I think that's probably the (cheaper) modified sine ones (which you may very well want to avoid using anyway!). I seem to recall the better quality pure sine ones are more like 92-95% percent efficient. I'll check on that though.
Ok, but the 12v charger needs to be a step up converter. That itself is going to have losses. If I have some time tomorrow, I'll see if I can find some typical figures, but I've got a feeling that there isn't going to be much in it. Depending on the design, the efficiency of the 12v charger could even possibly be worse! (although I imagine Bosch will have a pretty good design)
Also, wouldn't the 12v charger need somewhere around 13A to give the equivalent of the standard 4A mains charger?
I'm sure some cigarette lighter plug fuses are only 10A. So I imagine the 12v charger is certainly going to be a slower charge system.
So I know what you're saying, but it wouldn't surprise me if at the end of the day, the actual total battery drain is not going to be a million miles different whichever way you decide, and its probably the convenience of the solution that's going to be the higher priority deciding factor.
(and it probably is the case that whichever way you charge, you might find that its going to drain a leisure battery too much -although you did say you had three!)
But I'm interested what your final findings are. I have a similar dilemma, and my solution has been a pure sine wave inverter, but use it to charge a pair of batteries when actually travelling from one place to another and the engine is running!
