Is it normal for standalone battery voltage reading to go down daily?

Question

I have a battery that was kept in car boot for a while. I checked it after a few months it was showing 4.5 volts. I connected jump leads from Donor car directly to this battery and re revved it for 20 minutes or so. After an hour checked it was showing 11.5 volts. Next day 11 volts. Next day 10.3 volts.

Gonna still monitor it but just wondering is it normal for reading to change so much over night like this? Could it be because I didn’t fully charge the battery to 12.7 so it depleted at an accelerated rate, or could the difference be due to temperature over the days etc? Or is it more suggestive there’s a problem with this battery since it’s unconnected and it’s discharging too fast?

Thanks.

Answer 1

After revving for 20 minutes, the battery was still nearly dead.

As I explained in a previous answer, nearly dead lead-acid batteries have a very high internal resistance due to dilution of the electrolyte and will not take much charge current until they gain a reasonable amount of charge. I estimate that revving for 20 minutes gave you perhaps 1% to 2% charge.

So is it normal for a battery that is still nearly dead to have its voltage drop from one day to the next? Yes, perfectly normal.

If you want this battery to recover and accept a charge, put it on an automatic battery charger and leave it on until the charger says that the battery is fully charged. This might take 12-24 hours or more depending on the current capability of the charger and the size of the battery.

After fully charging, the battery voltage will drop (it will lose its surface charge) over the first day, and then it should hold nearly constant for a few weeks, except for very small changes in voltage related to temperature. If you find that the voltage continues to drop much after the first day, the battery has lost its ability to hold a charge and it should be replaced.

Answer 2

I’d say there’s probably something wrong with the battery. Without knowing where you are other than probably not USA since you said boot, idk how easy it is to get the battery properly charged and tested. To really give you a solid answer, we’d preferably have the battery make and model number, age, and temperature. The bci, electrical specs, battery chemistry and temp are a minimum though. In the USA, unless a person is in a remote location, it’s too easy to drop off a battery and get it charged and tested to even bother guessing. I can tell you if the battery is so dead that all the liquid is water and the plates are full, 20 minutes from another car’s alternator ain’t gonna do much. You need a specialized charging regime to shock the plates and hopefully get the water back to acid, then it can be charged normally, THEN tested and seen if it can hold a charge and deliver the rated current reliably. The cheapest testers can’t do that.