Stopping overvoltage with Solar on Lithium ion tool batteries for an audio application
18/3.6V = 5S cells so you propose 10S = 36V nominal 42V max.   

Ideally use an off the shelf solution for charging - either a commercial product or a charger IC.

The batteries will usually have had cell balancing - either within the battery pack or the charger. I have seen (now own) LiIon batteries (Dyson vacuum!) with NO specific balancing and they failed after a while entirely as you'd expect. 

I'd recommend 4.0V full charge per cell. Longer cycle life and slightly more forgiving of unbalanced charging. Somewhat per cycle 
 reduction in capacit. | You can make per cell dissipative top balancers with a TL431 and a few resistors per cell or add a transistor for as much balance load as you wish. 

You can buy Chinese N cell balance boards at OK prices and that is liable to be the easiest and probably cheapest path.