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Peak vs Cranking Amps: Which Jump Starter Number Matters?

By Garage Trials 3 min read Lesson 1 of 2

2 lessons in this series, 6 min in total
  1. Peak vs Cranking Amps: Which Jump Starter Number Matters? 3 min read
  2. How Many Amps Do You Need in a Jump Starter? 3 min read

This lesson draws on Rick Muscoplat’s two jump-pack guides, Battery University’s page on CCA testing, NOCO’s GB40 and GB70 data sheets and Clore’s JNC660 product page. Read how we research.

What each amp rating measures

Jump starter boxes use several different “amp” numbers. They don’t measure the same thing, and only some of them follow a standard test.

Rating What it measures Test conditions Fixed, published conditions?
Peak amps Highest burst of current the maker claims Set by each maker No
Cranking amps (CA) Current held for 30 seconds 32°F (0°C), voltage stays at or above 7.2V Yes
Cold cranking amps (CCA) Current held for 30 seconds 0°F (-18°C), voltage stays above 7.2V Yes (SAE J537)
Starting or starting-assist amps Current delivered to help start an engine The maker’s own test procedure No
Engine-size rating Largest gas and diesel engines the maker supports Set by each maker No

The CA and CCA definitions come from Muscoplat’s guides and Battery University. Under SAE J537, Battery University says, the battery is fully charged and then held at 0°F (-18°C) for 24 hours before the test. Muscoplat says starting-assist amps can be useful too, but only when the brand states its test conditions.

Why peak amps don’t compare across brands

There’s no standardized test for peak amps, according to Muscoplat. Each maker sets its own method, so the number can’t tell you how one pack compares with another.

Clore’s own numbers show how far apart two ratings can sit on a single unit. Its product page lists the JNC660 at 1700 peak amps and 425 cranking amps. Divide 1700 by 425 and you get 4: the peak figure is four times the cranking figure.

NOCO lists peak current for the GB40 (1000A) and GB70 (2000A) but no cranking figure, and gives engine-size limits instead. So a 1700A Clore and a 1000A NOCO aren’t on the same scale, and the larger number alone doesn’t show which pack is stronger.

One more mix-up: UL 2743 is a safety standard for portable power packs, according to Muscoplat. A UL 2743 listing tells you about safety, not starting power.

Which number to compare instead

If the brand lists cold cranking amps, start there. CCA is measured at 0°F (-18°C), so it’s the closest match to a hard winter start.

Some lithium packs don’t list CA or CCA at all. Those are car battery ratings, with current held for 30 seconds, and each jump starter brand picks its own ratings. NOCO lists peak current and engine sizes for the GB40 and GB70, and Muscoplat notes that some lithium packs list starting-assist amps.

Whichever rating a brand uses, check that it states the test conditions. Then compare in this order:

  1. CCA, when both packs list it.
  2. CA, when CCA is missing.
  3. The maker’s engine-size rating, checked against your own engine, when neither is listed.
  4. Never peak amps from two different brands.

With that order in mind, check how many amps your engine needs for its size and fuel type.

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