Connecting different-sized lithium batteries in parallel.

[Rick Law]

The cold irony is, the larger the capacity the more danger they are.  We are carrying larger and larger bombs in our pocket just so we don't have to recharge our phones twice a day.

I would love to have a cell like Terminator 2 and 3 portrait - run the robot for 120 years on two cells.  But I would hate to see how it blows up, at least as portrait in T3.  I actually doubt Hydrogen Fuel Cell (as portrait in T3) could deliver that much energy. That they are fuel cells implying they get energy from the molecular bonds.  One probably have to go down to atomic level to get that much energy to keep Arnold running around for over 100 years.