What to look for in a Jump Starter?

[Nigel]

The newer LiPo ones are definitely the way to go, but bear in mind that they have temperature limits for use and storage, so in really cold weather they may go hoverboard on you, leaving you with a lithium fire and a dead battery too The thing is that after several cycles of a car starting battery getting run down completely, it's ability to hold a charge will be greatly diminished and it will need to be replaced, so avoidance is the best method to use. I'm always near someone with a running car so I just carry jumper cables- they store forever with no maintenance, work at all temperatures, and work with any size engine. In my almost 6 decades here on Earth, I have found that most people will help out if you ask nicely

And here's a trick which I had to try to believe- you can use a cordless tool battery to jump-start a car battery. You must know which terminals to use on the tool battery but otherwise just hook it up, wait a few minutes, then start up and go. Tool battery voltage is irrelevant; any will do this if the tool battery is charged up. One of my winter projects is going to be taking a dead cordless drill and turning it into a jump-starter for my workvan since there's always some charged tool batteries in there

Phil

Actually, the tool battery voltage is very important:

• If it is less than 12 volts then it may charge your tool battery from the almost dead car battery making things worse!
• If it is less than 12.8 volts then it will have very little effect.
• If it is more than 16 volts then there is a risk of blowing up your car electronics, and a new car CPU is very expensive.

Anything between 13 volts and 15 volts should work well, and a 17 volt battery can be used safely if you don't charge it higher than 15 volts, a lithium battery is best stored at 50% charge anyway.

Need to be careful though, if there is nothing limiting the current then a lithium battery can easily melt wires, and it is possible the car battery has failed in a way where your external battery can cause sparks internally to the car battery which can then ignite hydrogen gas that has collected at the top of the battery causing an explosion and spray of battery acid - keep your eyes well away when you make the connections!

 

[SawMaster]

Actually, the tool battery voltage is very important:

• If it is less than 12 volts then it may charge your tool battery from the almost dead car battery making things worse!
• If it is less than 12.8 volts then it will have very little effect.
• If it is more than 16 volts then there is a risk of blowing up your car electronics, and a new car CPU is very expensive.

Anything between 13 volts and 15 volts should work well, and a 17 volt battery can be used safely if you don't charge it higher than 15 volts, a lithium battery is best stored at 50% charge anyway.

Need to be careful though, if there is nothing limiting the current then a lithium battery can easily melt wires, and it is possible the car battery has failed in a way where your external battery can cause sparks internally to the car battery which can then ignite hydrogen gas that has collected at the top of the battery causing an explosion and spray of battery acid - keep your eyes well away when you make the connections!

You are forgetting one very important fact here- the car battery is dead. So in that state, even a 12V tool battery will add to it, and it will pull down the tool battery far enough while you're sitting there waiting with the car not running to eliminate any danger of over-voltage. Lots of real-world experience from other folks in the contractor's world proves this, although it may not work on theoretical armchairs

Phil

 

[Nigel]

You are forgetting one very important fact here- the car battery is dead. So in that state, even a 12V tool battery will add to it, and it will pull down the tool battery far enough while you're sitting there waiting with the car not running to eliminate any danger of over-voltage. Lots of real-world experience from other folks in the contractor's world proves this, although it may not work on theoretical armchairs

Phil

Depends on what is wrong with your car battery, a good car battery that is flat will indeed pull down the voltage of any NiCad or nimh battery. Lithium batteries can supply huge currents so an almost dead car battery may not be able to pull them down, some of those little lithium starter packs claim to be able to produce over 1000A, 10 times the car alternator is quite common, even a good car battery has a limit on how fast it can charge. Yes, there are plenty of people who have done it successfully and will post that they have, the ones that wrote their car off by destroying the computers probably don't post so often!

 

[SawMaster]

Yes, those starter packs behave like that, but LiIon tool batteries do not have that kind of capacity, and they all have current output regulation to prevent problems. The guys my information comes from do talk of our failures, for that is how we learn from and help each other. A few did mention melting the insulation when some wires they used were too small, but that's about it, and most of them do drive newer work trucks, unlike my and my penchant for the old stuff

I do highly respect your knowledge and insight @Nigel but with this one, I have the actual experience and you don't. Let's end this part of the convesation, for I would prefer that those folks lacking our deeper understanding of this would go buy a safe and tested jump-start pack and not dive into waters deeper than they can handle when they have a better option

All my best you you my friend,
Phil

 

[Nigel]

Yes, those starter packs behave like that, but LiIon tool batteries do not have that kind of capacity, and they all have current output regulation to prevent problems. The guys my information comes from do talk of our failures, for that is how we learn from and help each other. A few did mention melting the insulation when some wires they used were too small, but that's about it, and most of them do drive newer work trucks, unlike my and my penchant for the old stuff

I do highly respect your knowledge and insight @Nigel but with this one, I have the actual experience and you don't. Let's end this part of the convesation, for I would prefer that those folks lacking our deeper understanding of this would go buy a safe and tested jump-start pack and not dive into waters deeper than they can handle when they have a better option

All my best you you my friend,
Phil

Good luck with your experiments, and please don't use a lithium 48v lawn mower battery or 54 volt hammer drill battery, even if they do provide protection, it will be too late!

