With the colder, darker days starting to roll in, your car’s battery could be feeling the strain. Advertisement - Article continues belowRather than calling the breakdown services or begging for a jump start from a passer-by, booster batteries pack a punch that belies their compact size. Even the smallest can start the average car, while the biggest will be able to power up larger vehicles multiple times. How we testedAll the packs were fully charged and then attached to a weak 12V battery on a 1.6-litre diesel that was immobilised to prevent it running. We engaged the starter motor for five seconds, five times with a one-minute break between each try.