Cub Cadet zero turn mower starting issues

A friend has the above captioned mower which I borrowed. I tried to start it but the engine would not turn over....checked the battery and it needed a charge....down to about 8v. I charged it overnight and it reads 12.8v on a tester, but it only turned the engine over about three times and then quit. Re-checked the battery and it still reads 12.7v.....anyone any ideas as to what the problem is?
Thanks.