Can You Leave an OBD2 Scanner Plugged In Without Draining the Battery?
This lesson draws on OBDLink’s product and comparison pages, FIXD’s sensor FAQ, Garmin’s dash cam manual and an owner’s figures posted on a Mustang Mach-E forum. How we research explains the method.
Why an adapter can drain a parked car
The OBD2 port can keep its power after you switch the car off. Garmin’s Dash Cam X310 manual describes its Constant Power Cable as an “always-on” 12V adapter that connects to the OBD II port and can power dash cams for a set time after you turn off the vehicle. Anything left in that port can draw from the battery while the car sits.
How much it draws is the whole question. An adapter designed to stay in the port drops into a low-power sleep mode soon after the car shuts down and wakes when it is needed. OBDLink’s own comparison page says a low-power sleep mode is essential for any OBD device sold as safe to leave plugged in.
That draw comes on top of the car’s normal parasitic drain, which will a car battery die sitting all winter explains.
The sleep figures we could find
| Adapter | Awake | Asleep | Who says so |
|---|---|---|---|
| OBDLink MX+ | 62 mA | 2 mA (BatterySaver mode) | OBDLink product page |
| OBDLink MX+, owner post | 37-40 mA | 1-2 mA | Mach-E forum owner, 2023 |
| FIXD sensor | No figure | “Low-energy mode”, no figure | FIXD sensor FAQ |
| BlueDriver | No figure | No figure | None found from BlueDriver |
The owner who posted the MX+ figures says it sleeps after 10 minutes with no Bluetooth or CAN activity and wakes when the car does, for example when a door opens. The same post says it won’t drain the battery as long as no app is actively pulling data, and advises closing the app when you are done.
OBDLink’s comparison page claims BlueDriver draws 19.3 mA asleep, about ten times the MX+. That is a competitor’s figure, and we found no figure from BlueDriver itself to check it against.
What 2 mA vs 62 mA means over a month
Our calculation from OBDLink’s figures: a 30-day month is 720 hours. At 2 mA, a sleeping MX+ uses about 1.44 Ah a month. An adapter that stayed awake at 62 mA would use about 1.5 Ah every day, or roughly 45 Ah a month, three quarters of a 60 Ah battery on its own.
So the gap between an adapter that sleeps and one that never does is about thirtyfold. A sleeping adapter adds little next to the car’s own drain. One that stays awake would use about 21 Ah over two weeks of parking, on top of everything else the car draws.
If your adapter’s maker publishes no sleep figure, you can’t know what it draws while the car is parked. Treat it as one that stays awake.
When to leave it in and when to unplug
Leave it in when the car is driven daily or several times a week and the adapter’s maker publishes a sleep mode. FIXD says its sensor can stay plugged in all the time because it uses very little power when it isn’t talking to the app.
Unplug it when:
- The car will sit for weeks. FIXD’s own advice is to remove its sensor if the car sits for more than 3 weeks. For winter storage, plan how to keep a stored battery charged instead.
- The battery is already weak. Check its resting voltage first with a multimeter test.
- The adapter is a no-name unit with no published sleep current.
- You want to rule it out. If a battery goes flat for no clear reason, the adapter is the easiest thing to remove first.
An adapter that publishes its sleep current
If you want an adapter that lives in the port, pick one whose maker states a sleep current in milliamps, not just the words “sleep mode”. Of the adapters in this lesson, the OBDLink MX+ is the only one with a maker figure we could find: 2 mA in its BatterySaver mode.
OBDLink MX+
A Bluetooth OBD2 adapter with free iOS, Android and Windows apps that also works with third-party apps. OBDLink lists EOBD and JOBD support and a low-power sleep mode.
Key specs
- iOS, Android and Windows apps
- Works with third-party apps
- 1996+ US vehicles; EOBD and JOBD
- 3-year warranty
Paid link
View on Amazon (opens in a new tab)For BlueDriver and FIXD, and what FIXD’s paid Premium adds, see BlueDriver vs FIXD. A handheld scanner on a cable is unplugged after each use, so this question doesn’t arise: Bluetooth vs wired scanners compares the two. A dash cam on constant power raises the same question, usually at a higher draw: see does dash cam parking mode drain your battery.
