I'm making a secondary battery pack for a tiny drill, and I wanted this 18650 spare battery pack, always being charged (stored) so it can be always ready to use. However, the Li-ion batteries don't like it to be kept fully charged for a long time as it severely affects their capacity. It seems that a voltage of 3.7v~4v would be a good storage voltage for that purpose.
I'm using this simple TP4056 charging ICs, and their full-charge termination voltage is 4.2V.
Is there any trick to make the termination voltage lower than 4.2v?
I thought of using a diode in series with the battery, but I think it would be troublesome as the chip wouldn't be able to read the voltage back to check if it's charged or not.