An ailing battery can cause you grief, but a lithium jump pack can soon sort starting problems. Small enough to pop in a glovebox, they can get you on your way without the need for jump leads.
A larger battery size (usually measured in Ah) means generally easier starting and more reserve charging power for phones etc., but adds weight and cost. Some buyers just want emergency starting capability; others demand the versatility of extra sockets and laptop adaptors. Our tests focus on starting ability as we rate the cheaper gadget chargers on sale.
How we tested them
For our starting test, each device had to assist an ailing battery on a 1.6-litre petrol engine, turning it over five times for five seconds, with two minutes in between attempts.