The batteries inside the case are mostly the same: 18650 cells or something close to that. What varies is the number of cells and the plastic case they're wrapp

The batteries inside the case are mostly the same: 18650 cells or something close to that. What varies is the number of cells and the plastic case they're wrapped in. There exist case adapters that allow you to use e.g. a Makita battery on a B&D tool; you can find them on Amazon. As long as the voltages are close, they work. (Don't use adapters for charging! Only use them for powering equipment.) If you know what you're doing you can even replace the cells in the case yourself. But I must emphasize the caveat above; this can be incredibly dangerous if you are not an expert. Even if you are a seasoned EE, you might not know enough to be messing with high-energy stores that cannot be shut off.