I'm trying to replace the old NiMH Roomba battery with a bunch of 18650s recycled from old laptops. Lucky enough Roomba uses 12s NiMH battery, that operates in a voltage range of 12-18 V, and has quite similar characteristics to a 4s Li-ion battery: 12-16.8 V.

After adding a simple 4s protection board, it seems to work fine and generally does the job. There's one small caveat I'm trying to tackle - Roomba's charging circuit complains about the battery when the protection board cuts the current to prevent a cell from overcharging and the robot reports a "Charging Error 5".

I'm guessing that it's just because the battery is unable to reach a voltage higher than 16.8 V. I also know that people solve this issue somehow without modifying the Roomba itself - there are third-party Li-ion batteries available on the market. Is there any simple solution that'd allow preventing the error message?

I thought about connecting a capacitor to the output to allow charging to a higher voltage when the battery gets cut, but it doesn't work - it'd need to have a very high capacity to work properly, I think.