For one thing..you don't burn any fuel while the batteries recharge...and even though it is most unrealisitc to reharge while the diesels are idle .....it is a fact of life in SHIV World...sorta like the stars being closer to earth than the clouds...
Oh..forgot to add...after you are fully recharged you can then resume steaming with recharge batteries "off" which increases fuel range efficiency...enormously.
You could of course simply turn off recharging but some skippers don't feel at ease traveling with batteries not fully charged.