Is repairing an old laptop battery pack by replacing dead cells with new cells good Idea

 

image from

Is repairing an old laptop battery pack by replacing dead cells with new cells good Idea?

Some people replace dead cells with new cells in old laptop batteries.

Laptop batteries contains usually 18650 3.7 li-ion cells, capacity depends on the manufacturer, usually 2500mAh–3000mAh

Old laptop batteries anyway have low capacity compared to a new battery,

Approx → 1/2C of original C (C-capacity)

Original C (capacity) = 2900mAh

Then after 2 years of use, capacity will drop to somewhere around 1900mAh

what is a dead cell?→ A cell cannot charge anymore/zero capacity/ a cell short circuit itself/ a cell with very high internal resistance

Then the newly replaced battery has higher capacitance compared to other reduced capacity batteries

Laptop batteries are in series

Batteries connected in series must be as close to identical as possible,

To make sure that the individual cells in each battery charge and discharge together and are ‘balanced’ (have the same voltage).

Putting different capacity batteries is series will lead to disaster because the lower capacity battery will charge up faster and become grossly overcharged,

This will lead it to heat and release gasses that cannot be replaced — and perhaps even explode!

Batteries lose performance and may go out of balance as they age. even new batteries cannot be balanced 100%, but they do around >95%

Therefore for best performance, only combine batteries that have the same age and usage, the older weaker ones will not provide the expected capacity.