What causes low battery voltage?

Batteries are used for storing electricity and are used as power backup when the main source of electricity does not supply power.

Battery low voltage is a condition where the battery is not sufficiently charged for it to be able to power the system that it is connected to. 

The charging system of a car is the main power source that supplies electricity to the battery. It mainly uses an alternator as a generator. Some newer green rated vehicles also use renewable energy sources like solar power to charge the battery.

What is a car's charging system?

A car's charging system is the one responsible for restoring charge to the car's battery continously as the battery is being used.

It can sometimes be very difficult to diagnose a charging system that supplies low voltage thereby undercharging the battery. In some cases, alternators are replaced and the problem remains unresolved.

 

What charging voltage is too low?

The answer to this is "it depends" as this varies in different cars. Voltage output of different alternators and voltage regulators can vary. There are also different types of batteries which have different charging voltage characteristics.

For many cars, any charging voltage below 13.4 volts is too low.

Below are common reasons why car charging systems output low voltage to the battery.

1. Faulty alternator/voltage regulator

It is common for the alternator and voltage regulator to be built as a single unit. A weak alternator/voltage regulator can supply low voltage. There are some internal parts of the alternator that can cause this such as worn brushes, weak or faulty voltage regulator.

2. Corroded/loose connections

This causes poor or no electrical contacts where cables are joined or terminated. The remedy is to reterminate or clean with electrical contact cleaner or both and ensure that connections are tight.

3. Bad ground

A bad ground is mainly caused by corrosion at the ground terminals. Loosed connections can cause bad ground too. Resistance of ground cables should also be checked. Voltage drop occurs where there is bad ground.

4. Bad battery terminals

Battery terminals that are not tight or are corroded creates significant voltage drop at the battery terminals. Bad terminals should be replaced and the battery positive and negative posts be cleaned regularly.

5. Excessive electrical load

When the load on the charging system is higher that its capacity, there will be significant drop in charging voltage that can affect battery charging. Sometimes, this is as a result of fault of some electrical component. For example, when there is a fault in the electrical cooling system fan circuit and the fan begins to operate at maximum speed, it draws a large amount of current and causes voltage drop.

Depending on the capacity of the cooling system, this can affect battery charging negatively.

6. Undersized alternator

An alternator with low charging capacity that cannot deliver sufficient current and voltage may manifest in the form of low charging voltage. An alternative with capacity high enough for the car's electrical load and battery should be installed.

7. ECU

The charging system of some modern cars are controlled by the ECU. We have once changed the alternator of a car twice believing that they were faulty but we later realised that it was the ECU that was dialling down the alternator's output voltage. This is a normal operation but a hardware or software problem of the ECU can cause low voltage output from the charging system.

8. Slipping belt

When the serpentine belt's grip on the alternator's pulley is not good, it will be slipping on the pulley and cause the alternator not to turn at a low revolutions per minute and the consequence is low charging output.