Very often, the first sign of battery trouble is when your car won't start. But how do you tell if it is caused by the car battery not charging, or if it's something else?

A car that won't start can be caused by a fault with the alternator or some other part of the electrical charging system. Faulty injectors or a clogged fuel filter can also cause a car to struggle and splutter when starting.

Your car battery will cycle through thousands of charges and discharges over it's lifetime. It's main purpose is to start the car, but it can also be used to provide extra power to accessories when the engine is off. It is vital though, that the battery is recharged while the engine is running so that it is ready for the next start. 

Car batteries are designed to take a lot of punishment, and should provide years of trouble free engine starts.

Sometimes, the problem is not with the battery itself, but with how it gets its charge.

Car Battery Charging Basics

Cars have gotten a lot more complicated electrically over the years. When it comes to charging the battery though, it hasn't really changed.

Basically, when you turn the key in the ignition, the starter motor is powered by the battery and this starts the engine. It also provides the initial spark to the spark plugs of a petrol car. In some diesel cars it will power the glow plug heater if the temperature is below zero. But once the engine has started, the alternator should recharge the battery.

When the engine starts turning, the alternator is driven by a belt that converts mechanic power to electrical power.  This power is used to charge the battery, and to provide all of the power necessary to run things like the headlights, interior lights, the radio, and all other accessories.

7 Reasons Your Car Battery Is Not Charging

If you find that your car needs to be jump-started on more than one occasion, there are a few simple checks that might help fix the problem.

1. Bad Battery - Check the Car Battery Condition

  1. After you have jump-started the car, leave it connected to the donor car for a few minutes to give the battery a chance to charge. If your car stops after the jump cables are removed, then it is a problem with the car battery not charging.
  2. Before you check the battery, make sure the headlights and anything else that may drain the battery are turned off. Remove the keys from the ignition too, sometimes the key can activate part of the engine such as the fuel pump, in anticipation of the engine starting.
  3. Using a basic voltmeter/multimeter set to DC current, you can easily check if the battery is in good condition or not.
  4. Attach the positive lead to the positive battery terminal (usually red in color). Attach the negative lead to the negative terminal (brown or black in color)
  5. With the engine off, the reading should be 12.5 Volts, plus or minus 0.2 Volts. If it is less, then the battery has not charged properly, more then there may be a problem with the alternator
car battery not charging

Using a multimeter/voltmeter to check the voltage level in a car battery

2. Bad connections to battery - Check the Battery terminals and wiring

This can be difficult to do on modern cars where batteries are fitted into tight compartments in the trunk or engine bay. You may need to remove a few plastic covers first to get to the battery. It's usually best to check the vehicle handbook first.

If your car is a bit older, then the battery terminals may be corroded or may have come loose. Corroded terminals can be cleaned up using a wire brush or sand-paper. It's unlikely that the wiring has become damaged as it's usually fairly thick and well protected, but it is still worth checking on older cars.

3. Blown Fuse - Check the Fuse Box

Sometimes a simple fuse can be stopping your car from starting. Fuses can sometimes just blow, especially on older cars for no reason. They can become brittle and worn with age, and a cold engine start may be too much for them to handle.

To start with, check the fuse for the starter motor and the fuse for the alternator.

Again, check the user manual for more information about the location of the fusebox and individual fuses. 

If all of the above check out ok, then you are going to have to delve a bit deeper. Car electrics can be complicated so now could be a good time to hire a mechanic or auto electrician.

4. Faulty Alternator - Check the Alternator

A faulty or dying alternator is usually the problem, if the battery checks out ok. Symptoms of a faulty alternator include car not starting, headlights dimming or a charging system warning light on the dashboard.

So how do you check an alternator? While the engine is running, connect the voltmeter as before. The reading should be between 14 and15 Volts on most cars. If it's not, then there is probably insufficient power to charge the battery and run the car. 

If you suspect the alternator, then visually inspect it for damage or lose wiring. Again, it may be hard to get to so you may need to let an expert remove it for testing

5. Uncontrolled drain on the battery - Check any aftermarket devices

If your car has been fitted with an after-market radio, Bluetooth kit or security system, then one of these could be draining the battery. This is usually more difficult to pinpoint. But if your someone has been tinkering with the wiring in your car recently, then it could be causing a drain on the battery.

Most car accessories and after-market devices should shut down completely when the engine is turned off. Some may need to have a permanent live connected so that they retain some settings like the time or radio channel frequency. 

6.Damaged Drive Belt - Check Drive Belts and Pulleys

A serpentine belt (rubber drive belt) is used to drive the alternator. The speed of the alternator will change with the speed of the engine. 

car battery charging problems

Engine alternator and auxillary belt

Sometimes the belt can stretch or become frayed, especially in older cars. The belt can then lose its grip on the alternator pulley, causing it to slip. A slipping belt will not drive the alternator fast enough to keep up with the power needed by the car.

All of the belts are connected with the engine by pulleys. These pulleys are fairly tough, but they do have a limited life-span. If you can see the belt slipping, of there is a noise coming from the pulleys, then they may need to be changed.

7. Faulty ECU - Run a diagnostic to check for error codes

All cars have multiple ECUs (on board computers) that manage the numerous electrical systems that are needed to run the car. If one of these control units is faulty, then it can lead to a problem with the car battery.

A diagnostic scanner will be able to read any fault codes that have been recorded by the ECUs. You can purchase a scanner yourself, or have it done professionally by any mechanic. 

common Questions & Answers 

Here are a few typical problems that you can encounter when trying to troubleshoot a bad battery or alternator.

Q. I have replaced the alternator and the battery is still not charging?

If the battery is definitely in good condition, then there is probably a problem with vehicle's wiring. It may be necessary to hire a good auto electrician to perform a full diagnostic. A lose wire, or ungrounded neutral can play havoc with the charging system overall. 

Q. The alternator tested good but the battery keeps dying?

Test the battery, if it checks out ok, then there is something else draining the battery. After-market stereos (especially those that need a permanent live) can often drain a battery if not fitted properly. Also check that all lights (including interior lights) are turned off when you leave the car.

Q. The car battery won't fully charge?

This is probably caused by a faulty cell in the battery. Batteries have a limited life-span of around 3-4 years. Most car batteries cannot be serviced, the best solution is to replace the battery.

Q. The new car battery won't hold a charge?

Assuming the battery is not faulty, then there is something draining the power from the battery. A good auto electrician should be able to trace the cause.

Q. If the alternator is bad will a jump start work?

If the battery is fully charged, then yes it will, but not for long. If the alternator is not charging the battery, then the car will stall after a short time. You may be able to drive the car a short distance, if you can keep any unnecessary electrical systems turned off.

Q. I've fitted a new battery and new alternator but my car still dies?

This could be caused by a fault in the fuel management system. A problem with faulty injectors or a clogged fuel filter will cause a car to cut out. An OBD2 reader will be needed to help pinpoint the exact problem.

Q. My car battery wont hold charge after sitting overnight?

Assuming you didn't leave the headlights turned on!; this is caused by a faulty battery or a drain on the battery. Check the battery as explained earlier, and maybe get a good mechanic to run a diagnostic on the rest of the car.