An Emergency Battery/Starter for your Car

Murphy’s Law dictates that dead batteries always fail at the worst possible time and place.

We’ve all had a dead battery surprise us in our car.  There’s nothing worse than when you’re running late, you get into your car, attempt to start it, and instead of the engine happily springing into life as commanded, there’s nothing more than a sluggish groan and – even worse – possibly the clacking of the starter motor relay opening and closing.

Hopefully, when such a misfortune befalls us, we’re not far from a “Good Samaritan” with jumper cables and a source of power to start our vehicle.  But Murphy’s Law being what it is, that is far from guaranteed.  Some of us choose to carry our own jumper cables with us so that we only need to find someone willing to let us connect up to their battery, but even that ultimate piece of the puzzle can be at times a non-trivial task depending on where and how you’re parked, where your battery is, and how close to your battery your Good Samaritan can locate his vehicle and his battery.

Calling AAA or any other type of roadside service inevitably seems to involve a wait of an hour or more.  And, one last bit of gratuitous inconvenience – batteries seldom fail on warm sunny days and when we’re parked at interesting places where we can comfortably enjoy ourselves while waiting for help to arrive.

If you decided to go “off the grid” and into some area of beautiful nature, lacking cell phone service and with few other people, your dead battery may be more seriously inconvenient.  What do you do if you can’t start your car, your phone doesn’t work, and there’s no-one else nearby?

Until recently, there’s not been any practical solution to this “First World Problem”.  It seems these days car batteries are only expected to last around 4 – 6 years, and while sometimes you sense a battery that is slowly failing, other times you get caught out and suffer a nasty surprise.

But while car batteries seem, if anything, more fragile and short-lived than they used to be, the last few years have seen astonishing improvements in external battery packs.  These were originally intended as a way to top up one’s cell phone if/when it dies and provide enough emergency charge to finish off a day of use until such time as the phone can get to a regular charging location.  But the Lithium-ion batteries that power such devices are good for much more than just delivering a small trickle of power to recharge a cell phone.  With suitable changes to their controlling electronics and the internal/external wiring, they can deliver astonishingly high rates of current, albeit for short periods of time.  This is part of what has enabled electric vehicles to not only be practical but also to be capable of amazingly high performance and fast acceleration.

It has been claimed that these types of small lightweight external batteries can even be used as emergency batteries to start a car.  That’s a huge leap from charging a portable device to starting a car.   To put it in numbers, charging a phone is probably requiring about a 5W rate of power from the battery.  Starting a car needs somewhere in the 2,500 – 5,000W rate of power.  Yes, 500 – 1000 times greater.

Even if a tiny external battery were capable of that, one also has to wonder for how long it could deliver that amount of power.  For the sake of round numbers, if we consider a battery that stores 5000 mAh of power, that translates to about 25 Watt hours of power, not all of which is necessarily usable.  At a rate of 2500 Watts, that would see the battery totally discharge in 36 seconds (and of course, 18 seconds at the higher end of the range of power rates needed to start a vehicle).

But, while that is short, how long does it usually take to start your car?  A couple of seconds?  If it takes much longer, then you’ve probably got some other developing problem with your engine that is stressing your battery every time you start the vehicle and you should get it diagnosed and resolved.

So, in theory, if it is possible to upscale the power delivery of a typical external battery pack 500 times, then they might work.  For sure, having such a device in the glove compartment of one’s car would give great peace of mind, and would also make a messy complicated task much simpler, both when needing a jump-start oneself and when, in turn, helping out someone else.

So we decided to test a unit to see if it really would do as promised.  After an abject and total failure of the first unit, we chose a second make and model of unit in the hope that the first unit’s inability to get anywhere close to starting our car was not indicative of a total failure of all such units.

The myCharge Adventure Jump Start external battery pack

We liked the look of the myCharge “Adventure Jump Start” unit.  It claimed a 6600 mAh battery capacity, and said that it would start cars, trucks, boats and ATVs.  More specifically, it claimed to be able to deliver 200A of starting current, with a peak rising to 400A.  That seemed like it would be enough to start our 4.0L Straight-6 car (see the section below about how much power is needed), and while it was listed on their website for a pricey $100, we saw it on Amazon for a much happier $30.

The unit measured 5.8″ x 3.1″ x 0.73″ and weighed 11.5 ounces.  This gave it a slightly lower capacity of stored power per cubic inch and per ounce than other units we’ve tested, but this was not surprising because of what we guess to be some heavier duty electronics and connectors inside for when it is being used to start one’s car.

