[DIY Small Classroom]The basics of motorcycle operation-current and voltage


The battery is absolutely indispensable for the spark plug to produce sparks and for the headlights and other electrical equipment to operate. Except for some motorcycles without batteries, from light motorcycles to large heavy machines, basically most motorcycles are equipped with batteries, and the batteries can also be charged through the operation of the engine.

If you want to know the battery status, you usually check the terminal voltage. If you want to know the status more clearly, you need a closer inspection.

Motorcycle electrical equipment needs to consider both the voltage and current directions

002-2▲The characteristic of the 鉤錶 is that there are red and black test rods, and the instrument body also has a circle-shaped measuring instrument. The front end of the circle looks like a clothespin and can be opened. The magnetic field is measured through the wire that measures the current in the circle, and then converted into a current value.

003-1▲The current value measured by the circuit tester used for motorcycle maintenance and general electrical related work is only about 1 ampere, but this clamp can switch between 40 amperes and 200 amperes to measure large current values.

Although looking at motorcycles as a whole, you may find “electricity” quite complicated and difficult to understand, but you should have learned that “voltage” plus “current” is equivalent to electricity in the nature class of elementary school. Everyone should have heard the analogy of tap water! How much the faucet can be opened is the voltage, and the amount of water in the bucket is the current.

When checking the electrical equipment and battery of a motorcycle, the usual method is to detect the voltage of the battery terminal, and then let the engine run to confirm the voltage change.

Although the characteristics of the rectifier will also be affected, if it is a 12V battery, when the engine speed is around 4000~5000rpm, the voltage will rise to around 14V~15V.

However, the current value cannot be known at this time. In other words, the water bucket = the accumulated power in the battery is not known. Take the example of tap water. If the water pipe is thin, the water pressure will be high, but it is difficult to fill the bucket immediately. If the water pressure is thick, the water bucket can be filled in a short time.

If you apply this to the electrical equipment and battery of a motorcycle, you will understand that the voltage and current measurement becomes very important if you want to correctly grasp the state of the motorcycle.


1. When servicing electrical equipment, voltage and current must be considered at the same time.
2. Even if the voltage is high but the current is small, the power generation may be insufficient.

As long as there is a ring, it can measure the generated current

004-1▲The voltage can be measured by connecting the battery terminals to the test rod in DC mode. When the engine speed increases, the voltage will also increase. If you want to know whether the battery is charging or not, you can know clearly by measuring the current.

In the middle school physics class, you should have learned that the voltmeter circuit is connected in parallel, and the current meter is designed in series. Connect the red and black test rods of the circuit tester to the positive and negative terminals of the battery in parallel. on. On the other hand, when measuring the current of a series circuit, the circuit must be interrupted.

In addition, general circuit testers are not designed for measuring large currents, so they cannot be used to test electrical equipment for motorcycles and automobiles. What became very important at this time was this measuring instrument called “鉤錶”.

The clamp can measure the magnetic field generated when the current passes, and then convert it into the current value. Therefore, even if the circuit is not interrupted and connected, the current can be measured by clamping it from the outside of the wiring harness. It is very convenient to use.

From the word current (the flow of electrons), it can be seen that the properties of voltage and current are completely different. Just like the voltage of a dry battery is 1.5V and the voltage of a 12V battery is 12.8V, the voltage value is fixed regardless of whether electrons are flowing or not. In contrast, if electrons do not flow, the current is zero, and if they flow in a positive direction, they flow in the opposite direction and become negative.

Taking the charging equipment of a motorcycle as an example, if the current flowing from the alternator to the positive terminal of the battery through the rectifier is taken as the positive electrode, the current consumed and discharged by the headlights and ignition equipment is negative electrode.

The way of expressing the positive and negative poles will change according to the direction of the connection to the bolt, but because the charging current of the battery is usually measured most of the time, it is generally based on the charging current when talking about the positive pole.

By the way, some cars introduced in the 1970s also included an ammeter as standard equipment, and usually this ammeter also regards more power generation = the current when the battery is charged as positive, and when the power generation is low, it is listed as negative. .


1. The current can be measured as long as the wire is clamped with the clamp.
2. The current value will change the positive and negative poles due to the difference in charge and discharge.

If the engine speed increases but the current value remains at the negative electrode, be careful of the battery breakage

The vehicle prepared this time is a luxurious version with a Z1 with a streamlined design to a Z1-R with a linear design, and a fuel gauge and voltmeter in a huge front fairing.

Next, take the KAWASAKI Z1-R equipped with a galvanometer as a demonstration, and explain in detail how to use the clamp to measure current. The direction in which the clamp clamps the wiring is the way electrons flow from the rectifier to the battery terminal, that is to say, the current value becomes positive when the battery is charging.

The wire from the rectifier to the positive terminal of the battery is connected to the ring and then the engine is started. The current generated will gradually increase as the speed increases, and the maximum can reach about 7 amperes. In order to prevent overcharging, the current value at idling speed is mostly zero or becomes negative.

If you start the engine in this state, the idling speed will be a positive number of approximately 0.1 ampere, or a negative number slightly lower than zero, and the current should also increase as the engine speed rises. However, because the vehicle itself has a rectifier to measure and control the charging status of the battery, if the battery is in good condition, the charge will not be too large.

If the headlight that consumes a lot of current is turned on, the pointer of the ammeter in the dashboard will move towards a negative number. Compared to charging, the value of discharging should be larger. Although this is not the case for the current models that have always turned on the lights, the current of old cars and out-of-print cars that can switch their headlights arbitrarily is ±0 when idling, and when the headlights are turned on, the current is usually ±0. Is a negative number.

A large amount of current consumption is usually when using electrical equipment that consumes power such as headlights and brake lights. The battery voltage of 12.4V can make 55W headlights light up. In simple calculation, it consumes 4.4 amperes. If the power consumption at this time is greater than In the case of power generation, the measured current value will be a negative number, and the battery will also be discharged.


If you turn on the headlights and taillights at the same time, the battery will release 2.75 amps of electricity at idle. Assuming the battery voltage is 12.5V, the electrical equipment will consume about 35W. If the engine speed is increased, the battery whose current is changed from a negative number to a positive number can be charged. However, if you charge a smartphone instead of navigation or use electric clothing that supplies electricity from the battery, a motorcycle with weak power generation capacity will The battery overheats due to excessive power generation.

The engine speed rises and the power generation increases. As long as the current value is not negative, the battery will charge, but if the battery continues to discharge excessively or even consumes the current from the ignition device, the engine will stall.

In particular, if electronic accessories such as smart phone charging and electric clothing are added to old cars that did not exist in the 1970s, the chances of battery failure during riding will increase significantly.

When the terminal voltage rises to 13~14V, or when the battery is broken at night or using electric clothing, just try to measure the charging current, and you may be able to know the cause of the fault.


1. There is no problem even if the current at idling speed is small.
2. The current value that is proportional to the engine speed will increase.
3. The current that becomes a negative number after using the electric equipment will change to a positive number when the engine speed increases.

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Information provided by “Webike magazine”
“Webike Taiwan” editorial department compiled

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