On 21/07/2020 22:12, Theo wrote:
I think I might need a new car battery charger. However I'm failing to find
anything suitable and wonder if the team can recommend any models.

Requirement #1 is to be able to charge flat batteries.
My boot lid sometimes doesn't latch properly and leaves a dim little light
on - which if you don't drive for a week or two empties the battery. My
current 1980s dumb-charger can't handle delivering current into a low
voltage battery for more than a few seconds before the thermal trip goes[1].
I've seen smart chargers say 'we handle flat batteries 7.5V' which is no
use when the voltage drops below that.[2]

A high wattage resistor to limit the current flow when the battery
terminal voltage is very low would solve your problem. Assuming a rough
16v output and a 9v under initial charge you will need to drop about 7v
and at say 4A so a 50W rated 2 ohm resistor ought to cope. Or perhaps
better a string of two 15W 1 ohm resistors so that you can up the
current by clipping on up the chain or use them in parallel as it gains
voltage. That way you don't overload your charger but you do end up with
some pretty warm resistors dissipating the excess power as heat.

I would expect that mode to be a constant current mode, although they could
pulse or do other fancy things. But it should be able to deliver it
continuously without tripping out.

A voltage in series with a suitably rated current limiting resistor will
do - you don't really care about efficiency here.

Requirement #2 is a display of voltage and current so I can see what's going
on. Little LEDs saying 'half' or 'full' do not cut it.

DVM in series if you must.

Nice-to-have #3 is a means of terminating charge when it's full, but I could
live without that.


Are there any 'smart' chargers that aren't also 'too clever for their own
good' chargers?

Not really. Any smart charger will take one look at your battery and
decide that it is a Norwegian blue parrot analogue. Stone dead.

Lead acid batteries that have been to such low terminal voltages usually
have at least one weak cell in the chain that is essentially wrecked.

[1] Instead I've been using a old 32V 2A bench PSU in constant current mode,
but 2A isn't very much and the case gets very hot (suspect a linear PSU).
It also isn't great to leave in the engine bay as it isn't weatherproofed in
any way.

[2] You might say the battery has had it by this point, but it's an AGM
battery in a hybrid so it lives an easy life. It only needs to provide 20A
for a few seconds to open contactors, it isn't used to start the car. I
might look at a LiFePO4 SLA-replacement next time.

Swap the incandescent light for an LED and you will give yourself
another marginal factor of 10 safety factor on wrecking it.

--
Regards,
Martin Brown