In article ,
The Natural Philosopher wrote:
Lead acid cells have a *very* low resistance when discharged and without
some form of current limiting between a simple charger and the battery -
even just a resistor - they would draw a very large current indeed, far
more than the puny power supply in an alarm could hope to supply.


So what happens when the puny power supply comes across a discharged
7A/h battery?

It has some form of limiting built in to restrict the charge current to
what it can cope with. A simple lead acid battery charger is a DC source
of around 16 volts open circuit with a suitable series resistor. Of course
modern ones will have some electronics.

ALL lead acid chargers are current limited in some way. Not all are
capable of handling a direct short, but a fully dicsharged battery is
not a dead short.

Didn't say it was - I said it had a very low internal resistance - and any
practical lead acid battery would take more than a burglar alarm power
supply could provide without some form of limiting.

Its a plastic bin job. Its finished as a battery. No
lead acid battery is ever fully discharged in operational life. Only
right at the very end of its operational life :-)

--
*Middle age is when it takes longer to rest than to get tired.

Dave Plowman London SW 12
RIP Acorn