I take them out and pressure wash them, but I'll tell you what helped a lot on the white stuff:
I bought a ryobi 18v glue gun. Prior to having it I used a glue gun about twice a year. Now that I have a cordless one I use it many times a month.
As an experiment, I thoroughly coated the positive terminal cable connection with hot glue.
With the cable hooked to the battery, I just buried it with clear glue stick so I could see what was happening.
It is as clean as the day I applied it and no white stuff anywhere.
(of course it won't work for you now that I've said it out loud, but it sure worked on my 4x4 toy.)
Picture of toy included since this is wte.