However, it was sufficiently larger and heavier as to make it not so well suited to use as a portable travel charger for your electronics while traveling – in such cases, being able to save a quarter pound or more of weight and a few cubic inches of space can make a material difference to the weight and size of your carry-on.  Plus, with phones and tablets and other devices having larger and larger batteries inside them, these days we’d recommend getting an external battery pack with a 10,000 mAh or greater capacity.

But our reason for getting this unit was focused primarily on being able to have it as an emergency starter for our car.  We didn’t care about its capacity – as long as it would start our car when it had a dead battery, nothing much else mattered.

The unit came with a built-in micro-USB connecting cable to make it easy to charge devices, and a USB connecting cable to use to charge up the unit itself, plus a short set of jumper cables for car jump-starting (just over one foot long).  As well as the built-in USB connector, it had another regular USB port to use to connect other cables to other devices.

It also had a built-in flash-light, and showed its state of charge with four LEDs.

Before actually testing it to see if it would start our car, we cycled it a few times to “condition” the battery – fully charging it and then fully discharging it.  We noted, while doing so, that while the unit claimed a 6600 mAh capacity, it only accepted 5900 mAh of charge, and as for power then delivered from the battery, we were unable to get more than 4200 mAh, and on the last of the multiple times, running at a slow discharge rate of 1.05A so as to give it the best possible chance to do well, it only delivered 4050 mAh.

These results, while disappointing, were not shocking.  We’ve not yet encountered a single external battery power pack that truly delivers the capacity it promises, and delivering only 62% of its claimed capacity is sadly fairly normal.

We were surprised, when discharging it, to note the voltage dropped from its nominal 5V.  With a 1A load, it was 4.9V, and with a 2A load it dropped to 4.75V, and at its claimed maximum current output of 2.4A it was down to 4.7V.  We were able to run the unit at 2.6A for a while, too, although it is unlikely any normal device would “ask for” or accept that much current when charging.

The big issue though wasn’t to do with how well it could charge portable electronics.  This was a device that had a single prime mission – restarting our dead car.

So we flattened the car’s battery and confirmed the car battery wouldn’t turn over the engine at all.  Then we connected up the myCharge, and turned the ignition key.  The starter relay clacked and clicked as it went in and out, and the engine made a couple of brave but attempts to turn over and start, but failed ignominiously.  The car wouldn’t start, and didn’t even come close to starting.

To double-check, we completely discharged the myCharge unit and then completely recharged it again, and repeated the test.  This time, it wouldn’t even turn the engine over at all, and so, for a second time, completely failed to start the car.

To triple check, we repeated on a warmer day and after the car had been running earlier in the day, making for a much easier task to achieve.  Still no joy.

This was the point at which we became very thankful we were at home, with a regular battery charger alongside the vehicle, standing by.  But imagine if we’d relied on this unit, without testing it beforehand, and found ourselves stranded somewhere remote, in bad weather, and with no way to call for help.  That’s a potentially life-threatening situation, and we’re astonished that a US company would risk the potential liability and lawsuits that would follow if their unit failed to work as promised and save the day.

We asked myCharge about this.  Rather than express shock and horror and rush to suggest the unit must be faulty and send us a replacement, they gave us several sentences of vague references to things like storage temperature (normal), the age of the unit (brand new), and a suggestion to refer to their very brief and unhelpful Quick Service Guide for “how to operate this device for effective use in the event of an emergency”.  The quick service guide basically says “connect the unit to your car battery, start the car, disconnect the unit”.

It surely should go without saying that an emergency car battery/starter unit must work, reliably, every time, in an emergency.  And if the unit claims to be capable of starting cars and trucks, (and without any qualifiers or limitations) we’d expect our 4.0L Straight-6 engined car to be included as something it could start.

To be fair, we’re reasonably sure it would work for very small-engined cars and motor-bikes and quad-bikes.  But if you’ve something approaching our car’s six cylinder displacement or larger, or a diesel-powered vehicle, clearly it is not sufficiently powerful.

We also tested the unit’s self-discharge rate.  We fully charged it, then after two weeks, checked to see its state and how much extra charge it needed to get back to fully charged.  When pressing the button on the unit, all four LEDs lit up, suggesting the unit was at reasonably close to fully charged.  It then accepted 255 mAh of charge, which is about 4.4% of a full charge, and a self-discharge rate comparable to that of the Imazing charger we review here.  At that type of self-discharge rate, we guess it can probably still work, in the sense of how it does work, two months after it was last charged.  We’re continuing to test and will update this again in the future.

