Could a person keep the supercapacitors charged with 18650 batteries, then use the capacitors when needing to jump start the car?

Why add in the caps? There are many Li-Ion jump starters on the market that work very well.

 

Could a person keep the supercapacitors charged with 18650 batteries, then use the capacitors when needing to jump start the car?

As mentioned, now you're just re-inventing the wheel, but not a very good one. Could you use lithium batteries to keep capacitors charged? Sure, for a little while, the caps will suck down the batteries too over time. But if you're already putting lithium batteries in the mix, why not just get an off the shelf jump start pack as mentioned above. They work fantastic and you could not make something yourself as good as the commercial units for less than what they cost.

 

I have seen the ads for those quite small "jump start" packs, and the very first thing that is obvious is the VERY THIN LEADS to those clips. How are you going to deliver 100 amps thru what looks like #16 wire??? Only until it heats up and the insulation burns off. Just look at all of the other stuff those vendors offer and know that it is all a waste of money..
For a back-up battery, use an actual battery box like is used for some boats. Acid proof and with bolt-down attachment points. Then mount the battery in the trunk. Use a 40 anp fuse close to the ungrounded battery terminal, and about #6 or #8 wire to connect it to the solenoid that ties it to the normal battery, The charging scheme can be a suitable diode right off the alternator.
To activate the backup battery system, use a N.O. pushbutton rated for the contactor current. SO they will need to hold the button pressed for two or 3 minutes. THEN TRY STARTING.

OR, just replace the battery on a yearly basis, and remember to not leave the lights on. ADD a lights on warning chime.
Of course, I my older grand caravan (2017) the headlights switch off by an automatic scheme a few minutes after the engine is switched off. Also the parking lights. Really, that is STUPID, because the parking lights are to warn others about a car parked in the dark. In many rural areas it really does get really dark at night.

 

It's always around this time when the weather turns cold that people start finding dead batteries. Sometimes it is from leaving the lights on; other times it's from a slipping fan belt. I've seen that numerous times. Start the car all day every time. But run the car at night, shut it off - try to start it and the battery is not there. It's a slipping fan belt. The alternator doesn't spin fully and it slips, thus not charging the battery. It may produce enough current to run the lights, heater, fan, radio, but it's not charging the battery. If you're having battery issues, check the fan belt too! If you can turn the alternator without turning the motor then it's not charging the battery.

WARNING! DO NOT GRAB THE ALTERNATOR WHILE THE ENGINE IS RUNNING ! ! ! Shut the engine off then put a wrench on the pulley and turn it. If it turns and the motor does not then the belt is slipping!

Heed that warning please. I do not want to be responsible for someone grabbing an alternator pulley while the engine is running and have them lose fingers because of it.

Been driving since I was 15. That's 52 years. Have seen loose fan belts many times. I even replaced a battery thinking it was bad when in reality it was a slipping fan belt. The new battery ? ? ? Yeah, it was dead the first time I drove the car at night. Stupid fan belt. Super easy fix and I didn't need a battery at all. So in the 52 years I've been driving I've rarely had a dead battery. Mostly in the fog when I forgot to turn the lights off. My own fault!

 

I have seen the ads for those quite small "jump start" packs, and the very first thing that is obvious is the VERY THIN LEADS to those clips. How are you going to deliver 100 amps thru what looks like #16 wire??? Only until it heats up and the insulation burns off. Just look at all of the other stuff those vendors offer and know that it is all a waste of money..
For a back-up battery, use an actual battery box like is used for some boats. Acid proof and with bolt-down attachment points. Then mount the battery in the trunk. Use a 40 anp fuse close to the ungrounded battery terminal, and about #6 or #8 wire to connect it to the solenoid that ties it to the normal battery, The charging scheme can be a suitable diode right off the alternator.
To activate the backup battery system, use a N.O. pushbutton rated for the contactor current. SO they will need to hold the button pressed for two or 3 minutes. THEN TRY STARTING.

OR, just replace the battery on a yearly basis, and remember to not leave the lights on. ADD a lights on warning chime.
Of course, I my older grand caravan (2017) the headlights switch off by an automatic scheme a few minutes after the engine is switched off. Also the parking lights. Really, that is STUPID, because the parking lights are to warn others about a car parked in the dark. In many rural areas it really does get really dark at night.

