Best portable jump starters for a flat battery

A portable jump starter is a battery pack with clamps. It gives a flat car battery enough of a push to start the engine, without needing another car or someone to help. We chose these from published reviews and manufacturer specs; we haven't tested them ourselves yet.

Our picks

Best compact (US)

NOCO Boost Plus GB40

Approximate price: $100–$125

• 1,000A peak
• Petrol up to 6.0L, diesel up to 3.0L
• Spark-proof, reverse polarity protection
• USB power bank and LED flashlight

GearLab named it the best compact pick: it started a 4-liter V6 with a fully drained battery three times in a row in near-freezing weather. It is also rated IP65 against dust and water. GearLab notes it is not meant for big engines, and it measured less usable capacity than NOCO claims.

See it at no.co ↗

Best for most cars (UK)

Ring UltraBoost 1500

Approximate price: £99–£110

• 1,500A peak, 350A starting
• Petrol up to 6.0L, diesel up to 3.0L
• Boost button for completely flat batteries
• USB-C fast charging, storage case included

What Car? gave it five stars and its "best for performance" award, with 20 starts in its test on one charge. Testers liked its quality feel and clear battery display. Ring rates it to work from -20°C to 55°C (-4°F to 131°F).

See it at ringautomotive.com ↗

Best value (UK)

Sealey RoadStart RS800

Approximate price: £75–£100

• 800A peak, 400A cranking
• Petrol up to 6L, diesel up to 4L
• Force-start function
• Waterproof to IP66

What Car? rated it four stars and its best-value pick, with 19 starts in its test from a small pack. The testers found its charge-level lights too optimistic, so recharge it regularly rather than trusting the display.

See it at rapidonline.com ↗

What to look for

• Engine-size rating, not peak amps. GearLab points out there is no industry standard for measuring peak amps, so a "2,000A" pack is not always stronger than a "1,000A" one. Makers also rate packs by engine size, which is a better guide. Choose for the largest engine you will use it on.
• Diesel needs more. What Car? notes that diesel engines need more power than petrol engines of a similar size. Check the diesel rating separately.
• Big trucks and SUVs. For full-size pickups and large SUVs, GearLab suggests a larger, higher-quality lithium pack or a sealed lead-acid unit.
• Safety features. Look for reverse-polarity protection (it will not send power if the clamps are on the wrong way round) and spark-proof clamps. A force-start or boost button helps with a completely flat battery, but GearLab warns it can bypass those safety checks, so follow the manual carefully.
• Clamps and cables. Short leads can make it hard to reach the battery in some engine bays. Several testers criticized short or stiff clamps.
• Extras. Most packs double as a USB power bank and flashlight. Some add an air pump for tires (tyres), which is handy but makes the unit bigger.

Lithium safety and storage

Lithium packs are small and hold their charge well, but they need some care.

• Avoid heat. Battery Tender notes that car interiors can pass 140°F (60°C) in summer, and heat wears lithium batteries faster. One maker's manual warns that storing its pack inside a car in hot weather may make the battery overheat, rupture or ignite. In a heat wave, bring it indoors.
• Cold reduces power. Clore Automotive says a pack stored in the cold delivers less starting power. Ideal storage is mild room temperature, about 50–70°F (10–20°C).
• Keep it charged. Recharge after each use. If you rarely use it, top it up every three months or so, and monthly if it sits in a car in freezing weather.
• Check it for damage. Stop using a pack with a cracked case or damaged cables or clamps, and replace one that no longer starts a car it used to start easily.

For the step-by-step method, see how to jump start a car. If your car clicks but will not start even after a jump, the problem may not be the battery: see car clicks but won't start.