Choosing a jump starter: why amps matter more than mAh

A jump starter is the kind of purchase you need once a year and are then very glad you made. The tricky part is that the box mostly quotes numbers that do not matter.

Peak current is the number that counts

To turn an engine over, a lot of current has to flow briefly. For a petrol engine up to 2 litres, 400 to 600 amps is enough. Diesels ask more because compression is higher, so count on 800 amps and more on a large diesel.

The mAh figure only tells you how many starts you get before the pack is empty, not whether it will work. A jump starter with huge capacity but too little peak current leaves you standing in the cold.

What winter does

At minus five degrees a lead acid battery delivers roughly half its capacity, while the oil thickens and the starter motor has to pull harder. That is exactly when it goes wrong. So do not cut it fine: a starter that is just enough on paper for your engine is not enough in January.

What to look for

• Starts per charge. A good unit manages ten to twenty on one charge. Useful when you help someone and still need to get going yourself.
• Reverse polarity protection. Red to plus, black to minus. Swap them without protection and the starter is dead, sometimes more than that. Every serious unit has this, check that it is listed.
• Self discharge. A starter you put in the boot and ignore for six months has to still work then. Set a reminder to top it up twice a year, because draining lithium to zero kills the pack.
• Extra functions. USB outputs and a torch sound like marketing, but a light during a breakdown in the dark is exactly when you need it.

The portable 12000mAh 1600A jump starter sits well above what an average petrol car asks, so you keep margin in frost and on a diesel.

How to use it

Clamp red to the positive terminal first, then black to bare metal on the engine block rather than the battery negative. That avoids sparks right next to the battery, where hydrogen gas can sit. Start, then let the engine run at least twenty minutes so the alternator puts something back.