Diagnosing a problem with your car’s battery
by Touge Tonight Team
Your car is struggling to crank over and it seems as though the battery is dead. You get a jump of somebody or connect a portable jumper kit, it fires up and you take it for a lengthy drive to give it some juice.
The next morning, your battery is flat again. This article will help you to diagnose what could be wrong.
Possible reasons your battery keeps going flat:
– You left something on. Headlights, interior light, key in the ignition on the “On” or “Acc” position, etc
– The battery is not charging when you drive it
– There is drain on the battery whilst the car is off
– Your battery may require maintenance, has dropped a cell or will not hold charge anymore
Checking your batteries state of charge
First you will need to check your battery and the alternator. First, try turning on your headlights. If they appear dim or do not come on at all then you’re on the right track. If they appear at full brightness, then it may be a faulty starter motor, bad wiring to the starter or a faulty solenoid/relay.
To check your battery, you will require a multimeter. You could get away with a voltmeter, especially if you have one as a factory or after-market gauge in your car. However for the other tests listed here, a multimeter is indispensable. You want to select a range or scale that will clearly show you between 10-20 volts.
Connect the black (COM on the multimeter) to the battery’s negative (labelled – or black in colour) terminal. Connect the red (VΩmA on the multimeter) to the battery’s positive terminal (labelled + or red in colour). If you get these back to front on a multimeter, they show the same voltage, just as a negative. You’ll get -12.6V instead of 12.6V. It’s better to swap the terminals around as this may cause inaccurate readings later.
A fully charged battery should read around 12.6V, depending on the ambient temperature. Anything around or less than 12.4V and you’ve got a problem, the battery is discharged (or flat) and needs recharging. Keep in mind these readings are at around 26°C. The battery will drop around 0.01V for every 5°C below this.
Check your battery and charging system
Jump start your car and connect the volt/multimeter to the battery as before.
If your charging system is working correctly, at idle your car should produce between 13.5-14.5V. If you’re less than 13.5V on idle, your battery will not charge correctly. The common culprit will be your alternator, which you should have tested at an auto electricians to ensure it is outputting the correct current. If it is not, you should replace the alternator.
After the battery has been charged, you should have it tested to see if it can hold charge. This can be done by most auto workshops as it requires an electronic battery tester or conventional load tester.
Parasitic drain on the battery when car is off
If your battery is less than six years old, and you’re still trying to identify the issue, the next thing to investigate is how much load there is from the battery with your car fully switched off.
On older cars that lack complicated electronics, the load should be almost nothing. Maybe a few milliamps for a clock or digital radio. On newer cars however, the key off load may range from 15 to 50 milliamps to as much as 300 to 400 milliamps. Some cars will draw 850 to 1000 (1 amp) for around half an hour after the car is turned off.
Just be wary with changing or disconnecting the battery in some vehicles. Some Subaru’s have a keypad alarm that will require the vehicle to be towed to Subaru to have a ‘master code’ entered to disable the immobiliser. Other cars may require the security code to be entered on the stereo before this works, may require settings to relearn climate control, seating positions, remote central locking remotes, etc. You can avoid this by obtaining a back up battery that connects to the cigarette lighter or using an auxiliary battery whilst you replace your original.
To measure the load on your battery, you should open the bonnet or trunk (wherever the battery is stored) and wait approximately one hour. This is for newer cars as some modules will be reactivated by opening a door or bonnet and may cause a much higher reading than expected.
You will require a multimeter for this.
1. Set your multimeter to measure mA. If the meter is not auto-ranging, you want to go with 100 or 1000mV scale.
2. Connect up your backup battery (or not, if you’re willing to risk it which isn’t recommended) and then disconnect one of the terminals of the battery.
3. Disconnect one of the cables connected to your battery and connect one of the multimeter leads to this.
4. Connect the other lead of the multimeter to the battery terminal where you removed the cable from.
5. Note the reading on your multimeter.
Causes of excessive power drain may be things such as lights left on (trunk, glovebox, headlights, interior lights, map lights), relays stuck open or modules that are not powering down and going to sleep.
To find the cause of drain on the battery
If the reading is too high, it’s time to start identifying the cause.
Refer to your vehicles manual to locate the fuse boxes and relay locations. Generally there is one fuse box in the car and a fuse box in the engine bay with associated relays. You want to start with anything not related to the car’s necessary electronics such as it’s ECU or Computer, ABS modules. Start with heaters, defrosters, etc.
Pull out a fuse and observe the reading on the multimeter. If the current load drops, then the faulty equipment is located on this circuit. For example, many cars have a fuse for rear defroster. If the current drops after removing this fuse, you will need to check the relay, the switch and the defroster unit itself. Should the current not drop, you should replace that fuse and move to the next.
Alternative solutions
You may find a faulty alternator diode can cause the battery to run flat. The diodes in the alternator are designed to only allow charging of the battery, if one has failed it will allow the alternator to draw current from the battery when it is in the off position. You can check this using your multimeter to measure AC voltage. If you get a reading of any AC voltage, your alternator has a failed diode. Alternatively, if you do not have access to a multimeter, you can disconnect the alternator overnight and see if the battery still runs flat. If so, replace your alternator.
Your battery may require maintenance. Newer battery tend to be sealed and therefore can not be serviced, in this case you should replace the battery. If your battery is not sealed, you should see six to eight caps (usually yellow in color with a small hole for venting of gas). You should take off the caps and ensure there is adequate water in each cell. Do not overfill however!
Should you fail to diagnose the problem, you could either try replacing the battery or taking it to an auto electrician for further investigation.