I have the same problem. I check the output of the charger and I got 6.2 volts (the charger says the output is 5.2v). I remove the back panel and check again the charging port with the charger attached and there's a 6.2 v reading, but when I checked the battery terminals, there is no measurement at all.

Based on the test that I made, I conclude that the problem lies in the circuitry between the charging port and the battery terminals. I think the best solution to our problems is to bring our GBA SP unit to nintendo authorized dealer for repairs (if it's worth the cost).

@ oldturkey03:I didn't clean the battery contacts since there is a resistance on it. I didn't see any fuse inside of the unit.