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Tech Talk: Your Key to Key Off Drain

July 22nd, 2014 by

It’s pretty common for “key-off drain” from accessories to kill a perfectly good battery.

Gale Kimbrough is Mr. Battery.When the car and any onboard devices draw a small flow of current from the battery when the engine’s off, that’s key-off drain.

There already are multiple power-hungry, key-off parasitic devices on vehicles today. And there are even more planned for the future. Picking the right battery matters now more than ever. The wrong battery could mean a dead battery. Here at Interstate Batteries, we have some advice on helping customers choose the right battery for their needs.

Looking for Normal

I often get asked, “What is a normal key-off drain?”

It varies so much between vehicle models.

Let’s say you’ve got two customers, and they both leave their cars at the airport for eight days.

One drives a 2010 Ford Taurus, the Super High Output (SHO) model, with an elaborate, aftermarket alarm system. The other drives a 2011 Chevrolet Malibu, and uses various devices, including an elaborate alarm, an MP3 player, a sound system, etc.

Your battery may power all sorts of devices when the key is off.

Your battery may power all sorts of devices when the key is off.

The Taurus started out from the factory with 45 milliamps, and the Malibu started out with 25 milliamps. By using various devices, however, both drivers have brought each vehicle’s daily key-off drain to 2.4 amperehours after the 24-hour period.

Their cars, though, use different batteries with different Reserve Capacity ratings. That rating shows how many minutes a new battery can deliver 25 Amps until the terminal voltage reaches 10.5 volts. The Taurus uses a group 65 battery, which may have an RC of 130 minutes. The Malibu uses a group 90/T5 battery, which may have an RC of 95 minutes. Both batteries are good, and meet the original equipment specs.

Key off battery drain as explained by Interstate Batteries

Big Drain Over Eight Days

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

Eight days of 2.4 Amp-hours is going to make a big difference, and there is a chance one of the cars won’t start when they get back.

The Malibu battery has a lower RC rating. After draining 2.4 Amp-hours, multiplied by eight days, the Malibu battery only has about 66% of its original charge left. The Taurus battery’s charge has also dropped to 75%. The Malibu driver probably will need a jump-start to get home.

Key-off drain can be a battery killer. The more you understand how batteries deliver power and their ratings, the better you can advise your customers.

After all, your expert advice might save them before trouble starts.

Need help finding the right battery? Visit an Interstate Batteries Dealer near you.

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

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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