I have a pair of mountain bike lights...

Big 14.8V Lipo battery running with a current draw on it of about 1.5A....

If the connectors get very very wet is this enough current/voltage to cause the water to become conductive? (assuming rain water)


Can I use any old water resistant grease to keep the water out or do I need a specific one for electrical connectors?

You might get a small leakage current, but rainwater has minimal ionic content so it's not like it's all going to short out. The main problem will be galvanic corrosion of the contacts and wire, since even a very small current can cause quite rapid corrosion.

A little waterpump grease or Silicon water dispersent spray on the plug should keep most of the water out

Connecter Protecter grease from Maplins , made for the job

http://hypertextbook.com/facts/2006/SamTetruashvili.shtml

suggests 18 megaohm per centimetre so not much current going to be flowing even with closer connectors.
I expect mud and other contaminants would reduce the resistance but not much.

Been using a standard wall-wart style power connector on a set of halogen bike lights (12v, 60W) for about a decade in all weathers. No notable corrosion issues so far. As noted above the resistance of water is massive in comparison.