Andrew Gabriel formulated the question :
If the battery is very flat, it assumes it's a 6V one, and because
it will instantly rise to the fully charged voltage for a 6V battery,
it won't get very far. You can get around this by connecting another
12V battery (or some other 12V power source) in parallel just for a
few seconds. (OTOH, if a 12V battery is down to 6V, your chances of
charging it again are not too high anyway.)

I built and designed for myself a rather fancy heavy duty automatic
pulse charger. That will not start to charge unless the battery voltage
is within limits, to get around it I modified it by fitting an override
button to force the charge to start.


It would be brilliant if it had some way to make it start charging
automatically after a power failure. You could then use it to build
a very cheap but effective backup power unit.

Really that is its only disadvantage, but a fairly common one on this
type of charger. I have another similar charger which does not need a
button to be pressed - its disadvantage is that if there is a power
cut, the charger will draw power from the battery and discharge it - I
use this one for my bike. Fine if it just a short power outage, but
there have been occasions when I have accidently tripped the socket
circuit in the garage and next time I go in a week later, the bike
battery is completely flat.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk