How can I fix my charger

GFI being tripped implies that the device is pulling too much current.

If it was working fine prior to this happening on the same plug, then it is likely that either there is a failure in your outlet's GFI relay, or a short circuit in the power supply of your charger, which pulls too much current.

if this is the case, start bytaking a multimeter across the output of the charger, and verify the proper voltage is applied.

then insert the multimeter into the circuit, and verify the proper current is running through it.

If both of these are correct, you can then move on to diagnosing the device itself, as the charger will have tested fine.

diagnosing the device is much more difficult, you will need to track down a short circuit. If something is physically damaged, that is the best place to start, however, you can run into a situation where something has fried internally, and shows no signs of failure. From here, start with the battery. verify its function, much like the charger, then move on, and ironman your way through checking component by component for continuity.

looking for a short is much more difficult than looking for a cold circuit, as you need to connect each node to every other node. from there, you can verify if there is a short, and fix it, as long as it is not board level.

best of luck, and happy hunting.