To make one further comment as politely as we can, let’s just say we are puzzled as to how this unit has amassed a 4.8 star rating on Amazon with all six reviewers happily reporting great success and no problems starting cars with it.  As we commented a couple of weeks ago, Amazon combines a reluctance to publish negative reviews with an unseemly eagerness to accept any type of positive review.

The Type S Jump Starter Portable Power Bank

Our experience with the myCharge unit rather soured us of the concept of using a small battery pack to start our car.  But then, earlier this week at Costco, we saw another similar unit, the Type S Jump Starter, which claimed a larger 10,000 mAh capacity, a similar 400A peak current, and a $60 price.  Knowing that if it also failed, we could happily return it to Costco for a full credit, and having underlying confidence in just about everything Costco sells, we decided to try again and hope for a better result.

The Type S unit (manufacturer site here) is larger and heavier than the myCharge unit, but as noted when discussing the myCharge unit, size and weight doesn’t bother us for a unit that will be in the car, not in our carry-on.  It measures 6.6″ x 3.0″ x 1.1″, and weighs 15.5 ounces.  It comes with a nice 33″ long heavy-duty USB connector cable for recharging purposes, and a set of short (1 ft) jumper cables for starting a vehicle.  It had a really helpful extra item – a cigarette lighter charger so you could recharge/top up your unit in the car itself, and a nice carry case in which to keep everything together.  A good and thoughtful complete set of accessories.  It has a one year warranty, and they’ll double it to two years if you register your unit with them (and probably expose yourself to occasional promotional emailings).

We really like that the unit both has a well written detailed user guide, and also prints the essential instructions on the unit and jumper cables themselves, directly.  So if you lose the manual (and the first thing we do, with anything we buy, is always download a copy of the manual PDF and save it on our computer) you can still figure out what to do and how to do it from the printed instructions on the unit itself.

It boldly claims that it will jump-start any car, truck, boat or motorcycle, and so we set about preparing to test this.

The unit accepted a bit more than 10,000 mAh of charge (being the first unit we’ve ever tested that actually took in more power than its rated capacity), and when discharged, delivered 6550 mAh of power.  Again, this is much less than claimed, but is proportionally similar to most other units tested, and indeed, slightly better (as a percentage of rated power) than the myCharge unit.  We noted with approval that when providing power, it maintained 5.2V when giving 1A of power and 5.1V when proving 2A, and only dropped to 5.0V at 2.4A of current.

After a couple of conditioning cycles, we connected it to the car and turned the starter key….

We interrupt the article at this fateful point to ask for your help and support.  The unit cost us $60 (plus $6 sales tax) to buy.  We’ve spent several days working with it, and of course, this is a repeat of the testing for the earlier myCharge unit too.  And then – ooops – I killed my car battery by discharging it too deeply.

Does this type of detailed review and purchasing advice have some value to you?  Would you like to know if you can indeed rely on this unit, or if, after buying one, you might have to subsequently make use of Costco’s famous return policy after suffering a failure to start in an emergency situation?

If this information does have value, and if you’d like to read the next section, please consider becoming a Travel Insider Supporter.  Simply click the link and choose whatever level of support you feel fair and appropriate, and you’ll then instantly get access to the extra section of this report, plus extra sections of other reports and entire special reports for supporters only.  Thank you.

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And now, on with the balance of the article.  There’s still plenty more.

How Much Power Does Your Vehicle Truly Need to Start?

That’s an easy question to ask and a hard one to answer.  A big variable is temperature.  If your engine is cold, then the oil is thicker, and the friction is greater, and it will require more power to be turned over and started than if it is warm and everything is freely moving.

(Note that temperature also comes into play with the emergency battery as well.  It should be nicely warm; neither too hot nor too cold.  If it is very cold and doesn’t immediately work, you might want to warm it up in your hands for 15 minutes or longer and then try again.)

Bigger engines also need more power to start than smaller ones, because there is greater weight and greater friction/surface contact inside a larger engine.  More cylinders, more swept area, greater weight of moving parts, and so on.

One common rule of thumb is to allow for an Amp of power for every cubic inch of engine displacement, and two Amps if the engine is a diesel.  Most engines these days are measured in liters rather than cubic inches, so to help you do the conversion, one liter is the same as 61 cubic inches.  If you’ve a 3 liter engine, then this rule of thumb advocates 183 Amps of current available for the starter motor.

To put it another way, a device that can deliver 250 CCA is probably good to start a vehicle with an engine size up to about 250 cubic inches or 4.1 liters.