They work quite well. I have this small one, $100 on Amazon. I've used it to start my 2.8L diesel truck when I left something on that drained the battery completely dead. It will also start my small 50HP boat all by itself. The cables are actually pretty beefy, more copper than I was expecting. This guy on YouTube does some really good reviews of several models. They really do work.

 

OK, the device Mr.S has must be quite different from the ones in the pictures in the ads that I have seen. And I am quite impressed that it actually worked.
I try to always carry an extra set of jumper cables, one longer and heavier set for my use, and a lesser set to leave with those who I come across needing a jump start. Failed jumper cables are often left behind and usually simple to fix, so it is not costing me money to do this. It is a simple way to recycle and at the same time give a helping hand.

 

Hi.
Once, someone discarded one of these. I cracked opened to see the guts and found 30Volts worth of rechargeable AA cells * (20 in series) in it. So plugging such pack into the cigarette lighter applied 30V for a couple of minutes to the barely cranking battery probably enough to give it some 'vitamin' to restore its state to a crankable energy. They were advertised a few years ago. And should be reports about these gadgets.

* Unusual 'Viatek' brand; labeled "not for consumer, only for propietary application" or alike. They did not perform when used somewhere else but did not know their age/condition before I got them...
Seems to be what the OP is after.

...with a toggle switch or some other means without having to pop the hood...

No switch ; but plugging into the cigar outlet.

----> https://www.asseenontvlive.com/product/mighty-jump/

 

Consider what the application of 30 volts DC would do to every inadequately protected bit of electronics in an average vehicle!! Quite a bit of destruction, certainly. Consider the damage that was done from the tow trucks giving folks 24 volt jump starts a few years back.
The second battery in the trunk scheme would be much more reliable and certainly safer to use. (details in post #23)
AND consider that in the past at least one US made car had the battery installed under the back seat.

 

Consider what the application of 30 volts DC would do to every inadequately protected bit of electronics in an average vehicle!! Quite a bit of destruction, certainly. Consider the damage that was done from the tow trucks giving folks 24 volt jump starts a few years back.
The second battery in the trunk scheme would be much more reliable and certainly safer to use. (details in post #23)
AND consider that in the past at least one US made car had the battery installed under the back seat.

I suspect the design, unwisely, is banking on AA batteries not being able to supply enough current to over voltage a system with a dead car battery attached. But as you mentioned, if the connection to the car battery is broken, or if the battery is brick dead (super high impedance) then there is a chance that the electrical system could see more than 12V, if something didn't gobble up the current first.

 

My computer must be spying on me because this showed up in my YouTube stream today, but this is great info for someone who has never used one of these portable lithium jumpers before (hint: the title is click bait, they're not failing, people don't read the insructions):

 

I am thinking that the reason these are being promoted is because folks have gotten wise to the dollar cables being sold for $25, and so the market vanished. And what a simple way to dispose of all those batteries that do not pass the quality inspection.

 

I am thinking that the reason these are being promoted is because folks have gotten wise to the dollar cables being sold for $25, and so the market vanished. And what a simple way to dispose of all those batteries that do not pass the quality inspection.

If you are referring to the Lithium jump starters, the reason they are being promoted Is because the good ones are very functional and replace heavy SLA versions, providing better performance. There is junk out there, but there are also many good devices that work very well indeed.

The video below, from Project Farm, is the latest of the several he's made over the years. Keep in mind that he reads the manufacturer's claims without comment because he is going to verify them—or not—later in the video.

 

@ "Y", It seems that all of what I see advertised are the junk items. Your experiences differ! That is quite believable. Certainly that is reasonable. And perfectly OK.

 

@ "Y", It seems that all of what I see advertised are the junk items. Your experiences differ! That is quite believable. Certainly that is reasonable. And perfectly OK.

I find that these days, for the most part I avoid advertisements and just assume they're all junk. When it's time to buy something, go digging to get real information. The Project Farm channel on YouTube is actually a really good reference, for all sorts of things. Watch some of his videos, he does some pretty good testing.