I was looking at some of these little Li-Ion car battery jump kits and started wondering how exactly they worked. Obviously it won't charge the battery to full but can dump enough power through to get the engine going. I'd think that the 18 or 20v battery packs would work similarly to this if in a pinch but would have to keep an eye on the pack to make sure it doesn't overheat.

My question is how voltage works while charging a battery. If a 12v needs charged and a 24v charger is used (I know this isn't recommended and can be dangerous, but this is theoretical), does it charge 2x as fast as if the same current is used with 12v?

So if 12v @ 5 amps is used vs 24v @ 5 amps, will the battery have 2x the charge after the same period of charging? I know this is how electricity works with motor usage but when dealing with electrolytic charging, does the extra voltage still stay in the battery or is it lost as heat or gas generation?

I know some batteries hold a charge higher than their stated voltage after charging for a short time, but the "bulk' of the charge is at the stated voltage. Is this directly related to the charging voltage or is it related to the battery characteristics?

 

I was looking at some of these little Li-Ion car battery jump kits and started wondering how exactly they worked. Obviously it won't charge the battery to full but can dump enough power through to get the engine going. I'd think that the 18 or 20v battery packs would work similarly to this if in a pinch but would have to keep an eye on the pack to make sure it doesn't overheat.

My question is how voltage works while charging a battery. If a 12v needs charged and a 24v charger is used (I know this isn't recommended and can be dangerous, but this is theoretical), does it charge 2x as fast as if the same current is used with 12v?

So if 12v @ 5 amps is used vs 24v @ 5 amps, will the battery have 2x the charge after the same period of charging? I know this is how electricity works with motor usage but when dealing with electrolytic charging, does the extra voltage still stay in the battery or is it lost as heat or gas generation?

I know some batteries hold a charge higher than their stated voltage after charging for a short time, but the "bulk' of the charge is at the stated voltage. Is this directly related to the charging voltage or is it related to the battery characteristics?

One of the magazine columnists tried one out - he says you need 2 to ensure getting going under all circumstances.

TBF: he ran his battery fully flat before trying one of those to start it.

 

The first thing to look at is the actual charging voltage that that the dead battery needs which for a typical automotive battery that's ~14 - 15 volts but for short burst boosting 18 - 20 is not a problem.

Second is the Ah (Amp hour) rating and theoretically what it would take to put say 5 - 10% of that back into the battery from another battery including the relevant energy transfer loses which under a high rate fast charge boost condition could easily be over 50%.

As for the source battery voltage that's largely irrelevant if a proper voltage conversion stage is being used like having a simple buck converter to drop the source battery voltage down into a range the charging battery and related systems can handle which on a typical automotive application anything under 18 - 20 volts at load is close enough to call it good.

Now for the source battery current supply rate that's entirely up to what you want to design around and largely irrelevant to the automotive battery being that for short burst hard charging cycles they can easily handle being hit with current levels equal to anything they can supply when fully charged like being hit for 30 seconds to a minute of input current equal to their cranking amp rating.