[nota]

Originally Posted by dad911

Use Hobby batteries. Cheap, universal charger. 3s = 12v, 6s=24v

https://hobbyking.com/en_us/batteries/lipo-batteries-all-brands.html

I would not put the ryobi batteries in parallel, they will not discharge evenly and may generate large currents internally trying to charge each other.

it was suggested to use large diodes to prevent that
doable as that would be part of the wiring on the unit not the battery
but if both full charged why should cross charging happen
they auto cutoff to prevent low volts [when the cross charging may happen]
worse case a bike or lawnmower it is not a big fire problem

batterys removed when stored or to recharge as they have other uses
and are likely low when use is over in the bike or mower

I like the ideal of multi use/roll batterys
and trying to make these 18 volt battery's I have work