Lithium Ion 18650 cells seem fully charged, but cut out after 10-20 seconds

[conandrum]

Hi

I have a Dyson (V6) aftermarket battery from February 2020.
Been working OK until a couple of months ago when I noticed the working time severely reduced to a couple of minutes.
My previous and original battery lasted almost 2.5 years. So 6/7 months with this battery seems strange.
I opened it up tonight and measured voltages across each cell and all of them are around 4.16V x 6 = 24.xxV
When I put it on the charger, the blue charging light comes on and goes out in under 5 minutes, still I get 10-20 seconds work out of it (on low power mode).

The cells are Pros18650P0 3.6v 19D26 3O (see photo)

Since February, they were hardly used.
In actual fact, I was away from home between 20 March to 20 May (3 months) and I do not remember if I left the charger switched on or not but most probably I switched if off.
Maybe this has contributed to them losing enough voltage to adversely affect them?

I will order another one from Dyson this time.
However, if you have any recommendations on things I could try to get them up and running again, let me know.

Thanks

 

[itsmoked]

At least one cell is bad. It has almost no capacity which doesn't show up looking with a voltmeter at a cell pack without a load on it. To find it you need to put the pack under load and then measure all the cell voltages. Over and over for a few minutes until the bad one surfaces.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[EdStainless]

The only other option (to a bad cell) is a failure of the charge protection circuitry.
You have to disconnect each cell to measure it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[conandrum]

Thanks guys.
I did some crude testing and saw that 2 cells seem to drop to 3.5V within 20s. The rest remain around 3.9V.
I think when these cells hit 3.5V Dyson cuts out. Does this seem like expected behavior in your opinion?

I ordered a OPUS BT-C3100 so I can check them individually.
I will update this thread when it arrives.
Any further tips appreciated.