recharging 18V battery back in parallel
Checking yonder google tells me that it's difficult and unwise for NiCd: http://www.powerstream.com/NiCd.htm, and unlikely to work for Lithium batteries unless you match the internal cells very carefully: https://electronics.stackexchange.com/questions/126673/charging-li-ion-batteries-in-parallel.

Either one could be overcome, but not for the minimal expense of just buying a cheap lawn tractor battery from Walmart, and putting a trickle charger on it.