 

[country_hick]

I have a set of jumper cables and a 110 volt car charger. I see no reason to have a portable charger.

Usually someone will offer to give you a boost if they or you have a set of jumper cables. If you are at home the normal battery charger will get you going.

 

[c4rc4m]

A guy from the AA told me, that the battery ones were a waste of time unless you keep them constantly trickle charged. They use them and that's what they do - their vans are fitted with a charging port and the starters are on permanent charge.

Most batteries, Lipo included have a charge range in which they like to be kept. Outside of this range they start to degrade / age must faster and the number of life cycles and / or available power drops. You probably could take it out and top it up periodically as 1 member suggested above with eg a weekly charge.

 

[Rajagra]

A guy from the AA told me, that the battery ones were a waste of time unless you keep them constantly trickle charged.

I've had a couple of lead acid jump starters over the years, they both died before I needed to use them. I would never buy a similar one today.
I did buy a Li-ion jump starter* more recently. I got to use it once (outside my house, but it was still the quickest solution.) It started my van easily. I expected the charge to have dropped a lot after, but according to the LEDs it was still nearly full. And that's despite never having charged it since the initial charge, I had mislaid the mains charger.
The best thing about these is they don't need to be kept at full charge (in fact that's worse for the cells), and the self-discharge is very slow.

* https://www.amazon.co.uk/gp/B00YDZR3ZI

 

[Nigel]

I've had a couple of lead acid jump starters over the years, they both died before I needed to use them. I would never buy a similar one today.
I did buy a Li-ion jump starter* more recently. I got to use it once (outside my house, but it was still the quickest solution.) It started my van easily. I expected the charge to have dropped a lot after, but according to the LEDs it was still nearly full. And that's despite never having charged it since the initial charge, I had mislaid the mains charger.
The best thing about these is they don't need to be kept at full charge (in fact that's worse for the cells), and the self-discharge is very slow.

* https://www.amazon.co.uk/gp/B00YDZR3ZI

A 20000mAh USB power bank contains about 74 Wh (Watt hours) of power.

A car starter motor uses around 1KW of power, but only for about 1 second, once the engine is turning the power requirement drops significantly even if it hasn't started, but most modern cars will start in around a second anyway given good starting current. 1KW for 1 second = 1000 / (60*60) = 0.28 Wh.

So the 20000mAh power bank should be able to start the car 74 / 0.28 = 264 times.

264 times doesn't seem right, what have I overlooked?

Last time I used a battery to start a car, I used a standard USB power bank with a DC-DC step up converter plugged into the USB port to give 13.8v - a good charging voltage for a car battery. The power bank only gives out 10W, but that is enough to charge the car battery, and nearly twice what my proper car battery charger gives out. A 10 minute charge is more than enough to easily start the car. The DC=DC converter needs to have a 10W current limit so that it doesn't overload the power bank and cause it to turn off, ie it needs to be a CV-CC converter, that also means there is no chance of melted cables or damage to the car, almost perfectly safe.

 

[Rajagra]

264 times doesn't seem right, what have I overlooked?

If only reality matched theory, eh?
Whenever I've seen jump starters make an actual claim about how many starts they can provide, it's been single digits. Maybe they're just covering their backs, or maybe it's taking account of longer start times, bad condition vehicle batteries, etc.etc.
And they do lie about the specs. This guy tests how much current they can put out, maybe he has other videos looking into how many starts they can give.

 

[Nigel]

If only reality matched theory, eh?
Whenever I've seen jump starters make an actual claim about how many starts they can provide, it's been single digits. Maybe they're just covering their backs, or maybe it's taking account of longer start times, bad condition vehicle batteries, etc.etc.
And they do lie about the specs. This guy tests how much current they can put out, maybe he has other videos looking into how many starts they can give.

Should probably take into account things like the fuel pump which although less power hungry, will be running for rather longer than the starter motor, then a fair amount of power will go into charging the battery and then get used by things like the alarm system. But it seems that the starter motor calculation is about right, seems likely that you might get close to 100 starts, which does match your experience.

