Gnyff

I have an old LT drill that I've just got a "new", used battery for. The battery is a 5.2Ah ultraM (25592000) type. Seems to be working fine (more than I can say about the drill - must open it up to investigate soon). However, even when fully charged the battery blinks with the diode closest to the test button (when inserted in the drill). The cycle is something like 15s. The only description of "blinks" I can find relates to low battery - but this is going on while a push on the test button lights all four LEDs. So now I'm really curious: Anyone having an idea of what's going on?!?