Note that NiCd and Nmh batteries are fairly easy and tolerant in charging. Lithium batteries are most definitely not easy and tolerant.
If doing lithium, you'll need a processor with a calibrated voltage measuring capability. Arduino would work if you make sure to calibrate it.
Next you need a current limited power supply. The current has to be less than the capacity of the cell, half the capacity is very safe.
The voltage has to be limited by cell number and type. A typical 3.7V lithium polymer cell has a limit of 4.2 V. Many people use the (hopefully) built-in protection circuit to limit the voltage...that's not the smartest thing you could do.
So, my programming steps...
if battery voltage over 3.7V, go to top
// continue here
reset and pause timer
if timer > 1 hour, turn off charge and go to top
if battery below 4.1V, turn on charge and continue timer
if battery above 4.15V, turn off charge and pause timer
go to charge
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