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I left my 2014 Hyundai Elantra running with the heat on for about 80 minutes (having intended only 30 or so), with the intention of helping dry out the backseat after an incident with a rainstorm and a partially open rear window.

When I returned to the car, it has died and now I can't turn it over or start it - I had half a tank of gas and I can't imagine that I used up all the gas. Is there any way that it drained down the battery (even while running) to the point where it now can't start?

Not sure whether to simply be asking co-workers for a jump start or to be looking for a ride to a gas station; I don't think it's giving me an accurate gas reading since it won't turn over or start. Key fob won't open trunk either so it does seem like the battery is drained all the way down.

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    Did you have any additional load plugged in (hair dryer, fan, light bulbs)? If so, it's possible you were drawing more load than your alternator could compensate for at idle. – TMN Jul 6 '16 at 16:26
  • I like TMN's line of thinking... heater aside, could you tell us exactly what was running with the car idling? – Zaid Jul 6 '16 at 16:48
  • Good question - nothing extra, unless the radio was still trying to play music from my Bluetooth phone (which had been removed from the car at that point). Nothing external, just had the heat on with the fans at half-strength . . . – blaster Jul 6 '16 at 17:46
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A dead battery or a weak/dead alternator are the usual suspects with these symptoms. Here's what you should do:

  1. Get yourself a $15 multimeter (WalMart has them).
  2. Set it to "VDC20" and put the leads on your battery terminals.
  3. A healthy battery will read 12.4V or higher, anything less than 11.8V will be insufficient to start the car.
  4. If you are reading less than 5-6V, your battery is completely dead and needs to be replaced. Anything above 5-6V but less than 12V, your battery is just drained and needs a boost.
  5. After a boost or new battery, get the car started with everything else off (lights, heater, rear defrost, radio) and measure again. A healthy alternator will get your battery reading about 14V.
  6. Turn on all your lights, heater full-blast, rear defrost and radio and measure again. A healthy alternator will get your battery reading about 13V.
  7. If either of your tests with the car on are reading lower than these numbers, you have a bad or weak alternator and should have it replaced.
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    "A healthy alternator will get your battery reading about __V."--this wording is slightly misleading. To my understanding, when the car is running, it's the alternator (not the battery) that you're reading, no matter where you read it. (This higher voltage is what charges the battery.) If you intend to have the user read the alternator by measuring the voltage across the battery, I'd suggest "A healthy alternator will read about __ V, measured at the battery." – Mathieu K. Jul 7 '16 at 3:02

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