On 22/07/2020 22:57, Theo wrote:
jon wrote:
I used an old 19v x 4A laptop power supply with a couple of 55W headlight
bulbs in series to charge my flat battery. It started at 2.5 amps and
dropped to 1.8 amps over a couple of days, giving a full charge.

That's a handy idea. I have some 12V 20A PSUs, and a headlight bulb in
series would limit the initial current to 4.5A which would be sensible.
That would be enough to get it off the floor, and then a regular charger can
take over. It wouldn't be the fastest but it would be good enough to begin
with.

12v is nowhere near enough to make the lead acid battery charge. A lead
acid charger needs to source at least 14v (usually about 17v no load).

Putting the headlamp in series with your existing dumb charger will
probably solve the problem by stopping it overheating. You might
actually need to find a 6v 20W filament bulb to match the voltage drop.

--
Regards,
Martin Brown