The best way to keep your batteries charged, if possible, is to just plug in your chargers right next to your bike. You don't even have to take the batteries off the bike that way - just disconnect them from the light and plug them right into the charger, right there on the bike. I just make a habit of doing it every time I use the lights at all.

One thing to watch out for, though, is that if you leave the lights hooked up to the battery for a long time, even though the light isn't on, it will eventually completely drain the battery. So...don't do that, like I did and find out the hard way. :-)