I have bikes with both 6 and 12V electrics and normally use a 'smart' charger when needed.
However, I've found that these will not charge a fully discharged battery - if the detected battery voltage is below a minimum threshold they will just display an error code.
I accidentally left the pilot/tail lights switched on and my 6V Bantam battery is totally flat and I'm currently (!) unable to charge it.
Does anyone know of a simple, old-fashioned transformer/rectifier type charger that will deliver 6V - or an alternative way to get some charge into my battery (bearing in mind that I'm in a field in Germany for the next week!) - I was wondering if a 5V phone charger might get enough into it for the smart charger to then be happy to take over....
Cheers,
Bernie