Grinder keeps stalling under load - battery or motor issue?

Hey all, my 4-1/2" brushless grinder (brand X) stalls when I try to cut 1/4" steel. Battery shows full charge, but it just stops spinning under load. Tried different blades, cleaned the vents. Is it the battery BMS or motor controller? I've checked voltage at the terminals, it drops to 16V under load from 20V. Thoughts?

Sounds like a weak battery cell or a bad connection. I've had that happen when one cell in the pack goes bad. Does it happen with other batteries? If you have another pack, test that. Also check the contacts on the tool and battery for corrosion/black marks. If it's just that battery, might be time to rebuild or replace it.

Could be the tool's electronics too. Some brushless motors have hall sensors that can fail. Have you noticed any vibration or weird noise before it stalls? Also try running it without load - does it ramp up smoothly? If it's jerky or slow, could be a controller issue. But the voltage drop sounds significant - wouldn't rule out battery.

It runs fine without load. Only happens when I push it. I don't have another battery, unfortunately. I'll try cleaning the contacts really well. If that doesn't work, I guess I'll have to buy a new battery. But I'm worried it's the tool itself since it's still under warranty. Thanks for the tips!

If it's under warranty, just RMA it. Don't mess with it yourself unless you want to void it. But if you're curious, you could try a known good battery from a friend or at a store. That'll narrow it down. My bet is on battery - those packs fail more often than motors. Good luck!