Quick answer
The usual causes are an old or failing battery, a charging fault, short trips that never recharge the battery, or a parasitic drain: something drawing power while the car is parked, such as a light left on, a faulty module or poorly wired accessories like dashcams and stereos. Test the battery and charging voltage first. If both are fine, a current draw test (parasitic drain test) will show whether something is draining it overnight.
Is this a DIY job?
- Difficulty: Easy for the first checks; Medium for the drain test
- Time: 15 minutes for the checks; 1 to 2 hours for a drain test (mostly waiting)
- Tools: digital multimeter (with a 10 A range), battery charger, torch
- Skills: basic multimeter use
Anyone can do the first checks. A drain test is a careful DIY job. Finding an intermittent drain or a module that won’t go to sleep is often quicker with an auto electrician’s equipment.
Step by step
- Look for the obvious. Check the interior, boot and glovebox lights go off, the headlights and parking lights are off, and nothing is plugged into a 12V socket that stays live. Check the doors, boot and bonnet are fully closed.
- Think about how you drive. Lots of short trips, or a car that sits for a week or more, may simply never recharge the battery. A smart charger or battery maintainer can keep it topped up.
- Think about recent changes. Was a dashcam, stereo, alarm, tracker, fridge or other accessory added? Accessories wired straight to the battery, or to a fuse that’s always live, are a common cause.
- Charge and test the battery. Charge it fully, let it rest for a few hours, and measure the voltage. Then have it load-tested (many battery retailers do this). A battery that won’t hold charge needs replacing. (see How do I test my car battery?)
- Test the charging system. With the engine running, the voltage across the battery should be roughly 13.5 to 14.8 V on most cars (smart alternators can read lower at times). (see How do I know if my alternator needs replacing?)
- Check the overnight drop. Fully charge the battery, drive the car, park it overnight and measure the voltage next morning before starting. A big fall points to a drain.
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Do a parasitic drain test (Medium). Summary: – Switch everything off, close the doors (latch the door switches closed with a screwdriver if you need the doors open to reach the fuse box), and lock the keys well away from the car. – Set the multimeter to DC amps on its 10 A range. Disconnect the negative battery terminal and connect the meter between the negative cable and the negative post. – Leave the meter connected and wait for the car’s modules to go to sleep. Disconnecting the battery wakes them, and on many cars they take 20 to 45 minutes to settle again. – Read the current. On many cars a normal sleeping draw is about 20 to 50 mA. Over about 50 mA is worth investigating; over about 100 mA is usually a fault. Check your vehicle’s specification if you have it. – If it’s high, pull fuses one at a time and watch the meter. When the reading drops, that fuse feeds the circuit with the drain. – Don’t start the car or switch on the ignition while the meter is connected. The meter’s fuse (and possibly the meter) will blow.
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Fix the cause. Common finds: a boot or glovebox light that stays on, a faulty door or bonnet switch, an aftermarket accessory wired to permanent power, a stuck relay, or a module that keeps waking up.
- Recharge and recheck. After the fix, charge the battery fully and repeat the overnight check.
Specs table
Overnight and sleep-current guide (12V cars)
| Check | Reading | What it suggests |
|---|---|---|
| Resting voltage, fully charged | 12.6 V or more | Battery charged |
| Resting voltage after one night | Small drop from the evening reading | Normal |
| Resting voltage after one night | Below about 12.4 V from full | Investigate: drain or weak battery |
| Sleep current after modules sleep | About 20 to 50 mA | Normal on many cars |
| Sleep current | Over about 50 mA | Worth investigating |
| Sleep current | Over about 100 mA | Usually a fault |
| Charging voltage, engine running | About 13.5 to 14.8 V | Normal (conventional alternator) |
Vehicles with lots of electronics, alarms or trackers can sit at the top of the normal range. Some take longer than 45 minutes to fully sleep.
What you’ll need
- digital multimeter
- smart battery charger or battery maintainer
- fuse puller
- If the battery has failed: car battery
- For accessories: low voltage cut off or dashcam hardwire kit (with low-voltage protection)
- For vehicles parked for long periods: battery isolator switch (turned off, it isolates the battery completely so nothing can drain it; best fitted by an auto electrician)
When to call an auto electrician
- The battery and charging test fine, but it still goes flat.
- The drain is intermittent, or the reading never settles because modules keep waking.
- The drain disappears when you pull a fuse that feeds a module or many circuits.
- Accessories have been added by someone else and you’re not sure how they’re wired. Unfused or poorly wired accessories are a fire risk.
- It’s a hybrid or EV. The 12V battery is charged from the high-voltage system, which is professional-only.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: How long can a car sit before the battery goes flat? A: There’s no single answer. It depends on the battery’s size and condition, the temperature and the car’s sleep current. A weak battery or a high drain can go flat in days. If the car will sit for long periods, a battery maintainer is the safest option.
Q: Can a dashcam drain my battery? A: Yes, especially if it runs in parking mode. Use a hardwire kit with a low-voltage cut-off, and fuse it properly.
Q: Why is my new battery going flat? A: A new battery won’t fix a charging fault or a drain. Test the charging voltage and do a drain test. (see What does the battery light on my dashboard mean?)
Q: Does disconnecting the battery stop a drain? A: Yes, but it resets the radio, windows and other settings, and on some cars causes warning lights until systems relearn. A battery isolator or maintainer is better for storage.
Sources
- Battery Tender, “Parasitic battery drain: how to find and fix it” – normal 20–50 mA draw, 75–100 mA abnormal, module sleep time, test method and cautions. https://www.batterytender.com/blogs/battery-tender-blog/parasitic-battery-drain-how-to-find-and-fix-it (checked October 2026)
- JS Alternators, “How to test your alternator” – resting and charging voltage ranges; smart charging systems. https://js-alternators.com/blogs/blog/how-to-test-alternator (checked October 2026)
- RACV, “Car battery replacement” – lifespan depends on installation, how often the car is driven and climate. https://racv.com.au/on-the-road/roadside-assistance/car-battery-replacement.html (checked October 2026)
- Fluke, “How to find parasitic battery drain with a multimeter” – meter in series at the negative cable on the 10 A range; let modules sleep (up to about 45 minutes); under 50 mA normal, over 100 mA suspect; don’t crank with the meter in current mode. https://www.fluke.com/en-au/learn/blog/digital-multimeters/how-to-find-parasitic-battery-drain-with-a-multimeter (checked October 2026)