What is a normal parasitic battery drain, and how do you find one?

Every modern car draws some current with the key out. Varta calls these loads "silent consumers": alarm systems, clocks and keyless entry stay in standby and slowly discharge the battery even though "the power consumption is only small" (Varta). That is normal. A parasitic drain becomes a problem when something keeps drawing far more than it should after the car has gone to sleep.

What is a normal parasitic draw?

Carmakers publish a limit for each model in their service information. The clearest public example is from Subaru's dealer bulletins:

Subaru adds that "vehicles with more advanced electronics may have a slightly higher but still acceptable reading." Treat 70 mA as a reference point, not a universal rule, and look up your own car's figure.

To see why tens of milliamps matter, see how long a car can sit before the battery dies. At 70 mA, a 60 Ah battery loses about 1.7 Ah a day; at 300 mA it loses about 7 Ah a day (our arithmetic).

First, rule out the easy causes

Subaru's 2025 service update says a draw test should "ONLY be performed after all other common causes for a dead battery have been ruled out":

• Test the battery. A weak battery can look like a drain. Subaru says weak batteries "mimic parasitic draw."
• Test the charging system. If the alternator isn't recharging the battery, no amount of drain hunting will help. See the voltage chart for normal running voltages.
• Check the obvious. A dome light left on, a door not fully latched, a key fob left in the car, or a charger left in a power socket.
• Think about how the car is used. Subaru notes that a car used only for short, infrequent trips "may not be driven long enough to fully recharge the battery."
• Ask whether it has happened more than once. In Subaru's words, "a one-time dead battery does not indicate a parasitic draw." If it has happened more than once, work through the four usual reasons a battery keeps dying before hunting for a drain.

Why you must wait for the car to fall asleep

Subaru calls skipping this wait "a frequent and critical failure" in draw testing. After you switch off and lock up, the body control module, telematics and infotainment keep running self-tests and network traffic. Subaru says that power-down "takes a minimum of 20 minutes on most models", and its telematics procedure tells technicians to wait 25–55 minutes. A reading taken before then looks like a drain but isn't.

Things that keep a car awake during the test:

• The key fob nearby. Subaru says to keep it at least 1.5 m (5 ft) away, and 4.6 m (15 ft) in its telematics procedure.
• A door, hood or trunk switch that thinks something is open. Close the latches.
• Weak cell signal. A telematics module in an underground garage can stay awake "drawing up to 140 mA for 14 days."
• Reconnecting or pulling fuses. Either can wake modules and restart the clock.

How to measure parasitic draw

Safety first. Wear eye protection and keep sparks and flames away: batteries give off explosive hydrogen gas (East Penn). Remove metal jewelry. Some cars also need care before you touch the battery: Toyota Canada says that on models with a battery state sensor on the negative terminal it is "highly recommended not to disconnect the battery terminals under any circumstances" (Toyota Canada). If your manual says that, use the fuse voltage-drop method below instead, or leave the job to a shop.

Method 1: ammeter in series. This is the service-manual method:

1. Prepare the car: lights off, doors, hood and trunk latched, fob well away, aftermarket devices, including any OBD2 adapter left in the port, unplugged.
2. Set a digital multimeter to its 10 A (or 20 A) DC range, with the leads in the amps sockets.
3. Connect the meter in series between the negative battery post and the negative cable, so all current flows through it.
4. Wait for the car to sleep (see above), then read the current.
5. Don't start the car, or switch on heavy loads, with the meter in series. All of that current would pass through the meter, far beyond its 10 A or 20 A rating.

Method 2: voltage drop across fuses. Subaru describes this as a less intrusive option that doesn't break the battery connection or wake any modules. Set the meter to millivolts and probe the test points on each fuse. A reading clearly above the 0.1–0.2 mV an idle fuse shows means current is flowing through it. Subaru's example: a 10 A fuse carrying 175 mA shows about 1.5 mV, against 0.1–0.2 mV with no current. Fuse-drop charts convert the reading to milliamps for each fuse size.

How to trace the drain to a circuit

Subaru's isolation steps, in short:

1. With the ammeter connected and the car asleep, pull one fuse at a time, starting with the under-hood fuse box, and record the reading after each.
2. Don't put fuses back in as you go. Reinserting one can wake a module and spoil the test.
3. When the reading drops sharply (Subaru's example: from 300 mA to 40 mA), you've found the circuit. Drains can be cumulative, so review every reading.
4. Use the wiring diagram to list everything on that circuit, and test those parts individually: a faulty module, a shorted wire or a stuck relay.

If the draw stays high and no single fuse brings it down, Subaru says the car's CAN data network may still be awake. Common causes are a fob inside the car, a faulty door, hood or trunk switch, or a module that keeps sending messages.

Drains that only happen sometimes

Some drains hide from the test. GM's bulletin PIT5745D covers dead batteries on many 2018–2022 GM models that happened only after a night outside below 4 °C (40 °F). A seat memory module could wake the vehicle's low-speed data bus in the cold, and the fault "is not easily duplicated in the service bay." GM's OnStar bulletin notes the module sometimes wakes on purpose to upload data, which is why it asks technicians to watch a high reading for at least 10 minutes before blaming it.

A one-off measurement in a warm garage can miss both. Cold also leaves less starting power to absorb a drain; see why car batteries die in cold weather. Logging the battery's voltage over several nights shows whether the drain happens at all, and when.