Paul wrote:
He's bringing it off the floor though.

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.

Indeed, and that's part of the attraction of this 'redneck charger':

- the supply is 12V so at no time will it ever overcharge a battery

- if the terminal voltage is low and the battery accepts current, the lamp
will light. The lamp prevents overcurrent.

- as the terminal voltage rises the lamp will dim. The lamp is a (very non
linear) indicator of the current and thus the terminal voltage

- if the terminal voltage shoots up (because the battery is knackered) the
lamp will light briefly and then fade out

- because it won't overcharge it's safe to leave it on for a long period.
In the end case the 12V out and 12V battery voltage balance and no current
flows. That's roughly the 25% SoC condition so the battery is within
nominal limits at this point.

- once it's reached 12V it's safe to apply a 'normal' charger (or in this case
leave the car powered up and it'll charge it from the traction battery,
starting the engine when that needs topping up)


It would probably need a bit of tinkering to get the right bulb to cover all
the conditions though. Probably a voltmeter is still useful as a better
indicator than the lamp itself.

Theo