So I came across this thing and i don’t get it.
https://www.amazon.com/autowit-Portable-Batteryless-Supercap-Ultra-Safe/dp/B07F8MJW8W
I keep a small Jumpstarter on my bike that looks like its about the same size, but mine definately contains a battery.
Nothing I’ve learned as an electrician has prepared me to figure out whats going on here unless this thing is really just a small battery (disguised by huge amounts of bullshit). So what exactly is going on here assuming the stat of the art didn’t wizz by me while I wasnt looking?
It’s batteryless in the sense that it doesn’t come with a battery. You have two choices, either you charge the caps up ahead of time from an external power source or you rely on the remaining power in the car battery to charge it (and then it lets it all out at once, much faster than the battery can).
Electroboom has used super capacitors to jump start cars with regular household (A, C etc) batteries by charging the caps with the batteries and then using the caps to start the car.
Don’t forget, once your car battery dips below something like 10 or 11 volts it’s not going to start, but there’s still energy in there. It makes sense if you have to let the caps charge for X minutes and then discharge them for Y seconds. However, I’d feel better just getting a jump pack for about the same price.
I get how its done. What I dont get is how it works.
Don’t take this as fact as I am not an electrician other than 110v smarts and 220v is to be avoided. But a capacitor can download energy at a much faster rate than a battery.
I have something similar as the OP K2500 asked about. It’s saved my butt a few times for my plow truck. And, the one I have can also charge cell phones. Kinda handy when the power is out. Also, such a system is VERY much more handy than pulling out the jumper cables and organizing cars to get the jump done.
I know caps can charge/discharge faster than batteries. I know marx generators can generate higher discharge voltages than the supply voltage. I know this thing looks very small. I think I know capacitors have a significantly lower energy density than batteries. I dont know how to tie any of this together or if any of it means anything as it relates to the product at hand.
There’s not really much more to tie together. The capacitors in the unit are capable of very high discharge rates, so a small unit can drive a starter motor. The “dead” car battery still has energy in it, but the voltage is too low to start the car. The unit here charges with the car battery slowly (over 3 minutes, according to the slides), and then discharges at an appropriate voltage over a few seconds.
The device needs a DC-DC boost converter to increase the 9 V (or whatever the dead battery is at) to the 12-14 V the caps are at. Aside from that, the caps are likely connected directly to the clamps for maximum current.
I dont see its dimentions listed at Amazon. The pictures don’t really show its size either. One of the slides says 600A instant. Trying to math this out from half that at 12v over 2 seconds using online calclators is giving me 100 farads and 1200 coulombs. I doubt a cap(s) that size would be hand held but I’ve never seen one. I also doubt I’m mathing this out correctly. I dont doubt for a minute that I’m missing something both crucial as well as obvious.
Yeah, I think that 20F is more like it.
But, that still can deliver 600A for a half a second or so, and then taper off.
Probably enough if the car would normally start on the first crank.
So I just found out these exist.
Amazon.com.
I guess my next mission is to try to cobble one of these together for less than 50$ without injuring myself. Thank yall for your replies!
You can do better than that. It’s hard to tell the size of the device, but 5 of these would fit in a handheld case and have 80 F capacity @ 13.5 V.
The internal resistance is a bit high but I suspect the 700 A refers to a short-circuit case anyway. The above caps would actually do 1200 A in a short circuit. Probably the linked device uses cheaper caps, though.
What happens if the car doesn’t start right away?
You charge it up and try again.
I’ve got this one and used it twice on other folks vehicles. Took about 1 minute to charge up and then we started the truck.
I have two other LiIon battery jump starters and this is nice because I don’t have to remember to keep it charged.
I would like to know what’s actually in these guys. Might be a cheap source of supercapacitors, though only if it’s well over 100 F. A few tens of farads wouldn’t be worth it. Anyone willing to do a teardown 
?
I buy 10F, 2.7v ultracaps for $.50 ea. So, 100F at 13.5v would be 250 caps = $125, so no real savings.
As Joey_P mentioned upthread, Electroboom did this:
He used six of these capacitors, 400F (and $12.21) each. Said it was good for a single start on his 4-cyl car.
The problem with caps is that the terminal voltage plummets as they discharge, unlike a battery.
The energy in caps is 1/2 C.(V squared) so you need a converter to maintain cranking voltage. Trouble is that the current from the caps needs to rise as the voltage falls so things get hot quickly.
If the caps are just making up for a weak battery then maybe the above would work, but super caps have about a tenth of the energy density of a lead acid battery so it’ll be touch and go I think.