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The High Cost of Free Power From Your Car Battery

January 27th, 2014 by


We all like gadgets. Nowadays, most gadgets are portable—and they usually plug into cars for power. As people carry more gizmos more places, their usual solution for power needs is the cigarette lighter. But if they think they’re getting free power, they’re wrong.

Gale Kimbrough, aka Mr. Battery.

Gale Kimbrough, aka Mr. Battery.

A car battery‘s designed to discharge hundreds of Amps in just a few seconds (ie: start a car) and then recharge from the alternator. On the other hand, most devices draw a steady stream of electricity. Let’s look at the math.

What if you turn off the engine and watch a two-hour movie in your car? Assume the DVD player draws 1.5 AC Amps or 16.5 DC Amps.

Car batteries have a reserve capacity (RC) rating, and if your battery’s is 100 minutes, that calculates to about 60 Ampere-hours. The two-hour movie would draw a total of 33 Amp-hours off the battery, discharging it to a 45% state of charge.

What is Reserve Capacity?

Battery Council International defines it as “the number of minutes a new, fully-charged battery at 80°F (27°C) can be discharged at 25 amps and maintain a voltage equal to or higher than 1.75 volts per cell” (i.e., 10.5 volts for a 12-volt battery). This rating represents the time the battery will continue to operate essential accessories in the event of a charging system failure. More battery definitions

Typically, a car battery that low won’t start. Every time a car battery discharges to 50% state of charge, the battery’s life shortens by about 250 engine starts.

Even if a car is left running, “free power” puts a strain on the alternator and battery. Let’s assume a car comes with a 75-Amp alternator and idles at about 650 RPMs.

By playing the DVD, you just added a 16.5 Amp load that the vehicle’s manufacturer didn’t plan on.

The alternator’s operating at about 50% power output due to the idling engine RPMs. At half power, the alternator puts out 37.5 Amps to power the ignition, lighting, air conditioning and/or heater, internal computer and electrical sensors—all needing about 25-30 Amps.

Add in a two-hour movie while the engine’s on idle, and the alternator’s drawing battery power instead of recharging it. Do enough of this “power-sharing,” and the battery may run out of power sooner rather than later.

Is your battery built to withstand the extra power needs you place on it? Visit an Interstate Batteries dealer or Interstate All Battery Center and ask about our new MT7 AGM battery, which is made for today’s power-hungry and accessories-laden vehicles.

By Gale Kimbrough (11 Posts)

As Manager of Interstate Batteries’ Engineering and Technical Services Group, Gale Kimbrough oversees technical training, product engineering, testing and validating electrical/electronic components.

Posted in Automotive, Mr. Battery, Technology, Tips


About The Outrageously Dependable Blog

Interstate Batteries® sells more than 16,000 kinds of batteries—from AA alkalines and automotive batteries to critical power solutions, and everything in between. Combine professional battery services, recycling programs and the largest battery distribution network in North America, and you’ll find Interstate has EVERY BATTERY FOR EVERY NEED®. Learn more about Interstate Batteries or shop online!

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