Question from inbox:

I have a question about problem 3.3 in the original Cram book. My  question is regarding the battery charge inefficiency. The problem  has 1.2, but should this be 0.2? I am confused where the 1.2 is coming from.

Forget the 30A DC motor and let us make the problem more simple. Lets say a battery charger is supplying 40 amps to a battery with 100  Ah rating. Let us assume this is an ideal world with no losses. The  time to charge the battery would be 100 AH/40A=2.5 hours. Now if we switch to the real world we know there are going to be losses. So right away we know that the answer has to be greater than  2.5 hours. In other words, it is going to take me longer to charge  this battery because the world is not ideal. So using deductive logic  you know that multiplying by 0.2 makes no sense. 2.5 hours *0.2= 0.5 hours. If in the ideal world it takes me 2.5 hours to charge the battery,  obviously in the real world (with losses) it could not possibly take  less time than that. Put another way, due to the losses it is going to take me 20% more  (1.2 multiplier) time to get it fully charged. This is where the 1.2  comes from.