Extra Battery Dilemma.


Esteemed Pedelecer
May 21, 2017
inverters are quoted as typically 85% efficient.
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.

I was hoping to get the 12V charger, so I can go direct from 12V to charging the bike, with lower losses.
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!


Jul 13, 2017
Perthshire Scotland
Agreed Jo,

and given the other constraints you have in the campervan, ie. how much water you can carry and how long before you need to empty the grey and black tanks, I would envisage having to visit civilization every few days, so I would simply recharge my "two" bike batteries. After all if I go away for 4 or 5 days, I don't want to be cycling all the time, a guy has to have some time off.

I'll keep the inverter for the microwave and my porridge ;)


Feb 22, 2017
Chandler's Ford
Toured Europe in a Hymer Motorhome for 3 months each year for 10 years. Never stayed on campsites just cycling & walking the dog in the hills & mountains.

4 X 125Ah leisure batteries, 150W solar panels, Sterling B2B charger and an inverter that I can't remember the details of.

The batteries lasted 10 years.

Mains use:- washing machine, spin dryer, bread maker, toaster, satellite TV, charging computer & phone etc.

Took a Honda generator but left it behind after 2 trips as I never used it.

The fastest of the charging systems was the Sterling B2B. A friend copied my set up and lived in his MH full time. When batteries were getting low he just ran the van engine.