I want to get a jump starter power-bank for my car. There are two types, one with 15v and the other with 12v both 10000mAh.
Does the difference of voltage make difference?
You should use a 12V power-bank as the electrical system on your vehicle is 12v.
Do not use a supply which provides 15v as this could harm other components in the electrical system such as various sensors or control modules.
The important thing to look for is not the capacity of the battery (10,000mah) but the CCA value. the Cold Cranking Amps is a measure of the number of amps the battery can deliver in 30s without the voltage falling below a certain threshold. this is important because the starter motor consumes a lot of amps very quickly to turn over the engine. If the power-bank cannot supply enough amps @ 12v the motor will not turn over no matter how large the capacity of the power-bank.
This more sounds like cluelessness of the seller. They sometimes just don't know anything about what they are selling and copy/paste numbers.
15v is probably the input voltage (from the charger of jump starter) to the jump starter. Because many 12v jump starters need a 15v charger. While I have seen many jump starters without any output voltage ratings, they are designed for 12v systems so they will work the correct voltages.
In either case, even if it was 15v, it wouldn't have been too high. Normally the car alternator can charge the battery up to 14.7v and many unregulated cheap chargers can go over 15v when charging the battery.
Yes, some are 14.8 volts. They do this by adding one more Li-ion cell to the bank (4x3.6v) When fully charged to 4.2v the reading will be 16.8 which is too high to run 12 accessories. When you connect it to a dead battery the extra voltage helps to charge the battery but the vehicle system never goes much above 12 because of the dead battery. Most of the chargers use 3 cells which only charge up to a max of 12.8v. Its hard to tell which ones are which. if the nominal voltage listed on the pack is 11.1 then its a three cell.