells, even when no external power was applied.
Method
I will take note of all banks of cells voltages.
Disconnect 1 wire from an end bank.
Discharge end bank 1/10th Volt.
Reconnect wire to end bank.
Using a 100th V capable meter I will note all bank voltages at 1 hour, 2 hours, 4 hours, 8 hours, 24 hour, 48 hour etc.

Results?
If my hypothesis is correct, I should observe a gradual decline in the voltages of the non-diminished banks.
I do not expect to see any voltage increase in the 3rd, drained, bank.

Will post results next week.

 

[lxgoldsmith]

So how did it work?

 

[265chemic]

I'd be interested to hear the results of this too - have a Dell 87wh 3s3p freshly pulled apart and it is off between packs. They read 1.79 / 1.80 / 1.84 so very flat I imagine, they were probably sitting for a couple of years

 

[dogman dan]

Just my opinion,, but a typical bms is designed to work pretty good with pretty good cells.

Old recycled cells though,, they just might self discharge, or just drift out of balance so much on each discharge, so as to be very hard for any typical bleed down bms to handle.

Best approach for this type of cells may be a charger capable of balancing each cell group every cycle. Like a 12s rc charger,, that charges each cell group through the balance wires.

Or manually doing the same. You may quickly realize one particular cell group is the high capacity one, and give it a full charge each cycle. Or,,, add one cell to your lower capacity group.

And also,, no real point in charging them past what they can hold by much. If they just drop to 4v every time you remove the charger, just charge to 4.05v,, not 4.2v.

 

[DrkAngel]

The Dell cells I (short term) tested for auto-balancing demonstrated no appreciable self correcting function.
Long term ... I will post up, if I can find it.

More probable is that top quality cells, of excellent quality control, depreciated voltage identically over extended period. = very good cells, if only of limited discharge rate capability!