Replacing lithium batteries is not as simple as replacing the earlier types (NiCd, Nimh) of rechargeable batteries.
A lithium battery as used in rechargeable hearing aids consists of the lithium battery cell itself, an integrated circuit (IC) to prevent overcharging and another integrated circuit to halt the battery output when its charge has dropped below a certain level.
The battery unit that is visible when the hearing aid is opened up normally has these 2 integrated circuits and associated wiring hidden inside the unit with the battery cell.
I took apart a Widex hearing aid accessory, a Com-Dex, and then dismantled the battery unit and found these two integrated circuits (I have assumed Widex uses the same types of rechargeable battery units in the hearing aids and their accessories).
So replacing a rechargeable battery in a hearing aid is difficult because I would have to source a battery unit that had these integrated circuits built in and is of the right shape to fit into the hearing aid (The battery unit in the Com-dex was a peculiar shape designed to fit into the Com-dex body).
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