Jump starter safety

The genuinely dangerous version of jump starting was two cars, a set of thin cables and no protection against a reversed connection. That version is mostly gone. What follows is what is left, in roughly descending order of how likely it is to hurt you.

The electrical risks on this page are small. Standing at the front of a vehicle on the shoulder of a road, at night, in the rain, while cars pass at speed, is not small. Before you open the hood:

• Get the vehicle as far off the roadway as it will go. A flat tire or a dead battery is worth ruining a wheel to move clear of a live lane.
• Hazard lights on, immediately.
• Put out reflective triangles well back from the vehicle — far enough that traffic has time to react, not just to see you.
• Wear something visible. This is the argument for a hi-vis vest living in the trunk kit.
• On a narrow shoulder with fast traffic, consider waiting for assistance rather than working at the roadside at all.

Reverse-polarity protection stops the pack delivering current into a wrong connection. It does not stop the clamps themselves from shorting against each other or against bodywork once the pack has engaged. That short carries serious current and will produce heat and sparks.

• Switch the pack off before connecting or disconnecting.
• Never let the two clamps touch while the pack is on.
• Keep the clamps clear of metal they are not meant to be on — brake lines, fuel lines, the bonnet catch.
• Disconnect the black clamp first, so a dropped red clamp cannot short to the body.

A lead-acid battery vents hydrogen, particularly when it has been charging or has been deeply discharged. Hydrogen is flammable and a spark at the terminal is a genuine ignition source. This is the specific risk behind a rule that sounds arbitrary: connect the final negative clamp to an unpainted engine ground rather than the battery's negative post where the manufacturer directs, so that any small spark happens away from the battery's vents.

Follow your pack's and vehicle's manuals — some specify the battery post, some a chassis point, and the manufacturer's instruction wins over a general rule.

Lithium cells are safe in normal use and unforgiving once physically damaged. Retire a pack rather than using it if:

• The case is swollen, deformed or split.
• It gets hot rather than warm while charging.
• It has been dropped hard, crushed, or submerged.
• It smells of anything, or has ever vented.

Dispose of lithium packs at a battery recycling point — never in household waste, where a crushed cell is a fire risk to someone else. The FAA's guidance on damaged batteries is blunt on the underlying point: damaged or recalled batteries and battery-powered devices likely to spark or generate dangerous heat must not be carried aboard an aircraft.

Crank for three to five seconds, then stop for about 30 seconds. Continuous cranking overheats the starter motor, which is an expensive failure and an avoidable one. Three properly executed attempts is the sensible limit; past that the problem is not a discharged battery. How to use a jump starter has the full sequence.

A pack stored in a hot vehicle loses capacity permanently, and a cell degraded by heat is the cell most likely to misbehave. Keeping the pack out of direct sun is both a longevity measure and a safety one — see storage and heat.