There are two different types of lithium-ion batteries Jump Starter : Lithium Cobalt Oxide (LiCoO2) and Lithium Iron Phosphate(LiFePO4).
Lithium Cobalt Oxide, one of the most common Li-ions, has the chemical symbols LiCoO2 and the abbreviation LCO. And Lithium Iron Phosphate, has the chemical symbols LiFePO4 and the abbreviation LFP.
Which one will you vote for?
For car starting, probably the LiFePO4.
LiFePO4 any day for automotive type applications.
In 4S configuration it has an almost identical voltage as a 12V Lead-acid system, compared to the 11.1V nominal of a 3S LiCoO2. Additionally LiFePO4 is much better suited to high current draws compared to standard LiCoO2, and they are also better at taking abuse from overcharge or overdischarge. The only real drawback is a lower capacity, but for car jump starting that doesn’t really matter, plus LiFePO4 can handle way more charge/discharge cycles.
Why? Does LiCoO2 Battery Jump Starter could not car starting?
No, it probably works fine, but the standard characteristics of LiFePO4 are better suited to car starting than the LiCoO2.
Here, I just did a web search for “LiFePO4 vs LiCoO2” and found this post, which might be interesting to you.
Confession time: I didn’t read the whole thing. I skimmed it a bit and saw it said a few things about the different chemistries that seemed to be right, so I linked it here.
Sorry. :person_facepalming:
My post above gives a rough idea of why LFP would be better for jump starting than LCO. Basically, closer voltage to the target system, higher current, handles abuse better.
So are there choices, with these car starter things? Do they tell you what the chemistry is?
I have one, and I don’t know what chemistry it is… From Amazon…
For 
starters, LiCoO2 is a particular battery chemistry which, to the best of my knowledge, is unsuitable for this purpose unless a quite big battery were to be assembled for this purpose: it is not a high drain chemistry.
Now, with this in mind I'll take the liberty to reformulate the original question from the thread creator: lithium ion or LiFePO4?
A properly sized LiFePO4 battery would do, of course.
Now, what about lithium ion? My answer is that, since we aim to make/operate a jump starter device, I'd say the extra voltage of a lithium ion 4SxP configuration could be of help. This is because the vehicle's battery is certainly going to be in weak/bad shape if it can't start the engine, thus it'll start draining power from the jump starter readily after connection, and having extra voltage can indeed help. Of course, this means the li-ion pack will be pulled down real hard, so I'd pick a nice deal of very competent high drain cells like Sony VTC5As, LG HB6s or thereabouts. With regards to how much in-parallel cells, I'd go no less than 6 to guarantee decent punch. Additionally, a thick as @#$% jumper cable, and the wisdom to avoid abusing the jump starter battery before it overheats and give up if the stuff is not enough, because the cells are likely going to be discharged beyond their maximum continuous discharge rating. 
Lithium ion is the ‘dangerous’ one, though, right?
people are always saying how safe li iron phosphate is, for instance - I assume it is different from that
is this the same stuff that blows up in vapers’ face or in their pants…?
wle
Problem is, almost all of the lithium ion jump packs use a 3S configuration, and 4S Li-Ion is not advisable, the 16.8V full charge could damage automotive electronics which are usually designed with a 12V nominal, 14.4V peak in mind. They may be tolerant to more, but I don’t want to find out the hard way when it could mean a dead car until computers get replaced.
With that, the 4S LFP setup is 12.8V nominal, 14.4V full charge, basically perfect. There is a reason you find drop in LFP replacements for lead acid setups. They can also deliver current much better than any Li-Ion.
Yes, Li-Ion, Li-Po (same thing, just pouch cells), LiCoO2, and LiMnO2 are the dangerous lithium that you generally hear about catching fire.
LFP can catch fire too, but it can handle much more abuse before doing so. The only real downside to LFP is reduced capacity. For this application that isn’t really a problem though.
so car jumper packs are usually ‘’the dangerous kind’’ then…?
and the fact that they ride in cars in summer, when temps can get to 150F and above. i assume that makes things worse too,…?
Yes, usually they are Li-Ion based simply because of cost, and they can put a higher mAh number on it. Being in hot cars certainly doesn’t help things.
I think quality cells from one of the top four manufacturers of 18650 Li-Ions will not burn or explode from being used in the hot environment of a car. But, they certainly will lose some of their capacity a lot more quickly and lose some life expectancy in charge cycles from that kind of abuse.
Yeah, for good cells being in a hot environment by itself isn’t going to cause a catastrophic failure. I doubt many of the jump packs use quality cells though. That said, simply being in a hot car probably won’t cause crappy cells to go off either, but that’s not a risk I personally am willing to take. I would feel much better about doing it to a LiFePO4 of any quality.
i think they are not cylindrical cells, typically they are 3 flat metal case cells, instead of 15-20 18650s…
so maybe there is not so much room for quality and age differentiation…
wle
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How do LiFePo4 cells handle being stored at full charge? Do they hold a charge better then a lead acid battery? I’m just wondering if there’s any advantagy except for size, for choosing LiFePo4 above an extra lead acid battery.
They hold a charge just like a standard lithium ion cell, so if you charge it up and come back a year later it should be good to go. Same things as with lithium ion still apply though, long-term storage at full charge will degrade capacity.
That said, the most notable advantages are size and weight. If looking to build your own jump pack, something like this is a great start.
Naw. Almost all spec automotive equipment, including electronics, is guaranteed to tolerate up to 35V spikes caused by alternator load-dump.
Some tow trucks don’t bother with 12V jumps, but use 24V “hot shot” jumps (2 batteries in series). No slow-cranking, but a hit-hard spin that gets the engine started quickly. Connect it up, start it, pull the plug. In’n’out in 4-5sec, tops.
I wouldn’t recommend it, but if you have a dead battery in the target car, you don’t want that sopping up lots of current from the donor vehicle. So you hit it with twice the voltage and start the car fast. No waiting a minute or so with the usual way of jump-starting.