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There are many OBD-II devices out there that can scan and/or monitor real-time data about your car. Most of these devices are readable via USB or Bluetooth.

Either USB or Bluetooth, both require some amount of power. Will these devices drain the battery when used all the time?

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If the engine isn't running, then they surely will. I have an ELM 327 (Bluetooth) device which plugs into the OBDII port. Even when the vehicle is turned off, the device is active. Before I realized this, I had left it plugged into my car, came out the next day and the battery was dead. Inside the OBDII port, the power lead there is always on and gives voltage straight from the battery. If there's power there, the OBDII device you have hooked to it will draw power, so will drain the battery.

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  • straight from the battery Does this mean it would still drain the battery even if the engine -- including the alternator -- is running? – Abel Melquiades Callejo Jan 2 at 23:21
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    No. Everything in the car is powered "straight from the battery" when the alternator isn't running. The alternator is charging the battery at the same rate as everything else is discharging it, plus some more charge to top up the battery if it isn't already 100% charged. – alephzero Jan 3 at 0:31
  • @alephzero - I was suggesting as opposed to "keyed on". – Pᴀᴜʟsᴛᴇʀ2 Jan 3 at 0:56
  • @alephzero does that mean that if you want to use these devices all the time (engine on), it would be better to have a technician recalculate if the alternator can still carry the device? – Abel Melquiades Callejo Jan 3 at 2:11
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    @AbelMelquiadesCallejo - There's very little load from one of these devices. There's no issues using one of these devices. It would be less draw on the system then charging an iPhone. – Pᴀᴜʟsᴛᴇʀ2 Jan 3 at 2:43

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