battery charging

Switched everything to Lithium, and opted for DC-to-DC charging so didn't end up using this. Brand new, never used, everything still in the box, $100 OF. new on Amazon for $150.

I know, don't do it. But... some races are long enough to require (and allow) engine charging, but not engaging the prop. So the engine would run under little load while charging. How bad is this? I see posts elsewhere about a need for a lot of engine work after doing this on a sail to Hawaii...