These things are a little miracle. A jumpstarter you hold in one hand that I used to start the 6.7 liter diesel in my truck. How can this be? The truck has two lead acid batteries the size of toaster ovens. I left the damned lights on at a race track. Went to start it and click, click… …nothing. I took the little compressor/battery bank/flashlight/jumper from my motorcycle and connected it. Once it gave me a green light I hit the switch. Boom, it turned over faster than it does on a fully charged monster dual battery setup. I did NOT expect that to work. How is that possible?
The simple answer–the massive C rating and low internal resistance of NMC Lithium batteries. C rating means how fast you can charge it, so maybe we should call it a D rating since we care about discharging more than charging, but the number is the same, and that’s not how it’s described technically. So the C rating of a typical NMC battery used in a jumpstarter pack is 15-30C and in some cases 100C for short periods. NMC batteries are generally used since they store a lot of power in a small size and handle high burst rate discharges. So if pack is rated at 5000mah (milliamp-hours, so 5Ah) a 30C discharge rate means it can supply 150Amps, if it can do 100C in a burst that’s 500amps. Not for long of course. Even at 30C discharge a 5Ah battery cranking a motorcycle at 150 amps will only last about 2 minutes. In reality a start lasting 10 seconds at 150 amps (150 A × 10 s = 1,500 amp-seconds ÷ 3,600 = about 0.42 Ah (417 mAh)) chews up about 1/10th of the capacity–you get ten tries…maybe. But usually one is enough.
Here’s how they work–it’s more complicated than you think.
The box you plug into the compressor of main unit or the bulge in the middle of the cable contains a relay or MOSFET switch that only connects the pack when everything checks out and you turn the switch to start. It provides: –Reverse-polarity protection, so the clamps don’t spark or blow the guts out of your jumper box if you get them backwards.
–Short-circuit and overcurrent protection.
–Back-feed protection, so the vehicle’s alternator can’t charge the pack.
–Battery detection: it won’t switch on until it senses a battery at the clamps. A “boost” or override button bypasses this when the vehicle’s battery is almost completely dead. Once that little box is happy it turns the light green–but that’s about all that happens. The battery is in parallel with your dead or semi-dead battery, but you don’t want it to charge the battery, you want to start the vehicle. So when you turn the switch to start, the box senses the current draw (actually, the voltage drop) and triggers the jumper either closing a relay or firing some hefty MOSFETs. Bam–you have 150 or more amps going though some fairly wimpy wires and clamps. If that current flowed for more than a few seconds stuff would melt, but it doesn’t, so all good–apparently. I’m sure this isn’t violating the laws of physics, but it’s pushing at the edge of the box.
I put a clamp-on hall effect ammeter that can measure and lock inrush current on my little jumper box. On my F350 diesel truck I measured 468 amps with a device the manufacturer claimed was good for 150, so I’d say Your mileage will vary. Started my F350 just fine. I don’t think I should do it often, but it got me going. Honestly, it it fried after that one shot I would have thought it was worthwhile, but it didn’t. It’s fine, I’ve been using it two years since then. It survived about ten tests with the ammeter since I didn’t believe the numbers for the first 9 times, It’s started my Husky 701 a dozen times, filled dozens of tires, and it started my diesel tractor a half dozen or more times (I need to replace that battery).
Some manufacturers claim “1000A / 2000A peak” , that’s got to be mostly marketing but still…. ..I’m not gonna say they’re liars. A motorcycle starter draws only about 80–150 A, so even a small pack is plenty. The bike only draws what it needs, so a bigger pack won’t hurt it.
There are Supercapacitor jump starters. They’re a different design. I’ve never used one but they sound very cool. I’ve always wanted to add a super capacitor to a bike battery so if I left the lights on and killed it, I could use the tiny residual charge to pump a capacitor for a minute or two, then release that energy into the starter. 1000 amps for a second or two. That would get my cold-blooded Husky going. Probably snap it right to attention. I finally replaced the crappy AGM battery it had with Lithium, so I suspect it won’t need help anymore. It likes the 13.3 V of the Lithium–downright perky.