Finding a parasitic drain: why the battery is flat on Monday

A parasitic drain is current the car draws while it is switched off and locked. Every modern car has some — the ECU, the clock, the radio presets and the alarm all keep their memory alive — and that is normal. It becomes a fault when something stays awake that should have gone to sleep.

The scale tells you how urgent it is. A drain of a few tens of milliamps is the car working correctly. Something around an amp will flatten a healthy battery overnight, which is why this fault almost always presents as a car that started fine on Friday and will not turn over on Monday morning.

First, is it actually a drain?

A car that cranks slowly in the morning and fine in the afternoon has three plausible causes: a battery near the end of its life, a parasitic drain, or a charging system that is not putting back what the starter takes out. They feel identical from the driver's seat.

The old test still separates the first two and needs no equipment at all. Disconnect a battery terminal overnight, then reconnect it in the morning before starting.

• Starts normally — the battery holds charge on its own, so something in the car was flattening it. You have a drain.
• Still slow — the battery is not holding charge. Test or replace the battery; there may be no drain at all.

It is crude and it costs a night, but it answers the question before you spend anything, and it stops people replacing a good battery.

The measurement, and the mistake almost everyone makes

The classic workshop method is a test lamp in series between the battery post and its terminal: all the car's key-off current has to pass through the bulb, so a significant drain lights it.

It works, and it is what the original version of this article described. But it has a real limitation worth understanding: a test lamp cannot tell you how much. It will glow at a few hundred milliamps and glow at two amps, and it will show nothing at all at 60 mA — which on a car that should draw 30 mA is still enough to cause the complaint. It answers "is there a big one" and nothing else.

Use a multimeter on its 10 A current range, in series in the same place. Now you get a number, and the number is the whole diagnosis.

Set the meter to amps and move the lead to the 10 A socket before connecting it. A meter set to volts, or with its lead in the voltage socket, is a short circuit across the battery when wired in series. That is how people destroy meters, and occasionally start fires.

The thing the original article left out entirely

Modules do not go to sleep immediately, and measuring before they do gives a false result every time.

When you close the doors and walk away, the car stays substantially awake — body control module, radio, instrument cluster, alarm — for anywhere from about 10 minutes to over an hour, depending on the car. During that period a perfectly healthy vehicle can draw hundreds of milliamps. Measure then, and you will condemn a car that has nothing wrong with it.

So: connect the meter, then wait. Twenty to forty-five minutes is usual. Watch the reading fall in steps as modules drop out, and take your figure once it settles and stays settled.

Two practical consequences that follow from this and catch people out:

• Breaking the circuit restarts the clock. Every time you pull the meter out or take a fuse out and put it back, you wake the car and have to wait again. This is why methodically pulling fuses one at a time takes an entire afternoon.
• Opening a door wakes it too. Shut everything, and if you need to be inside the car, either leave a window down beforehand or latch the door catches closed by hand.

What a good figure looks like

For most modern cars, a settled key-off draw of roughly 20 to 50 mA is normal, and up to around 85 mA is not necessarily a problem on a car carrying more modules. Older and simpler cars — a carburettor Peykan with a clock and nothing else — should be lower still.

If you want a figure tied to the actual battery rather than a rule of thumb, a usable one is that the drain should not exceed the battery's amp-hour rating divided by 2.4. A 60 Ah battery gives about 25 mA, which is roughly where the general advice lands anyway.

Finding which circuit

Once you have a number that is too high, the job is to find which circuit it is on.

Pull fuses one at a time, watching the meter after each. When the reading drops to normal, the drain is on that circuit. Then put the fuse back, wait for the car to settle again, and confirm it returns — because a drain that does not come back was a module you woke up rather than the fault.

The faster method, if you have the tool, is a milliamp clamp meter around each fuse's feed or around the main battery cable, because it does not break the circuit and therefore does not restart the sleep timer. Failing that, measuring the millivolt drop across each fuse in place finds the loaded circuit without pulling anything — a fuse carrying current develops a small voltage across it, and one carrying none develops nothing.

What it usually turns out to be

Once you are down to a circuit, remember that a fuse usually feeds several things, so the circuit is not yet the fault.

The original gives a good worked example. On a Pride, pulling the "room" fuse often stops the drain — but that fuse feeds the clock memory, the radio memory and the oil-change counter and the interior and boot lamps. So the culprit is one of several, and in practice it is most often a lamp that never goes out: a boot light whose switch has stopped being pressed by the lid, a glovebox lamp, or a courtesy light held on by a door switch that has corroded closed.

A lamp you cannot see is the single most common answer to this fault, which is why it is worth opening the boot and the glovebox and watching what happens as you close them, before doing anything more clever.

After that, in rough order:

• Aftermarket electrics — alarms, immobilisers, stereos, dashcams and trackers, especially where the installer took a permanent live and did not use the switched feed. Anything fitted after the car was built is worth suspecting first, and the connectors used are often not to standard.
• A failed diode in the alternator, which lets current flow backwards from the battery through the rectifier. Distinctive because disconnecting the alternator's output lead removes the drain entirely.
• A chafed positive wire touching bodywork through damaged insulation — a partial short to earth rather than a dead one, so it drains without blowing the fuse.
• A relay stuck closed, which keeps whatever it feeds — commonly the fuel pump or a cooling fan — energised with the key out.
• A module that never sleeps, which is the hardest of these to find and the one where a repeated, patient measurement matters most, because the symptom is simply that the current never falls to where it should.

Video guides

Video, for the parts of this that are easier watched than read.