An Emergency Car Battery Starter That Actually Works

This Imazing IM25 unit will probably come to the rescue the next time your car battery dies.

Last week we reported on two small rechargeable batteries that claimed they could be used to jumpstart a car with a dead battery.  One of them utterly failed to do so, and the other one had ambiguous results.

We suggest you should (re)read that review, because it includes a more general discussion on the topic of emergency starting a car with a dead battery.  What follows here is simply another review without all the other discussion that can be found in the earlier article.

So this week we purchased a third unit with a claimed greater ability to deliver sufficient power to turn over a car engine and get it started.  Would it prove to be the best of the three and could we offer it as an unqualified recommendation?

This new unit is the Imazing IM 25 Car Jump Starter, and unlike the other two units, it had a higher 1000A or 1200A peak current rating (it had these differing numbers on different parts of Amazon’s listing, and the box it came in compromised and said “1000 – 1200A”).  The other two units both claimed 800A max.  Like the other two units, it claimed an ability to also provide a steady 400A.

The battery had a claimed capacity of 8,000 mAh, which put it in the middle between last week’s two units (with 6,600 and 10,000 mAh claimed capacities).  It actually tested to accept a charge of up to 7,800 mAh, and to deliver up to 4700 mAh of charge, which were slightly poorer percentages of its claimed 8,000 mAh capacity than the other two batteries.

Size wise, it came in the middle, and the same for weight.  Cost-wise, it was very fairly priced – $56, compared to $30 and $60 of last week’s two units.

And that $56 is nothing compared to the cost and hassle of having to call a road-side service company to come and start your car, or, even worse, being somewhere where you can’t call anyone, utterly totally stuck.  Consider the $56 not only very cheap insurance, but also a massively more convenient and quick solution when you eventually come to need it (which, as you know, you surely will).

The unit also came in the middle for the thickness of its jump-start cables – 0.22″.  The other two were 0.20″ and 0.24″ (these are the outside diameters including insulation, but are indicative of varying wire thicknesses inside).  This Imazing unit also had a slightly shorter cable length, but that is good, because you risk losing a lot of the precious power in longer thin cable runs – it was about 10″ long for the cables.

We liked that the unit had a USB-C input/output as well as two regular USB outputs.  Like the other two, it had a light, and showed its state of charge with four LEDs.

A nice extra feature on its external jumper cables was an LED display that showed the unit’s voltage and certain other conditions.

The IM25 comes in a nice little carry case, and also has a USB cable so you can charge it from any sort of USB power source.

So, after doing a series of conditioning full charges and discharges, we recreated the test condition on our 4.0L Straight-6 car engine (9.6V battery voltage, about as dead as you can get, and lower than the 9.9V I was using last week), and the weather was slightly colder (mid 50s rather than mid 60s).  We had, a couple of hours earlier, turned the engine over before killing the car battery, and so while the engine was cold, it wasn’t utterly totally dead cold.  In other words, similar or slightly more difficult conditions than we had the previous week.

We connected up the Imazing IM25, and heard an authoritative “click!” from somewhere as it clearly turned on a relay, and we saw the voltage display light up with an encouraging 13.3V.  Wow – if it could hold that voltage, and give us a sufficient shot of current, the car would be started in no time at all.

We quickly jumped behind the wheel, and turned the key in the ignition.  With only the slightest of hesitations, the car turned over and sprung into life.

Yes.  Full, complete success.  No ifs, ands, buts or maybes.  The engine turned over positively, and started.

Out of interest, we then topped up the IM25 to see how much of its stored charge it used to start the car.  It took 2300 mAh to recharge back to full, about one-third of a full charge.  So there is enough power in it for a longer slower start, or perhaps to start the car twice, maybe even three times, but we’d not want to risk that third time.  (This also means that as soon as you’ve started your car, urgently start charging up the unit in case the car stalls and you need to use it again shortly thereafter.)

Self-Discharge Rate

The other thing we were interested in was to test its self-discharge rate.  All batteries slowly lose their charge while stored, and we wondered how long the battery would hold its charge.  After one day, it took 138 mAh to top it up.  We then tested to see what the situation was after half a day, and it needed 57 mAh.  Next we tried two days, and this time it took 106mAh.

Clearly, there is some imprecision as to when exactly the battery decides it is full and stops accepting more charge.  It seems there’s the best part of 100 mAh or so of imprecision for when the battery decides it has had enough.

We next did a test for two weeks.  After two weeks of just sitting on the shelf, we first pressed the button on the battery pack to see how it reported its state of charge.  All four LEDs illuminated, saying that it thought it was reasonably full.  We then topped it up, and the unit took 284 mAh of charge.

In round figures, it required 3.6% of a full charge to top up after two weeks.  So that would suggest to us that the unit is probably good for a couple of months or more between needing top up charges.  This is a slightly lower self-discharge rate, in percent of full charge terms, than the myCharge unit we reviewed before.  We’re now doing another longer test and will update again after having done so.

Our suggestion is that you should top the unit up, which you can do in the car itself just by connecting it to any accessory/lighter type power charger.  If you don’t already have one, get one with two or three or more charging outlets, and reasonably high power output – ie 2A or more – so it will quickly charge phones, tablets, and other devices.  Remember also our recommendation that as soon as you’ve used the unit, you should start topping it up again, which is another reason to want to be able to do so quickly at 2A, rather than slowly at 0.5A.

This four port charger by RAVPower (including QC 3.0 capabilities) seems like a good choice and good price ($16).  We’ve had other RAVPower equipment before and been happy with it.


The Imazing IM 25 Car Jump Starter proved itself able to take a car with a deader-than-dead battery and “jump-start” it.  If you have a car with an engine not substantially larger than the 4.0L Straight-6 in my car, you can probably anticipate that it will work for you, too.

So we give it a qualified recommendation.  Only qualified?  Yes, because, as we said last week, you absolutely utterly must be able to rely on such a device, because for sure, if/when you need it, you truly will need it.  You have to be confident it will work as promised.

So our recommendation would be to buy one, charge and discharge it a couple of times to make sure it is fully optimized, then kill your car battery (easily done by leaving its lights on) until just after the point where it won’t start by itself.  Don’t go too far past that point, or else you’ll start to harm the battery.

Then, try the unit with your car.  If it works, keep it.  If it doesn’t, well, Amazon has a very convenient return policy.  We expect you’ll probably find the unit does indeed work as promised, and if it does, then keep it in your car, and perhaps set a calendar reminder twice a month to give the unit a quick top up charge.

2 thoughts on “An Emergency Car Battery Starter That Actually Works”

  1. How did you discharge the Imazing IM25 car jump starter to fully “optimize” it? Please explain, Thanks, Tom.

    1. Hi, Tom

      It is often recommended that you should fully charge and discharge a new battery a few times so as to “teach it” its capacity. So that’s what I did. 🙂

      To fully discharge it, I connected it to a constant current monitoring load. These are readily available on Amazon and elsewhere, for example :

      I use that unit, and assorted other ones to measure both current/charge in to units and current/charge out. They’re amazingly accurate and essential when testing/reviewing batteries.

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