Sounds like the battery is giving you a false reading. Which is extremely common for a battery that is going bad. As they go bad less and less of its capacity is actually accessible/working. So while it will say 50% of it is is good, it is likely that 50% is part that no longer holds a charge. Eventually none of it will hold a charge.
Your best bet, replace the battery. And don't go cheap aftermarket. They can be hit or miss with if they work correctly. It is better to go OEM.