# Jump starters - lithium ion

I just bought another one of these to replace he old one that got flooded. (BTW, it worked as advertised - easily jump starting a few boats this summer.) This time around I noticed that the paperwork says it can jump start a car 25 times on a charge but only charge a cell phone 5 times. Is that correct? That seems counter-intuitive to me. Maybe I’m mis-remembering what I read.

To start a car engine, you need a lot of power over a second or two.

To charge a phone, you need a little power over an hour or two.

A hybrid jump-starter/phone charger can do both, either outputting a lot of power for a short duration or a small amount of power for a longer duration. Either way, it has a set watt-hour energy capacity, and the output math is determined by the usage.

A car battery might need 12 V * 500 A * 1 second = 1.7 watt-hours (6,000 watt-seconds)

A phone battery might need 3.7V * 1.6 A * 1 hour = 5.9 watt-hours. In reality the charge voltage, amperage, and time will depend on what kind of phone and charge you’re using (USB-C can quick charge much quicker, for example), but the energy used is about similar.

You would not be able to fully charge your car battery from the jump-starter, only give it enough of a surge to start the engine. After that, your car alternator is supposed to charge the remainder.

Just to recap :

1. Starting a car at 500amps for 3 seconds at 12V = 500x12x(3/3600) = 5 Wh = 417 mAh

2. Apple iPhone 11 battery capacity = 3110 mAh.

Ratio = 3110/417 = 8

Factoring in the efficiency of USB charger (about 80%), this works out to about 6.4. So a ratio of 5 seems okay in the OP.

Slight tweak: The iPhone 11 probably uses a lithium-polymer battery with a nominal voltage of 3.7V, whereas the lead-acid car battery is probably 12.6V.

Between different batteries for the same device (e.g. different batteries for the same phone), it can be assumed that voltage is consistent because their chemistry is similar and/or there are chips that regulate voltage for consistency. So it’s fine to just use the amp-hour ratings. But if you’re comparing two dissimilar batteries, such as a car and phone, you need both the voltage and amperage to get the total watt-hour capacity.

A 12V, 70 Ah car battery has 840 Wh, whereas a 3.7V, 3.1 Ah phone battery only has 11 Wh of energy. That’s 76x the energy capacity*, not 8. The jump starter wouldn’t be able to fully charge the car battery 76x, but it might be able to jump-start it 8 times.

*In reality it’s a bit more complex than that, with various system inefficiencies involved, charge controllers setting different depth-of-discharge limits, etc. You’ll kill a car battery pretty quickly, by eroding its chemistry, if you fully discharge it every single time.

The jump starter is not being used to charge the car battery. So comparing the car battery Ah to a phone battery mAh doesn’t make sense in this situation.

I was referring to this part. The ratio wouldn’t be 8, but more like 2.3, because of the voltage difference:

Car: 417 mAh * 12V = 5 Wh
Phone: 3110 mAh * 3.7V = 11.5 Wh
Ratio: 11.5Wh / 5 Wh = 2.3

Leaving out the voltage changes the ratio quite a bit. Even though you don’t have to fully charge the car, you still have to spend that much energy from the jumpstarter.

You can probably get to 5 if the car only takes 1.5 seconds of max current to start, instead of 3 seconds.

Yes you are correct. I should have taken that into account

there’s also the fact that the jump starter is relying on there being some charge in the car’s battery, just not enough to get the job done. I don’t know if these LiIon packs are capable of starting a car with a completely flat battery. For one, the battery would immediately try to draw as much current as it could from the jump starter (if it’s not irreversibly sulfated, that is.)