Peak amps is the biggest number on every jump starter box and the one worth trusting least. It is not a third-party tested figure the way a car battery’s cold cranking amps rating is. On a lithium pack, the specs that predict whether your car turns over on a February morning are watt-hours and the engine displacement rating.
Peak amps is not a tested rating on a lithium pack
Cold cranking amps has a standard behind it. Power-Sonic, which manufactures sealed lead-acid batteries, notes that SAE J537 requires a 12 V battery to deliver its rated CCA for 30 seconds at -18C without its voltage dropping below 7.2 V.
That standard covers automotive lead-acid cells. Power-Sonic states that lithium batteries have no CCA standard at all, so cranking amps and watt-hours matter instead.
A 1,000 A peak claim is therefore the maker’s own instantaneous figure, measured however it chose. Two packs printing the same number are not making the same promise.
Watt-hours decide how many attempts you get
NOCO’s spec table for the GB40 lists 1,000 A peak alongside a 24 watt-hour lithium-ion cell, rated for gas engines up to 6.0 litres and diesels up to 3.0 litres, at 2.45 lb.
The rest of the range follows suit. The GBX45 is 1,250 A and 31 Wh, the GB50 is 1,500 A and 35 Wh, and the GB70 is 2,000 A and 56 Wh.
Peak amps doubles across that lineup while stored energy barely more than doubles. Energy is what drains after three or four failed cranks, the same logic behind reading what a watt hour number actually buys you on a power station.
NOCO also lists the GB40 recharging in three hours at 2.1 amps over USB. Treat that like any recharge time printed on a box, a best case under conditions the maker picked.
What actually fails first in the cold
Power-Sonic describes the mechanism: cold thickens engine fluids so the starter meets more resistance, while the electrolyte in a lead-acid battery grows more viscous, raising impedance and pulling down current and voltage.
Your dead battery is harder to help and your engine harder to spin at once. Clamp contact matters more too, since a loose bite on a frozen terminal wastes current the pack could deliver.
Notice what the spec sheet omits. The GB40 listing gives peak amps, watt-hours, engine ratings, IP65 sealing and a one-year warranty, and no cold-temperature operating range at all.
Match the engine first, then accept the trade-off
Displacement is the spec to buy against, the only figure the maker ties to a specific job.
The GB40 covers most four-cylinder cars but stops at a 3.0 litre diesel, and carries only that one-year warranty. The GBX45 is the lightest of the four at 2.16 lb, though its IP65 rating applies only with the ports closed.
The GB70 reaches 8.0 litre gas and 6.0 litre diesel, and you pay in bulk at 4.18 lb, close to double the GB40. Capability is never free, the same way an optimistic range claim quietly hides a heavier battery.
Can you fly with a battery jump starter?
The FAA caps spare lithium-ion batteries at 100 watt-hours in carry-on, citing 49 CFR 175.10(a)(18). Between 101 and 160 Wh needs airline approval and is limited to two spares. All four NOCO packs sit between 24 and 56 Wh, under the line. Confirm with your airline.
Does a higher peak amp number mean a stronger jump starter?
Not reliably. No CCA standard exists for lithium cells, so peak amps is self-reported and not comparable between brands. Compare watt-hours and the stated engine displacement limit instead.
How many starts does one charge give you?
NOCO advertises up to 20 jump starts for the GB40 on a full charge. That is a maker’s figure, and cold plus repeated failed cranks cuts it. A 56 Wh pack buys margin rather than raw amps.
Tricia Nelson writes about technology, science, and how-to guides for 3Zebras. She has a knack for turning dense, technical topics into clear explanations, covering everything from iPhone and Android troubleshooting to the science stories behind the headlines.