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My father in law tells me if I leave the radio on in my car when I shut off the engine this will cause a bit of battery drain when I start the car. He says this will actually reduce the life of my battery.

Is this true?

Is this only true with 'older' cars? If so, before what year?

I'm surprised a modern car doesn't have the electronics to control when the radio kicks in (if it is left in the on position).

  • I have a 1998 Jeep and a 1995 Camaro. In both cars, turning the key to "On" activates all the electronics and accessories; advancing the key to "Start" cuts off just about everything but a few dashboard annunciator lights. Dedicating battery power to cranking is an idea that has been around for quite some time already. – Anthony X Jun 20 '15 at 4:21
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This step is a carryover from days gone by and is not needed in modern vehicles. Batteries from 50 or 60+ years ago were not as powerful or reliable as modern ones. Older batteries had difficulty handling the load of the starter motor alone. Engines cranked longer before starting and were cranking large displacement engines. Any added load from wiper, radio heater fans, etc. while starting may have prevented the engine from starting. Realizing this modern cars disconnect everything that isn't needed to get the engine running during the crank cycle. You can test this yourself by leaving the radio on. You notice the radio plays with the ignition in the "on" position but shuts off in the "start" position.

  • Does this apply to things plugged into the "cigarette lighter" ports? I have a USB adapter that I have plugged in. Should I remove the adapter from the cigarette lighter port before starting the car? – milesmeow Jun 19 '15 at 16:02
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    As @Brian Knoblauch has stated unless your battery is on the edge of failure the USB draws such a small amount of current it won't make a difference. The exception would be if the outlet is "on" with the ignition off. The USB adapter will draw a small amount of current even if not being used. The draw is insignificant if the vehicle is driven daily but if it sat idle for a month it may make a difference in starting power available. – mikes Jun 20 '15 at 10:53
  • New car or not, I 100% always turn off my AC/heater components as I want those systems initializing and disengaging under their own control. Moreover, I always engage my parking brake before shifting into park—even on flat ground—because I am convinced letting an automatic rest on the transmission, then shifting out of park, is harder on the components than having it rest on the brake. This has no bearing on the electrical merits of your question so I'm posting it as a comment. However I believe it's relevant to this topic on a macro level. – elrobis Nov 13 '17 at 17:29
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It very much depends on how your radio is wired up. A typical car radio takes two feeds from the battery; switched live and memory live.

Switched live is usually attached to the ignition switch and powers the radio up when the ignition is switched on. Memory live always takes a feed of power from the battery and is used to store things like the current time, radio station presets and the position in the CD you were listening to so it can be resumed.

Some people prefer to have both switched and memory permanently live so that the radio will continue to play music when the keys are removed from the car. Others have them setup so that the radio only plays music when the keys are in the ignition. Indeed certain manufacturers; Kenwood in particular, provide plugs in their looms so that the configuration can quickly be changed.

If your car radio won't play music unless the keys are in the ignition then switching it off when parking will not make any difference to the battery drain. However, if your radio is wired to continue to play when the keys are removed then do switch it off to prevent excessive battery drain.

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It depends. Some (most?) cars shut off the accessories automatically in "start" mode, however you still have a brief drain even then as you flip past "on" to "start".

Does it matter? Most of the time no, it doesn't. It would only matter if your battery was already right on the edge. Reasons your battery might be borderline include old age and extreme temperatures (example, it's all my standard sized battery, even when new, can do to start my high compression car when it's -20F out).

1

Leaving your radio on so that you can hear it working with no key in the ignition is only possible on some cars. It's useful to be able to do this sometimes, but there is indeed a risk you will accidentally leave it on for a long period and eventually flatten the battery so you can't start it. With a healthy battery that would take weeks, but with an old battery it might make a difference in days.

On a car that always cuts the radio off when you remove the key, it's technically true that leaving the radio on during starting will drain some additional current, but it's tiny compared with the starter motor (milliamps instead of amps). It won't make any difference.

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