VL2020 was the original models and for a few years after.
CR2020 was the later miniSD models.
The new microSD models use a different sort being CR2025.
The CR2020 stuff differs from the rechargeable ones but you could probably replace diodes and resistors as appropriate to get charging going on (no GBA flash cart I have ever seen has had proper charge management for SRAM batteries, the EZ line being no exception), most would just replace the battery again though as it lasts for years -- when carts had a real time clock to power it took effort, half modern SRAM just needing data to be held is not a demanding task. I would be half willing to bet that the internal resistance/general shelf life of batteries plays a role of fair significance in it over just being exhausted by the SRAM.
Edit. Forgot to say if you only had a rechargeable in your kit or something then it would probably work just fine until it dies from deep discharge or something, said VL2020s are used by certain car key fobs though (some BMW if memory serves) so they are quite expensive as sellers realised people would happily pay to fix a key fob that the car companies would charge through the nose for to otherwise replace.
