Discharged car battery voltage question.

I have seen voltages of 3 or 4 volts on discharged batteries, and they won't power anything of any use. I don't know your source, but I would not trust it. At these low voltages, some smart chargers won't even charge them, they need a hard starting charge from a dumb charger.

I think the implication is not that it *cannot* supply any further power, but that it *should* not. No rechargeable battery should ever be discharged to zero, and different technologies have different recommended lower limits, below which progressively greater loss of capacity will occur. Lithium cells, for example, have a working range from about 3.0 Volts to 4.2 Volts fully charged, and reputable battery manufacturers will actually cut off the connection at 3 Volts. An apparently dead lithium cell may just be slightly below the cutoff voltage, and a suitable charger might wake it up.

State of charge is not just about voltage, it's about the ability to hold up that voltage under load.

You can have a small capacitor that's charged up to a high voltage, but as soon as you try to draw current from it the voltage collapses. The capacitor can't store very much energy so it can't deliver the current demanded of it. The only way it can 'respond' in order not to violate the conservation of energy is for the voltage to drop.

That's what will happen if you put a load on your very-depleted battery - it can't hold up its voltage to supply the current, so the voltage will drop.

A digital multimeter is a high impedance measuring device (typically 20 megohms) so it can measure voltages which 'aren't there' in the sense of having no current available to back them up. Automotive technicians use a test lamp as a way of telling if there's a 'real' or 'phantom' voltage there.

Which is not to say you couldn't power certain low power things from your flat battery - like you can power a quartz watch from a tiny button cell. After all, the battery may still be at 1% state-of-charge, and 1% of ~1kWh is still a big number. But you are not getting within orders of magnitude what the battery is rated for.

Also, the further you deplete the battery the more you shorten its life. So don't do this unless you want a scrap battery on your hands.

Theo

On 02/06/2026 13:38, Davey wrote:

(Long story, you don't have to read it all because the last sentence is the point I am making)

Many years ago (1960s) I had an offer of a classic car (a 1952 Triumph) if I could fix it and drive it away. The history was that the car's owner had had engine trouble. He had dismantled it to see what was visibly broken, bought replacements and then couldn't remember how to reassemble it. So he had put the new bits with all the old bits in a pile in the boot and had left it in the front garden, with the battery put beside the house doorstep.

I went along and looked at it, and the bodywork was still sound, the clutch still disengaged, the gear lever moved OK and the brake pedal felt that it was working properly. So it was only an engine repair job.

I took a bit more of the engine apart and found a couple of bits that needed replacing that the owner hadn't spotted. It looked as though he had had a lubrication problem. He had spotted that the pistons and big end bearings were wrecked but hadn't checked the main bearings. The Mayflower engine was a rare beast (it only went in the Mayflower and the Standard Flying Nine) but despite no internet in those days I did find a repair garage 40 miles away from the car which had them as new old stock. I then spent every Sunday when it wasn't raining getting the car back together. The second time I was there, I took the battery back home. It still had its acid in it so I put it on a dumb charger and left it. Then I found that the petrol pump had a split diaphragm and the flexible pipe from the pump to the carburettor was perished, and they needed replacing. Eventually I was getting ready to see if it would start. The battery had been on a permanent 24 hours a day charge, and it took 2 weeks before the charge rate got high enough to get the needle off zero, and another three weeks to get the needle to the charge rate the charger was designed to provide. I tested it with a headlamp bulb and it seemed to be holding the charge for the 10 minutes I left the bulb lit. I topped it up with distilled water because it looked a bit low, and left it on charge for another week before putting it back in the car and putting some fresh petrol in the tank. It turned the engine over a bit sluggishly, but kept turning long enough to pump petrol from the tank to the engine, and the engine started. Job done.

That battery was good enough to start an engine that started easily, but it wasn't giving as much power as I expected, so I took it along to a battery supplier and asked if refreshing the acid would perk it up. When he saw the battery he looked astonished and asked me how I had managed to keep it going for so long. Apparently the replacement recommended for that battery was already obsolete, and the one I brought it was well beyond needing an acid refresh. I left the old battery there and went home with a new one. But my experience does show that lead acid batteries can be recharged from completely flat if they are given long enough to charge.