Car battery manufacturers have tended to make their batteries with ever greater abilities to deliver high rates of power to start vehicles – what are often termed CCA – cold cranking Amps, and with numbers well above what a car actually needs, as a marketing tool.  “Buy this battery, it is only $25 more, and it offers you another 100 CCA”.  And car manufacturers are quite happy to overspecify the needs of their vehicle in order to “play safe” and not end up on the wrong end of a liability suit if a motorist was stranded somewhere with an inadequate battery based on a too-low manufacturer’s specification.

We’re not sure how old the “one CCA per cubic inch” rule of thumb is, and more modern cars seem to have less friction and so are probably a bit less demanding in their CCA needs.  But, better safe than sorry, and so for most gas (petrol) powered engines, you’d probably be wise to keep that rule of thumb in mind.

Some Types of Dead Batteries Are a Problem

Usually, a “dead” battery is simply one that is not holding a charge well, or which has slowly discharged, perhaps due to a light being left on or whatever.  But sometimes part of this failure is a “shorted cell” – one of the six separate cells in the battery has developed a short-circuit, and if you connect a new power source across the battery terminals, the shorted cell might cause the battery to “suck up” a lot of the current being provided by the new power source.

That’s not a big problem if you’ve an abundant supply of emergency current, but if you’re using a device like one of these units that are probably working at close to maximum capacity (if at all!) losing 20A or more of their power to the dead battery might make all the difference between starting your car or not.

If you can’t start the car “normally” with the external battery, you might want to try disconnecting your car battery’s positive (red) terminal and connecting the emergency charger to the black negative terminal and to the disconnected red positive wire, not the battery.  That way all its power goes to the car, and none is drained off by the faulty battery.

However, most of the units we’ve been testing and researching seem to have a “smart” connector that won’t activate its unit if it senses the battery voltage to be “too low” (or entirely missing).

If you can indeed charge with the car battery off-line (and need to do this), we’d probably recommend you reconnect the car battery before driving off.  It provides a voltage stabilizing and filtering float and without that, you risk having “dirty” power with voltage spikes and transients that could harm some of the electronics that all modern cars have.  Sure, the alternator provides all the power the car needs, but the battery is an important part of the power management system.

A Cautionary Note

If you’re doing this testing, be careful how deeply you discharge your battery.  As we found out to our cost, it is possible to kill a lead-acid battery by discharging it too deeply.  All the more reason to rely on our testing, and not risk your own battery by duplicating our tests.

Making Sure Your Emergency Battery is Fully Charged

The other thing to keep in mind is that all batteries have a slow but steady “self discharge” rate.  They flatten themselves just by sitting in storage, even if not used.

Different batteries have different discharge rates.  We’d suggest checking after a month to see what degree of charge your emergency battery still has and if it can still start the car.  If it can start the car, recharge it and check again after two months.  If it is still good after two months, we’d probably suggest that you top it up every month; but if it works after one month but not after two, that sounds more like a top up every two weeks just to be on absolutely the safe side.


We know that our vehicle batteries will die from time to time, and we also know, from past experience, that such failures can sometimes be very inconvenient.

New battery technology has now enabled a new solution to this problem – a small unit, typically weighing about a pound and about half the size of a pound of butter, which in theory is promised to be able to provide enough power to start your car when the car battery is dead.

But, based on our experience, these claims, while sometimes true, are sometimes absolutely not true.  If you choose to buy one of these units, we strongly urge you to test it immediately after buying it.  Fully charge and discharge it a couple of times to make sure the battery has been “conditioned”, and then discharge the battery in your car sufficiently that the car won’t start unassisted, and see if the unit will start your car.

If it does, great.  All you need to do now is keep it in the car and occasionally top up its charge.  Maybe once every couple of years, you should also check that it still is capable of restarting your car, because storing a battery fully charged for an extended time is not an optimum condition and causes it to age more quickly.

And then you can enjoy peace of mind, and also get the full absolute life out of your vehicle battery, running it until it completely dies, rather than replacing it when it starts to get tired.

One More Thing

We’ve now reviewed a third unit.  You can read that review here.

4 thoughts on “An Emergency Battery/Starter for your Car”

  1. David, ask what the AAA tech uses. I know they carry them here locally and they have jumped me twice with no problems. Both times my battery would not turn the engine over.

    1. Hi

      As I vaguely understand it, they have a portable unit on wheels that basically is a second lead-acid battery. I’ve seen those for sale at Costco too.

      My desire is not to have to wait an hour or longer for them to come and do their magic (as seems to be the case in WA state), and not to stick a second full size car battery in the trunk, but rather to have something simple, light, and inexpensive.

  2. Nope, it was a handheld unit both times. The first time I was disappointed because I did not think that little thing would work. And he put it on and bingo, it started right up. – Mike M

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