The battery has swollen a few years ago, but it still worked until last year, when it suddenly refused to recognize the battery, showing the message "Unknown battery status" and shutting down every time I connect the charging cable.
I had the solution in my head for a long time, but finally decided to try it, after the phone stopped working. So here's the basic concept:
- almost every rechargeable battery in every device consists of one (or more) battery cell and a circuit board, with some electronics, maybe a sensor or two.
- if a battery cell goes bad, it can be replaced and the device will continue to function as if nothing happened.
In this video I complete the procedure with a completely different type of battery cell, harvested from a power bank:
The result was as expected, the battery is recognized, BUT because the cell is not identical, it has slightly different min/max/charging voltages, the phone doesn't display the charge state accurately.
NOTE: in this case, you have to remove the battery shell from the phone, connect the new battery cell, then put it back in. Other devices may behave differently, they won't care in which order you do it or will require some sort of factory reset procedure, or whatever.
So the end result is that my phone works, BUT cannot charge and is no longer portable, as the battery doesn't fit into the phone. If I wanted it to charge, I'd have to find a battery cell with identical voltages, capacity doesn't really matter.
If you want the phone to be portable, just like new, then you have to find a battery cell that fits inside and has the proper nominal and charging voltages. With older phones this shouldn't be a problem, because an identical or slightly smaller cell (manufactured with recent technologies) will have a bigger capacity.
IMPORTANT: if you don't care about portability and charging, then you can buy any external power bank, even the cheapest ones have at least a ~2200mAh battery inside. You open it up, solder two wires to the battery cell (not to the 5V output!!!)... and solder the wires to the cellphone's battery pack electronics. You'll be able to "charge the phone" by charging the power bank.