I have the following keyboard, RT-UMK-100-BT. It suddenly wouldn't charge and as I didn't use it very often it got shelved until I had a need for it again. I opened it up to find the battery all swollen so I presume that's why it doesn't charge anymore. I cant find out what exact battery it takes. It does have 5c 300ma on the back though. Anyone got any idea what battery I could use.