I first heard about these a few weeks ago. Since, I've been looking into the pros and cons.
The principle is a vendor chains together 6 2.7V multi-hundred farad supercapacitors, a DC-DC boost circuit, and some battery cables, to make a jump starter pack.
The supercapacitors have very low internal resistance so they can discharge at very high current, enough to easily turn a starter motor.
The more cleaver thing is if you have a "dead" battery you use some of what is still in the dead battery to charge the supercapacitors. Seems they'll charge up at a 10A rate for a couple of minutes, get up to about 15V, then you switch from charge to jump mode and start the car.
A simple one, claiming 300A: ($99 refurbished)
A more advanced one, claiming 700A, having a meter that tells you the internal impedance of your car battery (which should be of interest to JHZR2): ($120 new)
A variation of this style, having an internal Li-Ion battery, for the case where the car battery is really dead as a doornail, the capacitors can charge from the internal battery: ($157.29 new)
Schumacher markets a DSR-108 and a DSR-109, still within the consumer budget.
Here's an OzCharge, 500A, on Amazon, for $245.99
Then, for heavy/fleet use, Chicago Pneumatic markets two models, starting at $1500 (guess)
Once you know about these you can search for videos of people using them.
Seems the pros are:
- Not carrying around Li-Ion batteries in the trunk (no vent with flame issues)
- A better idea, for the frequent user. Very high discharge rates are hard on Li-Ion batteries.
Seems the cons are:
- Higher cost
- I see shelf life of capacitors are 10 years, so I don't know how long the jump pack will really last.
- Linear voltage discharge of caps vs. flat voltage discharge of Li-Ion batteries mean you have to buy bigger capacitors, so enough voltage will be there to continue to turn over engine.
- If you only need a few jumps in 5 years, the Li-Ion jump packs are probably good enough, as long as you don't cheap-out too much
- Heavier and much lower energy density than Li-Ion batteries.
An unknown is you're supposed to balance-charge the capacitors, same as Li-Ion batteries, when they're hooked in series. Maybe that doesn't matter much to a very occasional-use jump pack. It seems to matter to a frequent-use supercapacitor bank that's used for other things, like for a stop-start circuit, or regenerative braking.