Rick's Free Auto Repair Advice

How to buy a jumper pack

How to shop for a jumper pack

Every big box and auto parts store sells “jumper packs.” Some use a lead-acid battery while others use a lithium battery. Unfortunately, most of the inexpensive lithium packs are just souped-up cell phone power banks masquerading as jumper packs. They may start a dead car, but only if the car’s battery is just partially discharged. Most of the cheap ones won’t start a car or truck with a really dead battery. Worse yet, because they’re built with cheap batteries, they don’t last long. So how can you tell a good jumper pack from a cheap one? Start with the specs.

Jumper starter pack specifications

When you buy a car battery, it has a Cranking Amps (CA) and a Cold Cranking Amp (CCA) rating. CA and CCA tests are standardized within the industry, so you can rely on that rating.  If you find a spec for CA on a jumper pack, that means something.


Cold-cranking amps (CCA) is like CA except that the rating is based on amperage output at 0°F (-17.8°C).


Cranking amps (CA) refers to the number of amps a battery can output at 32°F (0°C) for 30-seconds while still maintaining at least 7.2 volts.


Starting amps is an industry-recognized term that’s defined as the amount of current a battery can sustain at 72°F for 30 seconds/

STARTING ASSIST AMPS for lithium jumper packs

Since lithium jumper packs can’t be used to crank an engine for more than 5 seconds. the other rating terms can’t be used. Instead, STARTING ASSIST AMPS is used and defined as the number of amps the jumper pack can deliver for 5-seconds at 32°F.

What about peak amps?

The peak amp rating on a jump starter is meaningless

There is NO industry standard for “peak amps.” So it’s a meaningless term. In fact, it’s worse than meaningless. Real battery performance tests require a minimum time period for the test and a minimum voltage the battery must maintain throughout the test. In addition, the tests must be conducted at a set temperature.

Yet, peak amps is how most cheap jumper packs are marketed.

“For years, brands have used peak amperage as a way to differentiate its power output. The downside, though, with peak amps is that it provides little in the way of actual starting power and can often mislead consumers.”—Rick Stanfield, NOCO’s Chief Technology Officer

Ironically, NOCO products are only rated in peak amps!

So ignore the peak amp rating. It’s worthless.

Lead-acid jump starter pack discharge rate versus a lithium starter pack

All batteries self-discharge as they sit unused and jumper pack batteries are no different. However, the rate of self-discharge differs between lead-acid batteries and lithium batteries. For example:

• Lead-acid batteries self-discharge at the rate of about 1% per day depending on the storage temperature—faster in hot weather, slower in cold weather. That means they must be recharged about once a month, just like the lead-acid battery in your car. If you leave a lead-acid jumper pack in your car for the entire winter, it’ll most likely be dead by the time you need it. Worse yet, deeply discharged lead-acid batteries form sulfate crystals that can destroy the battery if left in a discharged state long enough. In other words, charge it or lose it.

• Lithium battery starter packs, on the other hand, hold a charge much longer. But there are some caveats to that. Lithium batteries self-discharge in large drops. After about 30-days, most lithium batteries drop to about 70% of their charge. Within six months, they drop to 30% and stay at that level for about a year. If you leave a lithium starter pack in your car for the entire winter without recharging it, chances are you’ll only have about 50% charge.

Some lithium jumper pack companies claim their products hold a charge for over a year. That’s true but highly misleading since the charge level will be 50% or less within 6-months.

Lead-acid versus lithium battery replacement costs

A replacement battery for a beefy lead-acid style jumper pack costs about $60. I’m not aware of any lithium pack supplier that sells replacement lithium batteries, so you’ll have to junk the unit when it quits on you.

Lithium jumper packs can be dangerous

Due to their chemical composition, lithium batteries can be dangerous. You’ve probably read about lithium battery fires in Samsung phones. Lithium batteries develop microscopic structures called “dendrites” that can short out two cells and start a raging fire. In addition to dendrite formation, jumper packs with lithium batteries can start on fire if they:

• overheat while charging
• overheat while discharging
• are connected with reverse polarity
• are left connected to the dead vehicle’s battery after the vehicle starts
• are charged at too high a voltage or at too high an amperage.

UL steps into the picture with standards for jumper packs

Cheap jumper packs have been starting on fire while jumping. That’s why Underwriters Laboratories has developed a test and standard for jumper packs. It’s UL 2743  and you should be looking for the UL rating on the pack you buy.

Here are the safety features your lithium jumper pack must have:

• Reverse Polarity Protection— to protect your car AND the jump starter in case you hook up the cables backward.
• Backfeed Protection— to protect the battery from high voltage and amps once the dead vehicle starts and its alternator starts pumping out power.
• Overheat Protection— to prevent the lithium battery from starting on fire when jumping and when being recharged.
• Over Voltage Protection— to protect the lithium battery from high voltage damage once the dead vehicle starts up
• Under Voltage Protection— to prevent damage to the lithium cells if you try to jump-start when the jumper battery is below minimum voltage.
• Short Circuit Protection— to prevent the lithium battery from starting of fire if you accidentally touch the cable clamps together.
• Proper Connection Verification
• Low Voltage Override

Lithium Jumper packs have limited cranking times for safety reasons

Because high heat can start a lithium battery on fire, you can only crank the engine for a maximum of 6-seconds when using a lithium starter pack. If the engine doesn’t start in 6-seconds, you must stop and let the lithium battery cool for at least three minutes. Then you can try again for another 6-seconds. You can only repeat this process a total of four times. Then you must recharge the jumper pack.

