DIYbanter - Car battery charger

On 23/07/2020 13:26, Paul wrote:
Martin Brown wrote:
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.Â* 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.Â* 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.

He's bringing it off the floor though.

If the battery has been discharged to below half its nominal terminal
voltage for extended periods it is probably scrap metal anyway.

Not doing a complete charge cycle with this bodge charger.

Just lifting the battery high enough, so a
standard charger will accept the battery
and finish the job.

That will happen automatically as the battery terminal voltage rises and
the filament bulb no longer has sufficient current to maintain a white
hot filament its resistance will decrease allowing more current.

--
Regards,
Martin Brown