Recell laptop battery with 6x 18650 cells. Odd results !

[Nomad]

Hi. Calling all electrical types - I had 2 poorly laptop batteries, both from the same laptop, both have 6x 18650 cells.

Batt#1 had 1 pair of good cells, 2 pairs of dud cells, it would charge up but only showed 2% of expected capacity. My guess is the cells were the problem and the BMS controller was ok.

Batt#2 had all good cells, it would charge up but only showed less than 5% of expected capacity. My guess is the problem is the BMS controller as the cells were good.

I've fitted the good BMS onto the good cells, it works but stops charging when it gets up to 7% (24.8 volts).
Originally it stopped when it got to 23.7V so I guess it's now balanced hence the slight improvement.

Another odd thing, the battery status screen says the design capacity for the battery is 48.8 W/h but this battery pack has capacity of 330W/h. I'm confused !

Would anyone have any ideas or suggestions?

The new battery is a lot better than it was so I can use it but I'm curious why it won't charge up

 

[carbon]

There may be an option in the bios to reset the battery capacity

 

[mike 109444]

There may even be a button on the battery pack that also forceses a "reset" of the pack.
Or a setting in the manufactures setup/operating system that allows setting of the max charge percentage. (have this on a dell pad/notebook device.
Or operating system (Windows) battery status/monitoring may have settings for max performance / normal / low power consumption.

 

[Misterg]

the battery status screen says the design capacity for the battery is 48.8 W/h but this battery pack has capacity of 330W/h. I'm confused !

Six 18650 cells are going to be much closer to 48 watt hours than 330 watt hours:

48.8 / 6 = 8.1 watt hours per cell

8.1 watt hours @ (say) 3.7V = 2.2 amp hours, or 2200 mAh per cell.

Seems reasonable to me

 

[PhillipM]

s the design capacity for the battery is 48.8 W/h but this battery pack has capacity of 330W/h. I'm confused !

Something weird with your calcs there the battery pack would be massive for 330Wh, I don't think you'd even manage that on 6 x 21700 cells!

 

[Nomad]

Hi, thanks for the ideas.

I have 2 identical laptops (older Fujitsu Amilio Li3910), the one I'm using has a fresh version of Ubuntu installed and includes a good battery info screen.
The rather large (!) 330 W/h is from that screen, not my fag packet calculations so "something" is misreporting the hardware.
Notice the status. Charger is plugged in, battery is not charged, status is not charging (but it should be).

The history of 6 charge / discharge attempts shows the problem.
Charge then disconnect the power and run the battery down to 5%, every attempt to charge cuts out around 7%.

It's very consistent, I just don't know why it won't charge past 7%

For info, my original unmodified battery (battery 3) charges fine and shows a sensible number for the design charge so I'm sure the issue is with the new hybrid battery not with the laptop or CMOS settings etc. I don't see any reset buttons on the battery BMS.

 

[Kram]

I've fitted the good BMS onto the good cells, it works but stops charging when it gets up to 7% (24.8 volts).
Originally it stopped when it got to 23.7V so I guess it's now balanced hence the slight improvement.

24.8v is above 6x 4.1v = 24.6v which is a common maximum for Li-ion, and not far from 6x 4.2v = 25.2v which I would expect as the maximum for a modern 18650 cell. I would say its near full!

There are some cell types that go higher but those are uncommon, havent seen one in an 18650 and usually lipo and will be marked as such.

As somone experienced with li cells I would remove them, do a full charge and discharge test on each cell with a li charger outside of the laptop. That will tell you if they are good and what to expect.

It seems you do not know enough about the subject and are going by incorrect information your app is telling you. Overcharging or messing with li batteries is a good way to start fires!

 

[Pigeon_Droppings2]

As somone experienced with li cells I would remove them, do a full charge and discharge test on each cell with a li charger outside of the laptop

You can buy charger/testers that will do the charge and discharge test for you...I bought one...then put it in the loft!

 

[Nomad]

For comparison, I fitted the good (1 year old, unmodified) battery from my other laptop, info screen shows that has a reasonable expected charge - not 300 W/h.
This battery is showing 100% fully charged, stats page says it has 32.4 W/h.

Known good battery :

Using the 32.4 W/h from the good battery as a benchmark, the 24.4 W/h in my problem battery is well below full charge so I think there are 2 problems :
Problem battery won't accept charge above 24 W/h ( showing as 7%)
Problem battery is misreporting potential capacity as 300 W/h rather than sub 50 W/h.

Testing both batteries off the laptop with a multimeter the good battery shows 12.5V and the problem one 11.95V
For me that confirms (without using the stats page) that the problem battery is not yet fully charged
The laptop will run on the problem battery for several minutes with the power supply disconnected so is has some useful capacity but it's very low.

Good points about working with batteries being dangerous. I have a history with RC helicopters so am aware but thanks for the warning.
I would have liked to check the individual cells but they are welded together in pairs so i can only test them in pairs.
Nothing unusual about the way I'm charging, the batteries are charging while the laptop is plugged into the standard mains charger.

Apologies !
I've just re-read my original post and realised I used numbers from W/h stats page but said volts when I should have said W/h (no wonder @Kram was shocked)

 

[Parm]

How much is a ready made battery pack?

 

[Kram]

no wonder @Kram was shocked

No, that was not it... Every other fact you stated about the battery was wrong.

However from your post the pack voltage is 11.5/12.5v. With 6 cells that would be a 3 series 2 parallel.

If the pairs of cells are charged in series and not balanced, one pair could overcharge before the others are full, any charger should measure this and either balance them or cut off.

Now an old battery pack, the cells could be worn out and have a higher resistance than the others.
Balancing is usually by discharging a cell/pair, followed by charging the pack as a whole. If the resistance is high, the low cells will never charge fully.

I'd suggest bin it and get new battery pack.