MMM died during start up

[Dagger Thrasher]

Just a thought... have you tried recalibrating it to your radio/checking your radio's endpoints, trims etc? I'm just thinking that it's possible that your radio's throttle channel could've been knocked out of trim when you changed the cells, resulting in the MMM refusing to fully arm. It's a long shot, but worth checking out.

I had that problem with one of my MMs once...it took me ages to figure it out. Wouldn't respond at all until I realised I'd messed up the radio settings.