Battery won't complete charging

I have a stick up camera battery - less than a year old and has worked pretty flawlessly. Got the battery low notification for perhaps only the 2nd or 3rd time the other day so removed the battery and put it on charge. Orange & green lights lit up as normal... but it never reverted to just green to indicate fully charged. I left it the whole weekend then tried putting it back in the camera. Device Healthcheck showed 4% charge and Live view disabled. Changed cable, plug and socket to all known good kit, put back on charge and left for another 4-5 hours. Same two lights. Back in the camera - 2% battery. Is there a trick to get the battery to charge properly? It's less than a year old and has only been charged a couple of times so should be plenty of life in the battery.