RE: RC18MT charger?
The charger that comes with the RC18MT does not shut off. It will continue charging as long as it is plugged in. It is a very slow charger, so you won't destroy your battery if you leave it on for a couple hours extra. But it does not shut off on its own.
calvino is correct about the CX-15. It is rated at 2.5-4 amp charge rate. That would almost certainly damage your battery.