The car sat for two or three days, and when you sit down to start it the engine barely turns over or doesn't fire at all. You bought the battery just last year, you had the alternator checked, but the same story keeps repeating every week. In nine out of ten cases something in the car is quietly drawing current while the car is parked, and the job is to find what. That is called a parasitic draw, and it can be measured with a regular multimeter.
What a parasitic draw looks like in practice
The classic scenario: the car shuts down normally, everything switches off as it should, you lock it and walk away. The next morning it starts without a problem. But if the car sits for two or three days, the battery is so flat that the starter just clicks. You put it on a charger overnight, the car starts, you drive for an hour, the alternator tops it up and everything looks fine, until you leave it standing for a longer stretch again.
That is typical behaviour when something in the car refuses to go to sleep. Modern cars have dozens of control modules that gradually power down after locking, one by one, over several tens of seconds. If one module stays awake, it pulls current all night, and winter cold further reduces the battery's capacity so the drop happens even faster. A cold morning is not always a sign that the battery has failed.
What is a normal current draw at rest
Before any measurement, you have to know what numbers you are aiming for. On older cars (say up to model year 2010), the rest current after all modules have gone to sleep should be below 50 mA. On newer cars with more electronics (start-stop, telematics, keyless), the upper limit goes up to about 80 mA. Anything over 100 mA on a car that has been sitting quietly for half an hour is a sign that something is off.
The key condition is patience. If you lock the car and try to measure right away, you will see draws of 500 mA and up because the modules have not gone to sleep yet. You need to wait 20 to 30 minutes after locking, do not open the doors, do not press the boot release and do not turn on the lights. Every wake-up resets the sleep timer and everything starts over.
Step by step measurement with a multimeter
The multimeter is wired in series, meaning into the break in the current path between the negative battery terminal and the chassis. The procedure goes like this:
- The car has been locked for 20-30 minutes already. Do not open the doors.
- Disconnect the negative terminal from the battery.
- Set the multimeter to DC ammeter, first on the 10A scale (not on mA, because if there is a larger draw somewhere it will blow the internal fuse).
- Connect the red lead of the multimeter to the negative post of the battery, and the black lead to the disconnected terminal. Now the current is flowing through the multimeter.
- Wait a couple of minutes for the system to stabilise. Read the value.
- If the draw is low (under 500 mA), switch the multimeter to the mA scale for a more accurate reading.
If the multimeter shows, say, 250 mA after half an hour at rest, something is definitely pulling current. The next step is pulling fuses one by one. You take fuses out in order, watch what happens to the multimeter reading, and when the draw suddenly drops you have found the circuit with the culprit. Then you look at everything connected to that fuse and narrow it down from there.
The most common culprits we see in the shop
There are a handful of things that keep showing up from car to car:
- An aftermarket head unit poorly wired to constant 12V instead of ACC. The radio never sleeps, it keeps the amplifier and display in a standby state.
- A faulty switch on the boot light or under-bonnet light. The bulb stays on, you cannot see it because the boot is closed, and it pulls current all night.
- Aftermarket alarms or GPS trackers, especially the cheaper models from online shops. Poor sleep mode, sloppy build quality.
- Aftermarket seat heaters, parking heaters and inverters, often wired through a relay that stays energised permanently.
- An alternator with a leaking diode. Current leaks back into the alternator from the battery while the engine is off. You spot it by measuring AC ripple with the engine running, where on a healthy alternator the AC component on the battery terminals should be under 50 mV.
This is especially common on taxi vehicles and on cars that have had all sorts of electronics added on over the years. The more layers of installation, the more places where someone did not properly wire up the power and ground.
When the problem is the battery and when the electrical system
There is a simple test to separate the two. You disconnect the negative battery terminal in the evening, leave it overnight, and in the morning measure the voltage at the battery terminals themselves. If the battery is still around 12.6 V, it holds voltage when disconnected, so the problem is in the car because something is draining it while it is connected. If the voltage has dropped significantly even though it was disconnected, the battery itself is worn out and there is no point hunting for a parasitic draw.
In the shop we work in exactly that order. First we disconnect the battery, check whether it holds voltage, and look at its state under load (load test). Only once the battery proves to be sound do we move on to the multimeter and pulling fuses to find where the current is leaking. That way we don't burn a whole day searching through the wiring when in reality the battery was simply at the end of its life.
When to bring it in
If your battery is going flat even though it is new, if you have already replaced the battery and the problem came back, or if the rest current is consistently over 100 mA and you don't have the tools or time for a systematic search, there is no point guessing. An experienced auto electrician, working through the fuses and the wiring diagram, will identify the culprit relatively quickly, and you keep your sanity intact.
If you are not sure where current is leaking, stop by the shop or book an appointment. Better to check now than to be stuck in the morning before work with a car that won't start.