Here’s how to do it safely.
Don’t try to boost a damaged battery. Wear eye protection and gloves. Turn off all accessories.
Park the helper vehicle close to, but not touching, the disabled one. Open the hood of both cars.
It’s considered safest to start the helper car after making connections, but most folks leave it on throughout rather than risk a no-start in severe cold.
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Take one booster cable connector in each hand and have your assistant do the same. Hold each end clamp by the insulated portion, don’t let these touch each other and don’t let your body touch either vehicle when making connections.
Connections are made in a circular order. Just remember the following rhyme. (It’s a mnemonic, not poetry.)
‘Red from the dead, to red on the good.
Black from the good, to under the hood.’
Red from the dead. First, connect the red booster cable clamp to the dead battery’s positive (+) lead. On vehicles with a trunk or under-seat battery, or side battery terminals, you may find a bright red plastic cover labelled ‘+’ in the engine compartment. This is provided for you to make the positive booster connection. Flip open the cover to reveal the positive terminal.
To red on the good. Now, have your assistant connect the other red booster cable clamp to the positive lead of the good battery, or positive engine compartment terminal as described above.
Black from the good. While still at the good battery, have your assistant connect the black booster clamp to the negative (-) battery terminal. If not accessible, clamp to any solid, bare metal portion of the engine block.
To under the hood. Finally, connect the other black booster clamp to any bare metal part of the engine block, far away from the battery, on the car being boosted. Never make this last connection at the battery post. You don’t want to risk a spark and possible explosion from hydrogen gas emanating from the battery.
Let the dead battery charge for a few minutes, the longer the better, before attempting to start. This step is particularly important when boosting batteries that have been totally discharged in extremely cold weather.
You can rev the helper car a bit during this time if you wish, then try starting the disabled car. Once started, disconnect the booster leads in the reverse order.
Keep in mind that heavy corrosion on battery terminals may cause a no-start, as will extended storage without a trickle charger.
Batteries can lose over half of their starting capacity in severe cold. Average battery lifespan is about five years.
If you’re experiencing slow or weak starts, have your car’s electrical system checked before you’re left stranded.
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