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For lead acid batteries, figure 70% of the energy you put in will come back out. Wikipedia tells me that lithium ion batteries are 99.8% efficient, but has no citation and the temperature of my phone charging heavily make me suspicious of that number. Perhaps that is a theoretical maximum.

Motor efficiency is proving elusive on the web. But let's say 90%-95% for brushless DC motors. (square that for in and out)

The trick about batteries is that if you only want to use them for intense bursts you will need a huge energy capacity. You don't normally want to charge at a rate faster than say, 1 hour to get full. If you need to store a 3 second brake stomp and give it back 5 seconds later then you are going to have about 1200 times more batteries than you needed just to store the energy (to support the current).

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