Leaving the adapter plugged in: what it costs
Unplugging it every time is a nuisance. But an adapter left in the socket can flatten the battery over a week of standing.
On most cars the OBD-II socket is live all the time — even with the key out. So an adapter left in it keeps drawing current around the clock.
How much it draws
A good adapter goes to sleep once the phone disconnects and takes a few milliamps — less than the car itself spends on the alarm and module memory. A cheap clone does not sleep at all and pulls 30–50 mA continuously.
That difference decides everything. Fifty milliamps is more than an amp-hour a day. Over a week of standing such an adapter can flatten the battery to the point where the starter will not turn — especially in winter, especially on a battery over three years old.
What else it costs
- Noise on the bus. A clone that never sleeps keeps listening and sometimes answers requests meant for others. On some cars that produces random module faults that do not actually exist.
- Flat battery in frost. A cold battery has less capacity anyway; an extra consumer finishes it off.
- Mechanical damage. The socket is under the dashboard and the adapter is easy to kick. Breaking the socket costs more than three adapters.
How to check yours
- Leave the car overnight with the adapter plugged in.
- In the morning check the voltage right after switching the ignition on, before starting. A healthy battery shows about 12.4–12.7 V.
- Repeat the same night without the adapter and compare.
A couple of tenths of a volt is normal. Half a volt or more means the adapter is not sleeping.
What to do
- Unplug it — the most reliable and free option.
- Use a switch. Some adapters have a button; for the rest there are OBD-II extension leads with a switch.
- Leave it in, if the adapter is a good one and the car is driven daily: even fifty milliamps overnight are harmless; the trouble starts with long standing.
