Reader asks about jump-starting another vehicle

Reader asks about jump-starting another vehicle

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A small handheld tester is the preferred tool to use to check battery capacity and condition. Photo by James Halderman

Wheels: R.J. writes by email: “On my way out of the mall, a young man asked if I would help him start his truck. So I hooked the cables up and within five minutes he said I needed to rev the engine as the car would start faster if I did that. Still, I thought I would ask you if that made a difference.”

Halderman: He is right. A typical alternator only charges about 20 amps at idle but can charge over 100 amps when the engine is at 2500 RPM. Many vehicles can’t start when the temperatures are low due to several causes including:

• The engine oil is thicker when cold, making the starter require more electrical currents for the battery to crank the engine

• The low temperatures also slow the chemical reaction inside the battery so it unable to produce the amount of current it could at warmer temperatures.

• The capacity of a battery is affected by age so an older battery will not be able to supply what it once was able to when it was new.

To help prevent the need for jump-starting, it is wise to have the battery tested every year or replaced about every three to five years. There are also small batteries that can be used to jump-start another vehicle that are lightweight and can be kept in the trunk if needed.

Have an automotive-related question? Write to Jim at

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