The ANENG AN9002 multitester is connected via Bluetooth to a Samsung Galaxy A04 cellphone. The display shows the car battery voltage before starting, and until the engine is running. With a Bluetooth connection, the battery voltage can be monitored remotely with a graphic display on the cellphone.
The graphic display makes the multitester functions similarly to an oscilloscope. More details about the ANENG AN9002 digital multitester.
As seen in the graph, the initial voltage of the car battery is around 12.8 volts.
The car battery voltage when the contact is in the Acc (accessories) position is around 12.7 volts.
In the On contact position, the battery voltage drops to around 12.6 volts. There is a small curve in the graph when the contact is positioned to On. This is because the fuel pump, relays, and instruments on the dashboard (fuel meter, temperature, odometer, indicator lights, etc.) are activated.
When starting the engine, the battery voltage drops to 10.56 volts.
The battery voltage then rises to 14.75 volts when the alternator is active, because the alternator is driven or rotated by the engine.
Conditions such as those in the graph above are quite normal.
Watch YouTube video about car accu voltage before and after start.
In the above video the voltage is slightly different. When engine start the voltage drops to 10.39 volts. This condition is basically still normal.
Meanwhile, in the following graph, the battery voltage can be seen to drop more deeply.
At the beginning of the graph is when the contact is in the Acc position, with a battery voltage of around 12.7 volts.
The battery voltage drops to 9.22 volts when the engine is starting.
Then the voltage rises to 14.71 volts, when engine is running. This indicates that the alternator is still working well.
Some causes of a very low or dropping battery voltage when starting, include:
- The battery has a weak storage capacity. Although the initial voltage still looks normal at around 12.8 volts. But the battery current capacity has started to decrease. So the voltage drops when starting, due to lack of electric current. For example, the normal capacity of a battery is able to produce an electric current of 40 amperes for one hour (40 Ah, ampere-hours), generally on sedans. If the battery is old, its capacity will decrease. For example, it can only flow an electric current of 25 amps in one hour (25 Ah). Because there are dirt deposits on the battery cells, which prevents the battery cells from contacting the electrolyte (battery water).
- Both positive and negative battery connectors have poor connections. Maybe because they are loose, or because they are dirty.
- The starter motor is dirty. Especially in the carbon brush section. So it needs to be opened and cleaned. Dust from the carbon brushes collects. The carbon dust is an electrical conductor, and can cause internal current leakage in the starter. So that some of the electric current from the battery does not rotate the starter motor. But short circuits inside the starter.
It could also happen that the battery voltage does not drop when starting. Still above 11 volts. But the starter is difficult to turn. This could be an indication that the battery is still good. Some possible failures are:
- The positive wire from the battery connected to the starter solenoid is loose or dirty.
- The negative wire from the battery to the vehicle body is dirty, loose, or rusty. There is also a negative wire that is connected from the battery directly to the engine block, or from vehicle body to engine block.
- The wire from the ignition switch to the starter solenoid is loose or dirty. Causing the solenoid not to function properly.
- The switch in the solenoid is dirty or damaged (worn, burned). Some solenoids can be dissasembled to clean the switch contacts. While others must be replaced when faulty.
The function of the solenoid on the starter is as a relay to allow a large electric current flow directly from the battery to the starter. Because the current is very large, it cannot go through the ignition switch or other circuits on the vehicle. Large currents require large cable and connectors, and are well connected.
If the large current to the starter is resisted, the starter is unable to rotate properly, aka lack of power. And the battery only produces a small current. That is why the battery voltage not to drop much when the starter is turned on.
The solenoid also functions to shift the small gear (pinion gear) of the starter, so that it engages with the ring gear on the flywheel. So that the starter can rotate the crankshaft. When the starter does not work, the gear will be released from the flywheel. So that the starter will not rotate with the crankshaft, when the engine is running.
Signs of a bad starter solenoid include:
- Nothing happens when you turn the key
- A single “click” sound from the engine compartment or under the car
- Repeated “clicking” sounds
- Intermittent operation
- A spinning starter without an engaged flywheel
- A slow-cranking engine
All symptoms of a damaged solenoid as above, are usually indicated by the battery voltage not much dropping. When the starter is turned on.
The basic principles explained in this article can also be applied to motorcycles, pickup, minibus (MPV), jeep (SUV), and other vehicles. Especially those with 12 volts electrical circuits.
