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"Peak amps" on the box is a marketing number. This sizes a pack by what your engine actually needs to turn over.
Estimates for sizing a portable pack, not a replacement for your manufacturer's battery spec. Cranking-amp requirements scale with displacement, compression, and oil viscosity; always check the owner's manual for the battery's rated CCA.
Why peak amps is the wrong number
Most jump starters advertise peak amps — an instantaneous surge measured for a fraction of a second, under no meaningful load. It is close to meaningless for starting a car. What matters is sustained cranking amps delivered for several seconds while the starter motor turns a cold engine.
A rough working rule: a gasoline engine needs about 150 to 200 cranking amps per liter of displacement in mild weather. Diesels need roughly 1.7× that, because of much higher compression. Cold multiplies the requirement again, since oil thickens and the battery's own output falls.
Packs typically publish peak amps only. As a conversion, sustained cranking output is often around one third of the advertised peak — which is why a "2000 peak amp" pack can still fail to start a cold diesel.
What else decides whether a pack works
- Cable gauge. Thin clamps and cables throttle current regardless of the battery inside.
- State of charge. A lithium pack left in a trunk for a year may be at half capacity. Recharge twice a year.
- Temperature at the moment of use. A pack stored in a cold car delivers less than its rating.
Shopping for one? See our jump starter guides and data studies for measured prices and picks.