I've got a couple older Serfas lights (True 500 and 1500) from around 2013-ish. I haven't really used them too much. They still seem to hold a charge...but I don't know how long they would last. I mean I could just leave them on at full power and see how long the lights stay on. It there a quicker way to check how much charge they hold...or should I just save myself the trouble and just get new batteries?