What Size Jump Starter Do I Need?

For nearly every gas car on the road, a pack rated by its maker to 6.0L gasoline is enough, and the NOCO Boost GB40 is the cleanest example of one - 1000 peak amps, up to 6.0L gas and 3.0L diesel. Go up a size if your engine is bigger than that rating or if it is a diesel, because the diesel ceiling is always far lower than the gas one. Ignore peak amps entirely: it is the number that does not predict a start.

Almost every shopper arrives at this category with the wrong question. "How many amps do I need?" sounds like the right one, and it is the figure printed largest on every box. It is also the figure with nothing behind it.

Why peak amps is the wrong number

A battery's cold cranking amps rating is defined: the current a 12V battery delivers for thirty seconds at 0F while holding at least 7.2 volts. That is a specified test at a specified temperature, which is why the battery roundup ranks on it.

Peak amps on a lithium jump pack is not that. We pulled the live listing for every pack on this site and not one of them publishes a test method, a duration or a temperature behind its peak figure. There is no standard, so the number is not comparable between two brands, and a maker that wants a bigger one can simply print it.

You do not have to take that on trust, because the packs themselves give the game away. Lined up against the one figure makers do stand behind, the peak numbers go in a different direction:

Read the bolded row against the one above it. The Weego publishes more peak amps than the NOCO GB70 and rates a smaller gas engine and a diesel engine less than half the size. And the GOOLOO GT4000S claims double the Weego's peak amps while rating an engine nearly twice as big again. If peak amps measured starting capability, those rows could not look like that.

The one genuinely useful exception is also in that table: the Weego is the only pack we have found that publishes cranking amps - 440 CA against its 2100 peak. Cranking amps is a sustained figure rather than an instantaneous one, and it is roughly a fifth of the headline. That ratio is a good thing to carry around in your head.

The number to size on, and where to find yours

Every pack worth buying publishes a maximum engine displacement in liters, usually two of them - one for gasoline and a much lower one for diesel. That figure is the maker telling you, in writing, what it expects the pack to start. It is the specification to shop on.

Your engine size is on the trunk badge on a lot of cars, in the first page of the owner manual on all of them, and in the vehicle details on your registration in most states. If you know the car as a "2.0 turbo" or a "3.5 V6", you already have it: 2.0L and 3.5L.

Notice how much lower the diesel column runs. That is not conservatism, it is physics: a diesel has no spark, it relies on compression heat to ignite fuel, and the compression ratio is roughly double a gas engine's. The starter has to turn that over fast enough to build ignition temperature, in cold oil. A pack rated 8.0L gas being rated 6.0L diesel is the same pack describing two different jobs. Jump starters for SUVs and trucks goes further into the truck end of this.

How we judged the sizes below

Three criteria, in this order, and nothing else:

1. Does it publish an engine rating at all? A pack that publishes no displacement figure cannot be sized, which is why the Stanley is not a pick here despite a big peak number. "Up to V8" is a cylinder count, and a 6.7L diesel V8 is a different request from a 5.0L gas V8.
2. Is the rating a sensible margin above your engine? Buy the size that covers your car with room, not the size that exactly equals it. Lithium delivers less current cold, and the published ratings do not say at what temperature they were set.
3. Does it publish starts per charge? This is the quiet one that matters. A first attempt that fails is the normal case on a very flat battery, and a pack with 20 starts in it has margin for a second and third try. A pack that publishes nothing there is asking you to guess.

Not a criterion: peak amps. Not a criterion: the number of "safety protections" a listing counts, which is a self-reported tally with no third party involved.

Sizing up is not free, which is why this is a real decision

The obvious move is to buy the biggest pack and stop thinking. There are three costs to that, and all three are on the listings:

• Weight and bulk. The GB40 is 2.4 lb. The GB50 is 3.33 lb. The GB70 is 4.41 lb. A glovebox pack lives in the heated cabin; a trunk pack lives outside, in the coldest part of the vehicle, which is precisely where lithium performs worst.
• Recharge time. 3 hours for the GB40, 5 for the GB50, 6 for the GB70, per NOCO. The bigger pack you drained on a Monday morning is not ready for Monday evening.
• Money. The gap between the small and large ends of one brand's line is most of the price of a second pack - and a pack sized for your engine plus a maintainer prevents more no-starts than a bigger pack rescues.

One adjustment for the temperature you will actually use it at

None of these published ratings name a test temperature, and lithium cells deliver less current cold - which is the awkward fact at the center of this whole category. The practical response is not to buy a bigger pack; it is to keep the pack warm. A pack in the glovebox has spent the night at cabin temperature. The same pack in the trunk has spent it outside.

That is a reason to favor a pack you will actually carry inside the car, which argues for the smallest one that clears your engine rating with margin. Lithium vs lead-acid jump starters has the chemistry, and car batteries in cold weather has what the cold is doing to the battery you are trying to rescue.

The short version

Find your engine displacement. Buy a pack whose published gasoline rating is comfortably above it - 6.0L covers the large majority of cars - and if it is a diesel, shop the diesel column instead, which is always the lower of the two. Prefer a pack that also publishes starts per charge. Then put it in the glovebox, not the trunk, and test the battery so you never need it.