All of the MPPT controllers that I've seen are made for 12, 24, 48v, or else for odd specific voltages 14.4v, etc, have not seen one made for 18v yet.
Milwaukee Electric has an 18v charger that runs off a 12v cigarette lighter outlet, that's the closest I've seen so far.
Hello there,
From the other posts i take it that the tool uses Li-ion batteries where some are in series and possibly some in parallel.
If they are Li-ion, then the only way to do it is to either get an approved 12v charger (as i think others said) or else use a 12v to 120vac inverter for the car and use that with a solar panel and maybe lead acid battery.
The only other way i can think of is you would have to study the design of the pack to make a safe charger.
This is not just something we can take lightly because accidents with these calls are really nasty.
The reason this design would be so particular is because the cells are not charged in series, they are usually charged with a charger that can at least MONITOR each cell individually, that's why there are so many contacts. Each cell may even be charged separately or two at once that are in parallel, but in this case they are hard wired in parallel.
So the best advice is to use an approved 12v charger or use a 12v to 120vac inverter to power the regular charger. Small 12v to 120vac inverters are not that expensive either.
Sorry to say all this but if you do it any other way there is a very big risk involved.