I employ a 150 watt voltage booster regularly for various things, and have charged a 24v Nicad battery with it several times successfully, but then I also destroyed it by not discharging the battery much, then charging
it, forgetting and overcharging.
That 'gotta get it and keep it full' lead acid mentality came back to bite me.
had increased well over 150 watts and the cells were popping an crackling ominously, and continued to do so for a long time after I ripped the wires off.
I have a wattmeter inline to read Volts amps watts amp hours watt hours, but I should have had a temp sensor with an audible Alarm
on the battery.
Obviously Nicads have different requirements and less consequence if overcharged compared to lithium. I did have it on some brick pavers away from anything flammable at least, but I wish I also had a full face shield when I heard it crackling and snapping and needed to disconnect the boost converter.
I think If I were to try charging
a lithium power tool battery I'd make sure it was dead and put in ~65% of the rated capacity and then stop, at least initially.
These can be had for under 2$ slow boat
I put mine in a old inline powerbrick with a 60mm fan bridging the two heatsinks, and 45 amp anderson powerpoles on input and output, and use it as a Lead acid Battery to lead acid battery charger regularly. If I ever employ it on another chemistry again I will be doing so under much more rigorous observation than my most recent Nicad attempt.
The now batteryless powertool, I use an 18 and 22AH AGM
(same size, different weight and age) battery in series, on 8' of 10 awg. Less convenient, but way more power when heavily loaded than the Nicad ever had.
My voltage booster regularly takes float voltage back upto 14.8v for charging these two 12v AGMS separately from my main battery bank.
I tried charging them in series. One battery read 13.5v the other 15.5. Wont be charging them in series again.
There are lots of voltage converters that look like this one, some are buckers, some are boosters, and some are buck/boost. I have a buck boost with 2 potentiometers, the second being a current
limiting potentiometer which would be a better choice for Dc to DC battery charging than one with voltage only potentiometers, but I was unaware of this option when I installed the sub 3$ single
trimpot booster in the ventilated powerbrick casing several years back.
I replace the tiny trimpots with 22awg or thinner wires to a larger remote
fingertwist potentiometer on all of these, so I do not have to bust out the reading glasses and the jewelers screwdriver each time I need/want to adjust charge voltage / fan speed/ led brightness.
The potentiometers are usually 10K ohm, but on the 10 amp buck/boost converters I like best, they are 200K ohm, and I never found any 10 turn pots of that value. A few, on some 5 amp buckers, have been 50K ohm.