GBA micro - batterylenght

Ive just bought a GBA Micro and also a EFA Linker 2 1gb card.
Ive loaded my GBA Micro and the battery in the EFA Linker 2 (for saves) one time each.
What Ive read is that the battery lengh should be about 9-10 hours or something like that. Mine now shows red light after about 5h (absolutly not more).

What is wrong!?

probobly cause you play it went is charging

make sure you charge it to 100% once, without playing at the same time. then make sure you use it completly up after that 100% charge. that should get some more time out of it.