I have a Ryobi battery that I'd like to charge from 12V instead of 230V. The charger comes with a power supply that outputs 36V 1.7A, so I thought I could easily replace this with at step-up boost converter with the same specifications.
I am using this converter - I think it should be fully capable of the task.
When connected to my lab power supply, it almost seems to work. On the battery there are 4 steps to indicate charge (see picture.) When it charges one step, the current output on the lab supply goes from 3A to 0.7A in about an hour and then the charging seems to stop.
I imagine this is because the battery has four cells, and the current drops to "fill the cell entirely" before moving on to the next. (Just a guess.)
The problem is that charging seems to "hang" here.
If I compare with the 230V power supply, I see that the charging voltage is slightly higher at this state (38.3V.) If I adjust the boost converter to this voltage, then suddenly the charger returns to using about 2A, and the charging continues.
My thought was that since I only replaced the 230V power supply, I would not get into trouble that would possibly destroy the battery - is that true?
What can I do to make the charging "run" all the way?