Introduction

My car battery died recently, and I thought I’d try out an Energizer car jump-starter. This was a new product to me – I had not realized that such compact lithium ion jump starters were available! It reduces the need to find someone else’s car to do a jump-start.

In brief, it is a unit that looks like a USB power bank (it can be used as such) but with a rubber covered flap that has a connector to allow it to connect via a supplied adapter cable to a car battery too. It will supply the hundreds of amps of current needed briefly to start a car. My car is heavy, it has a 3 litre engine, and so I was not very confident the small Energizer unit could work. The battery was in poor condition, plus I had not driven the car for a long period (more than a month) and even the electric door-locks would not open automatically. Long story short, it did start the vehicle!

The photo here shows the relative size of the unit compared to a small 9V battery.

According to the specs the unit has 44.4Wh of capacity, and can supply a peak current of 500A at 12V, and a starting current of 300A. The unit comes with a USB charging cable and a cigarette lighter to USB adapter which I didn’t try out. I just charged the unit using my mains USB mobile phone charger.

The supplied red and black clips for the car battery are connected to a separate housing that plugs into the main unit (there is a connector under the rubber flap for it). An LED light indicates status.

Charging It

The Energizer Car Jump Starter is a high capacity device; it will take an extremely long time to charge, unless you use one of the latest USB chargers, that support high voltages (also known as Quick Charge, or Adaptive Fast Charging). The Energizer marketing material states it will take ‘up to 3 hours to charge’ but that is only with one of these more advanced chargers. A basic 5V-only charger will take much longer!

No USB mains charger is supplied, but a USB cable is, as well as a very small car cigarette lighter USB charger. I could not open it (without breaking it) to examine it, but the charger states 5V 2.1A, so it would take many hours of use to fully charge the Jump Starter from empty.

Using It

Using it is extremely easy, no power buttons need pressing. Just connect the red and black clips to the car battery, and the other end is plugged into the connector beneath the rubber cover. The LED stays lit green if the connection is ok. Then, just start the car! Once it is started, the clips can be disconnected.

Here is the connector beneath the rubber cover:

As mentioned earlier, the main unit can also be used as a USB power bank to charge mobile phones and so on.

Double-pressing the front panel button turns on the torch feature!

However, it’s not a very good torch. The beam spread is huge (almost 180 degrees) and it is not very bright either. Even your mobile phone camera torch feature will be much brighter. It’s a real shame, I would have loved for the torch to be brighter so that it became a genuinely useful feature.

Battery status can be observed through four pin-*** sized holes – very hard to see off-angle.

Adapter Cable

The car battery adapter cable consists of a couple of really nice alligator clips (with lots of surface area and sharp teeth), 8AWG 200 degree C rated cable, and an adapter connector box with some circuitry.

The adapter box connector has three terminals, but only two are used – the center one is not connected inside the adapter box.

The construction of it all looks really nice.

The alligator clips have the wires terminated extremely close to the jaws, to minimise resistance presumably. Interestingly, the red cable actually contains two separate wires. One is thinner, and goes to the opposite jaw on the alligator clip, so that the circuitry can know if the jaws are touching each other, and to measure the battery voltage. This is part of the protection functionality.

In the photo below, you can see the thinner red wire in the alligator clip.

Mini Teardown

I was curious to see what level of quality it was built to, so I took apart a portion of it – just the car adapter portion. I didn’t do the rest, because the main unit is sealed and I didn’t want to destroy it.

There are lots of MOSFETs! Only half are visible in the photo below; the remainder are on the other side of the PCB.

The actual MOSFETs used are all the same, they are all HOOYI Semiconductor HY4004 N-channel devices, rated at 138A at 100 deg C.

The underside reveals the remainder MOSFETs, and also the separated red wire that goes to the alligator clip is also visible here.

I didn’t trace out much of the circuit, but it is clear that the MOSFETs are being used in the following arrangement:

N-channel devices are being used because they are available with extremely low on resistance, compared to P-channel MOSFETs. There is a huge desire to minimize power loss in the circuitry and the cable and alligator clips, so that as much energy is quickly transferred into the car as possible, during the vehicle start. The MOSFETs are in two groups, connected so that the car battery can be isolated, with no current flowing through the MOSFET internal diodes when the MOSFETs are off. It is highly likely there is additional protection circuitry in the adapter box, or in the main power bank unit, in case of a faulty MOSFET.

The circuitry is fairly complex, and presumably it tests for the battery voltage being within a certain range, before switching on the MOSFETs. The 16-pin device is likely a microcontroller but I didn't recognise it.

How Does it Work?

I think it’s behaviour is complex; I didn’t understand it entirely, but it is clear that the device will not supply power to the car unless the alligator clips see a certain condition. Probing there reveals a small voltage (a couple of volts) is applied every 1.4 seconds, for 0.8 seconds. The green light on the adapter box flashes along with it. I simulated a car battery by connecting the alligator clips to an adjustable power source, with a 2.2k ohm resistor in series, and monitored the voltage with a multimeter. Once the voltage was raised to about 4.2V, the device switched on, and then 12.4V was available, and the green LED is lit constantly. This voltage remained even if the 4.2V was removed, but I have not tested to see if this is permanent, or if it reverts back to its original behaviour eventually. I suspect it times out after a while, if that voltage is removed. If the voltage is not removed, it seems to continue to supply 12V - perhaps indefinitely, but I only tested the presence of 12V for a period of 10 minutes.

I was also curious, could the device be used as a general purpose 12V DC power source? Given the above information, it was clear that an initial voltage would be needed to be seen, but the current is near-zero, so that could be feasible. Once the 12.4V is available, it is possible to draw large amounts of current; I put a 1.5A load on it, and it worked just fine (the voltage dropped over the space of ten minutes to 12.3V. Interestingly, the green LED briefly (for less than a second) switches off every four minutes. I’ve no idea why. It didn’t seem to interrupt the 12V output, but I didn’t check for any glitch with an oscilloscope. In summary, it is likely usable for (say) connecting to a 12V lamp, provided a starting voltage source is available, such as a 6V battery, and current draw is very low. A battery and a couple of diodes (in case one fails) would work. It may still have other timeouts or unusual behaviour, since it isn’t designed for this purpose. Still, it’s nice to know it could drive high power lamps. I may test with a 12V LED spotlight sometime. In theory it should be able to drive a 6W LED spotlight for many hours.

Summary

The Energizer Car Jump Starter is an interesting device. I’m still not convinced it is a good idea to use small Lithium Ion cells for starting a car, but it did work. I was happy to see that there is protection circuitry inside the device, for preventing damage; it will not apply power to the car unless it is connected correctly to the vehicle, and it sees a small positive voltage. After starting a car with the device, although it may work for a second jump start, you ought to charge it at the nearest convenient time.

The core functionality works well, but the torch feature is near-useless. As a USB power bank (to charge a mobile phone) it is adequate; I do not believe it supports Quick Charge in that scenario, and likely only supports the basic 5V fast charge outputs (also known as Dedicated Charging Port functionality). I didn’t test this though. Still, to provide some juice to charge and power a phone in an emergency, it would still be really great.

I wish the device came with a rugged case or box, because I'd be worried it could get knocked around in a car, which cannot be healthy for a device with so much energy stored.

For the hackers, it should be possible to adapt the device to function as a 12V supply for (say) powerful LED spotlights.

Thanks for reading!