Car battery charger

[Dave Plowman (News)]

If the battery is totally flat it will be damaged anyway. With such an expensive item best to charge it long before it is flat.

[Scott]

True, but OP must be planning to flatten the next battery as well (otherwise he wouldn't need the charger).

[Dave Plowman (News)]

You use the charger before the battery is flat?

[Scott]

An ordinary charger can do this, surely?

My understanding is that OP is looking for a special charger that can recharge flat battery (.Requirement #1 is to be able to charge flat batteries').

[Harry Bloomfield, Esq.]

Theo presented the following explanation :

I doubt that will work, LED's dim appreciable as the voltage reduces, so they have no clear cut off voltage. The only way to get it to work is to have the LED's very dim at full voltage, too dim to be useful.

[Harry Bloomfield, Esq.]

williamwright pretended :

With a current limited charger, it does tell you how the recharge is progressing.

[tabbypurr]

that's a crappy option, but it would sorta work & is simple. LED i will vary from max at car charging down to zero at 12v.

NT

[Theo]

I like it :)

Theo

[Theo]

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.

Theo

[williamwright]

Yes.

Bill

[williamwright]

I had to put a bleeper across my indicators in one van I had because the engine was so loud I couldn't hear the tick-tock thing.

Bill

[Tim+]

Our family?s first Renault 4 had non-self cancelling indicators and we fitted a bleeper for similar reasons.

Tim

[Harry Bloomfield, Esq.]

Theo brought next idea :

12.0v will not put any charge at all into a lead acid battery. Even 13.8v is only capable of bring a battery up to a 50% charge.

[Martin Brown]

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.

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

DVM in series if you must.

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.

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

[Martin Brown]

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.

[Paul]

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.

If the battery is 60Ah, and that part of the knee of the charging curve is 10% of that, or about 6Ah, you can figure out how long it'll take to get that far up the curve. And as long as the voltage is slightly greater than the battery state of charge, it'll charge. That's because coulombs are going into the battery, and "something gotta give". Now, if you applied X volts, and no current flowed, *then* I would agree that no chemical state change could occur. But if the coulombs are going in there, some redox reaction is bound to be working with that as a reactant.

And for a damaged battery, the battery is no longer "stiff", and the terminal voltage will shoot up rather quickly. I've had several batteries on my dumb charger, where the stupid thing says the battery is full, 30 seconds after a charge starts. And it's because the battery is up around 16V, instead of resisting the charging effort and measuring 11.6V. And with the dumb charger, the charger only pushes a bit less than an ampere, at 16V. So you're not really charging the battery all that fast any more either. This is why non-CC chargers "suck". And why the SMPS ones are better (consistent charging over time through the charge cycle - no goofy corner cases).

Paul

[Theo]

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

[Martin Brown]

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

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.

[Dave Plowman (News)]

Yes. The essence it to recharge a 'flat' lead acid as soon as possible. And of course before it gets too low. Given the high price of a new one, and how long they can last when looked after, makes sense to treat them kindly.