My car's battery got flat and the car doesn't start any more. I have a table top PSU rated for 0-15 V, 3.5 A max and 1.7 A continuous, and would like to use it to let the car start again. The battery is a Varta 540 126 034 rated as 12 V, 40 Ah, 340 A. Car is a Honda Jazz from 2006.

I am aware that just connecting the battery to the PSU and letting them go is not a good idea, because that might overcharge the battery and damage it. However, I don't need to carge it fully, just enough so that I can put it back on the car and let it start. Then the car alternator will take care of the rest. Is this feasible without specialized equipment? I have voltmeters and ampmeters.

Specifically, which voltage should I give it? Which max current? How much time to reasonably have enough charge to start the car, yet be sure that I am not damaging the battery or doing anything risky?

1 Answer 1


At 40 AH capacity that means your 1.5A 15V charger will take 26 hours to provide a 40 AH charge to the battery. You could try hooking it up for 2-3 hours and see if you can get enough charge to start the engine. The 1.5A charge current is going to be nearly "trickle" charge level for an auto battery.

  • It worked. In the end I kept the current a bit lower (between 0.5 and 1 A), because I had rather thin wires. After a couple of hours it was already able to light up the instruments and I probably would have been able to start pushing the car. I charged more or less ten hours in total, and that was enough to start the engine. Mar 26, 2020 at 11:08

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