 

[SawMaster]

@Nigel - Dunno what you might have overlooked, but even a fully-charged healthy car battery, which has far more power in it than a jump-pack, will be hard-pressed to do 100 cold starts without charging. Therefore your figures are wrong somewhere. In real life there are other things involved here, such as old and worn starters needing more current, the rise of any batteries internal resistance while being charged or discharged, how dead the starting battery is, the increased friction of an engine when cold versus when warmed up, etc. This is why I say that theories on paper do not work the same in real-world conditions, and it is why I didsdain any theories or calculations until they are proven in practice Every engineering fail has numbers proving it couldn't happen but it did anyway, which is why any safety-related engineering is always done to 150% or more, and in some instances 300%. Good engineers know better than to fully tryust raw or refined numbers and practice proves them right

Real-world results? Jump-packs may go 20 starts at best before they won't have enough juice left to handle a very dead battery, and most go about half that or less. This from used car dealers, junkyard people, the AAA battery techs I've spoken to, and from other people whom I believe from various other places. And watch that AAA guy; his main job is to sell and install batteries. One tried to use a "freshly charged" jump pack on my van to no avail, but when a bystander offered to try with jumper cables, ZOOM! An instant start. That AAA guy bowed his head in shame and slinked away. I called AAA shortly afterward and they credited me the event. 4 cams recording+3 days parked=1 dead battery, and that math you can trust Test it at your own risk

Phil

 

[Rajagra]

Engine starting is a brutal process, I think it defies the usual specs. Just ask anyone who's driven a car with a bad battery. You do everything possible to avoid turning off the engine. A lot of energy is probably wasted by internal resistance of the battery, which is insignificant at other times.

The guy whose video I linked has others about batteryless jump starters! Using supercapacitors. (Also hybrids with supercapacitor plus small battery.) Interesting idea. I think he opens one up to see what's inside. I wonder what spec the supercaps are. Imagine running a Dashcam off those!

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[Nigel]

@Nigel - Dunno what you might have overlooked, but even a fully-charged healthy car battery, which has far more power in it than a jump-pack, will be hard-pressed to do 100 cold starts without charging...

If I charge my empty 20,000mAh USB power pack from my full 80Ah Lead Acid caravan battery, then the lead acid battery will be about half full. The specs on lead acid batteries are quite misleading, they don't contain what you expect.

This small 8000mAh lithium jump starter claims to "start a V8 up to 20X on a single charge!", and I assume that is without assistance from a half dead lead acid battery since it is aimed at starting race "car, truck or powersports vehicles." Most jump start packs are intended to assist the existing battery, rather than do the full job and generally have too much voltage drop to do the job on their own, a poor set of jump leads will have the same problem, while a good set of thick ones will do the job easily.

The guy whose video I linked has others about batteryless jump starters! Using supercapacitors. (Also hybrids with supercapacitor plus small battery.) Interesting idea. I think he opens one up to see what's inside. I wonder what spec the supercaps are. Imagine running a Dashcam off those!

Super capacitors are only 2.7 volts, so you need a string of 6 in series to get a 16.2 volts super capacitor suitable for starting a car. You can replace a car battery with a 83F 16.2 volt super capacitor and it will work fine. The only real problem is that super capacitors that size have a lot of leakage and will self discharge to a useless voltage within 2 or 3 days, so they are not a practical replacement for a car battery.

 

[SawMaster]

The specs are misleading but there's a lot more power in a lead-acid battery than the direct amperage numbers indicate. On this side of the pond, car batteries aren't rated by straight amperage for that reason, as that number is highly misleading. Instead the rating here is "Cold Cranking Amperage" where your 80A rated battery might have well over 900 amps available, but only in short discharge bursts such as what is needed to start a car. Two lead-acid batteries of the same size with the same Amperage rating can have vastly different cold-cranking ratings based on how they are built. Placed under a lower longer load, the lead acid might deliver only 80A in total, but that's not how it's being used. The lithium pack excels at longer low loads which is why it's amperage rating seems astonishingly high. Lead-acid properties are perfect for car starting which is why they are used almost universally for that job, even when their total amperage rating seems too low by the numbers. Lithium can't deal with extremely heavy loads nearly as well which is why it isn't used there.

This difference is where your number-crunching went astray. Amperage does not have time calculated into it, and as that always varies amperage numbers do not work here except in a general way. This is why a 2A 12V tool battery can jump-start a car after a few minutes when a 2A mains charger cannot, even though both are rated at the same 2A. The tool battery can deliver well over 2A under load but the 2A charger cannot. This is why the results vary so wildly even when the number is identical. In it's pure form, Amperage is not the proper number to be using here, and even our Cold Cranking Amperage isn't totally accurate, but it's a lot closer to what happens in the real world.

Phil

 

[Rajagra]

Ideally we would focus on total energy capacity measured in Joules or Watt-hours (1J=1Ws, 1Wh=3600J)
Amp-hours is next best, but you can't directly convert that to energy capacity (using Ah x volts) since the voltage varies, especially under heavy load.
Peak current doesn't tell you much at all about energy capacity. And if voltage drops a lot during peak current that can also mean the peak power output can be lower than expected.

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