Mu Lithium battery weighs 10lbs and spins the motor over like nothing else ever has. I had two XS 16 volts in it and it was ok. Put in the single Lithium battery and it spins like nothing I have ever heard. That's a 605" BB with 60MM Babbit bearing, 15.8-1 compression and 1300+lbs of open valvespring pressure. In other words it takes a LOT of effort to spin her over.