Pic. 9.6. Measuring battery voltage with a multimeter. If the voltage is below 12.5 V, the battery should be charged
2. After 4–5 minutes, measure the voltage between the battery terminals. Red wire with tester terminal (multimeter) connect to the positive terminal of the battery, black to the negative (pic. 9.6). When measuring, all consumers must be turned off.
Pic. 9.7. The principle of connecting a multimeter when measuring current, voltage and resistance. Consumers are, for example, lamps, a starter or a radio
Pic. 9.8. Using a special tester, you can determine the condition of the battery by measuring its capacity. The measurement should not last more than 10 s, as the battery is under heavy load
Measurement of voltage, current and resistance
If you want to check the health of various electrical systems in your car, it is not necessary to immediately contact an electrician. There are many different devices on sale with which you can independently check the condition of the electrical system.
Probe with lamp (with needle contact). With it, you can check the presence of voltage in the electrical circuit. The brighter the lamp burns, the higher the voltage.
The needle pierces the insulation of the wire under test. The other terminal of the device on the wire is connected to «mass» (car body).
Attention! An incandescent lamp probe consumes a lot of power, so it cannot be used to test electronic devices (e.g. controller). To do this, you need a voltage probe with an LED.
Probe with LED. Depending on the version, this device determines the presence of direct and alternating voltage in the range of 6–700 V. Visually, the presence of voltage is determined by the ignition of the LED. Simple devices of this type are inexpensive.
Universal tester (multimeter). This device can measure voltage, current (constant and variable) and resistance. You can use inexpensive devices with a digital display. The power supply of such a device, as a rule, is carried out from an autonomous battery.
Voltage measurement. In order to measure, for example, the voltage of the battery at idle, connect the cable of the device with a negative contact to the negative terminal of the battery or fasten it to «mass». Connect the positive terminal to the positive terminal of the battery or the wire under test. If the device shows only 10.4 V, this indicates a short circuit in one of the battery cans. Also check the voltage of the battery when the starter is turned on, a value of less than 8 V indicates that the battery is not all right.
Current measurement. To do this, break the circuit and connect the device between open contacts. When measuring, it will be enough to disconnect the connector and connect the wire between its two contacts.
Attention! Always be aware of the thresholds that your tester can measure. Very high currents flow at the contacts of consumers such as a starter, which can damage the device during measurement.
Resistance measurement. In this case, the tester checks whether the current passes through a particular wire or switch. If the current flows freely, the resistance value will be zero. If there is a break in any place, the resistance value will tend to infinity. In addition, you can find out what internal resistance this or that device has.
When measuring the resistance of a wire, its ends must be stripped and connected to different terminals of the device (pic. 9.7). When setting measurement values, always start with the maximum value.
Measure battery capacity (pic. 9.8).