GBA SP damaging flash carts - likely - but how to diagnose?

I have reasonable suspicion that one of my GBA SP has so far damaged an GB Everdrive. Krikzz himself told me that wrong voltage coming from the battery my be the reason.

I have experience taking stuff apart and back together, but know next to nothing about electricity. I have two GBA SPs, one of them AGS-101 (bought shell-swapped) and a new spare battery. I have access to basic shop tooling, but don't know where to begin. Is measuring the battery output enough? Is there a possibility that the unit is improperly assembled and 'leaks' thus damaging the cart?

 

There is a catch that depresses on the game port when a GB cart is inserted rather than a GBA cart; that changes the voltage from 3v to 5v as triggers a step-up circuit - that might be the issue if the switch/catch is "on". Just a thought.

 

GBA cart uses 3v and GB/GBC cart uses 5v. So it would be under-volted if the switch is bad but shouldn't be damaging the cart.