Portable jump starters: what size do you actually need?

A portable jump starter means you can restart a dead battery without a second car. Picking one is mostly about matching it to your engine and ignoring the biggest number on the box.

Don't shop by peak amps alone

Most listings lead with "peak amps," but there's no single standard for how that number is measured, so it's hard to compare across brands. A more useful number is the engine size the maker says it can start, usually written as something like "up to 6.0L gas / 3.0L diesel."

Match it to your engine

• Pick a unit rated above your engine size, not right at it. A battery that's very dead, or a cold morning, needs extra headroom.
• Diesel engines need much more power than gas engines of the same size. Use the diesel rating if you drive one.
• If you have more than one car, size it for the biggest engine.

Safety features to insist on

• Reverse-polarity protection, so connecting the clamps backwards doesn't cause damage.
• Short-circuit, over-current and over-heat protection.
• Recognized safety certifications listed on the product, especially for the lithium battery.

Temperature and storage

Lithium batteries don't love extreme heat or cold. Check the operating and storage temperature range against your climate, and in very hot areas consider keeping it out of the car when parked in the sun for long periods. Recharge it every few months, even if you haven't used it.

Nice extras

• USB ports to charge your phone while you wait.
• A built-in flashlight for night breakdowns.
• A clear battery-level display, so you know it's charged before you need it.

Always follow the manual's connection order. If a battery is cracked, leaking or swollen, don't jump it; call for help.