Look for these safety features in a lithium  jumper pack

• UL 2743 Very few jumper packs have passed the new UL 2743 standard. If you’re in the market for a jumper pack, look for the UL 2743 seal on the jumper pack.

  • Short circuit protection — This one is pretty simple. The battery pack shut off if you accidentally touch the positive clamp to the negative clamp.
  • Reverse polarity shutdown — The best battery packs prevent current flow if you’ve attached the cables to the wrong posts. This is a critical feature and you shouldn’t buy a starter pack without it.
  • Over-voltage — Once the engine starts, its alternator will immediately start charging the battery and your jump starter. Alternator output can be as high as 15.5 volts. That voltage isn’t a problem for jumper packs equipped with lead-acid batteries, but it IS a MAJOR problem for battery packs fitted with a Lithium battery. That high voltage can cause a lithium battery to start on fire or even explode. If you’re buying a Lithium battery jumper pack, over-voltage protection is a MUST.
  • Over-current protection — Over current protection is similar to over-voltage protection. It prevents the jumper battery from overheating based on a high amperage output from the car’s alternator. Here’s why that’s important. These starter packs are designed to recharge at home with a low current (amps) power supply. Recharging at a low current prevents damage and ensures long battery life. The power supplies are designed to charge at around 2-5-amps maximum. Once you start the engine, a car’s alternator can easily output 30-60-amps depending on the engine’s RPM. No jump starter can handle that kind of recharging amperage for long without damaging the battery or starting a fire. That’s why many jumper pack companies state that you must remove the pack IMMEDIATELY after the engine starts. The better packs still say that in their instructions, but they also limit the possible damage by shutting down the pack if alternator amps are too high.
  • Over-temperature protection — When you attach a starter pack unit to a dead battery, you’re instantly draining the starter pack battery. Since lead-acid and lithium batteries produce electricity through a chemical reaction, rapid production of power creates heat—lots of it. High heat can permanently damage a lead-acid battery. But high heat in a lithium battery can cause it to explode. The best jumper packs prevent that possibility by automatically shutting off the pack if internal temperatures exceed a set value.
  • Over-charge protection — This feature allows you to leave your jumper pack connected to the recharging power supply without the worry of battery damage due to overcharging. The power supply/charger circuit monitors the battery’s state of charge and shuts off the charger when it’s full.

Finally, check out the jumper pack’s cable lengths

Manufacturers try to keep cable length as short as possible so they can use a thinner wire gauge. To equip the pack with longer cables, they must use a much heavier gauge wire, and copper costs money. Yet, you need enough length to attach the cables and set the jumper pack in a safe location while you leave to start the engine. You don’t want the pack to fall down into the engine compartment.

Here’s an example:

The popular NOCO GB40 ($99 on Amazon) and NOCO GB70 ($199 on Amazon) lithium jumper packs have approximately 8″ cables. Neither unit shows a UL 2743 listing

In contrast, the Clore JNC345 ($233 on Amazon) features 6-gauge 40″ cables and a UL 2743 listing

jumper pack, booster pack, how to jumpstart a car

This JNC660 jumper pack from Clore Automotive is rated at 425 Cold Cranking amps

Look at the jumper starter storage specs

If you think you can leave your jumper pack in the trunk when it’s -30F and expect it to work, think again. Lead-acid and lithium jump starters make power through a chemical reaction. Chemical reactions slow dramatically in cold weather. In most cases, you’ll see that lithium jump starters can’t be stored at temps below freezing. What does that mean for you? It means you’ll have to take it inside when you’re at home or work, OR you’ll have to warm it up before you use it to jump another vehicle. This is one area where lead-acid jumper packs perform better than lithium jump starters.

Lead-acid versus Lithium starter pack — which is best

Lead-acid jumper packs have been the industry standard. They generally pack more CA than any lithium battery. The faster self-discharge rate is the biggest downside. Bottom line, you’ll get more power out of a lead-acid jumper pack than any lithium pack. But you’ll have to recharge it more often to keep it full—as often as once a month. On the flip side, you can buy a replacement lead-acid battery for your jump pack when it wears out.

Lithium battery jump starters are smaller and lighter. They do self-discharge, but at a slower lower rate. However, just like lead-acid batteries, they output less when cold. Lithium packs are more expensive and you generally can’t replace the battery when it wears out.

Compare these Lithium jumper packs


Noco gb40

Noco GB40

($99 on Amazon).
1,000 peak amps. No listing for starting amps
No UL 2743 listing
Approximately 8″ cable length


Noco GB70

Noco GB70

($199 on Amazon)
2,000 peak amps. No listing for starting amps
No UL 2743 listing
Approximately 8″ cable length

Clore JNC345

Clore automotive JCN345 lithium jump starter

Clore automotive JCN345 lithium jump starter. 550 Start Assist Amps. 40″ cables

($233 on Amazon)
550 Starting Assist Amps (no peak amp listing)
6-gauge 40″ cables
UL 2743 listing

©, 2015 Rick Muscoplat


Posted on by Rick Muscoplat

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