Motorcycle Navigation: Mounting a GPS Device to Your Motorcycle, Part 3

Feb 13, 2017 View Comments by

Your GPS device was working fine, accurately reaching waypoint after waypoint. Then suddenly the screen turned black and showed no sign of life. The battery is dead, you realized. Murphy’s law followed and rain started pouring while you had no real idea how to continue.

This may have not happened to you yet, but ride long enough without a powered GPS and it most likely will.

Before we discuss the details of hooking up a GPS to the motorcycle’s electric power supply, it is imperative to establish some nomenclature, fundamental concepts, and standards. Electric power must be treated seriously because it could potentially cause major and even fatal damage to the rider as well as the motorcycle and attached electric devices.

Motorcycle power conversion

Modern motorcycles produce an alternating current (AC) from their alternators, the engine’s crankshaft indirectly turning a rotor shaft against a stator. This is immediately converted and regulated into direct current (DC) by the regulator-rectifier, which feeds it into the motorcycle’s battery and from there to other electrical components.

The motorcycle’s electrics use a nominal 12-volt DC system. The actual voltage value is above that, typically between 13 to 14.5V DC, to allow the battery to charge. Within the context of powering GPS devices, we are only dealing with DC voltage.

Voltage, Current, Resistance, and Power
A water circuit is a simple analogy that explains these terms quite well. Given a reservoir with a dam, the energy potential of the dam depends on its height. The taller the dam the more energy potential it has, and voltage is similar. Current is comparable to the amount of water that actually flows from the reservoir down the dam to the ground level. Resistance corresponds to the size of the dam’s floodgate opening that controls the water flow. Higher resistance means less flow; no resistance means a flood. Power is the amount of work that the current can perform per second, such as turning a water wheel.

Electric current flows from the source of energy (the BATT or positive pole, marked as +, and red) to the ground (the ground or negative pole, marked as —, and black). Polarity describes the direction of the flow.

Voltage, U, is measured in volts (using the letter V); current, I, in amperes (or amps, A); resistance, R, in ohms (Ω); and power, P, is measured with watts (W). There are a few equations that are very useful for understanding the relationship between these terms.

Ohm’s law: U = IR; voltage equals the current times the resistance.
Joule’s law: P = I2R = UI; power equals the voltage times the current.

If, for example, we have a smartphone that is used as a GPS device, and it requires .9 amps at 5V to charge, then we know that it could consume up to 4.5 watts (P = UI = 5Vx0.9A = 4.5W).

Shorts and Disconnects
Two of the most common electrical issues are short circuits and disconnects. A short circuit is a situation where the positive and negative poles have a direct path to each other, with very little or no resistance. Following Ohm’s law, the current would immediately surge to a huge value which causes extreme heat and sometimes even explosions (e.g., the Samsung Galaxy Note 7 battery fiasco). Motorcycles have fuses that protect against such situations, but if a metal bar accidently shorts the battery poles, no fuse would help.

If the wiring’s polarity is by mistake reversed, then the positive will touch the ground and short the circuit. That’s why it is critical to always pay attention and validate correct polarity.

SAE PowerletDisconnects happen when the circuit is broken between one component and another. Connections might come completely or partially loose, or wires might be internally cut. It is quite frustrating to have a non-charging GPS while not being able to figure out the root cause.

The best tool to investigate these issues is a digital multimeter that can measure voltage, polarity, current, and resistance. Many also have a special continuity mode that can verify a short circuit or a disconnect.

Connector Standards
Modular components require common standards to ensure compatibility by different vendors.

The SAE connector is often used to join automotive 12V lines. It provides a sturdy, quick disconnect, is water-resistant, and considered safe since the polarity of the connector is defined as an exposed pole being ground. There is no risk of a battery short circuit if the exposed ground pole touches the motorcycle’s grounded chassis.

USB PinsThe car cigarette socket is a pluggable 12V adapter that has wide support in the automotive industry. The positive voltage comes from the center pin, which is well protected, whereas the outer collar is the ground. It is a bit bulky and often doesn’t hold the male part very firmly. This could be an issue for dirt or adventure motorcycles.

The ISO 4165 standard (also known as Powerlet) is similar to the cigarette socket but has a more compact design that has become the de facto standard on modern motorcycles. It also holds better due to the multiple spring-like metal ground plates of the male connector.

USB is the universal standard for virtually all consumer devices today and comes in different shapes and sizes. However, USB runs on 5V DC and shares a similar logical pinout. There are four pins: two for data (D+ and D-), one for ground (–), and one for 5V positive (+). For charging, we just need to wire the (–) and (+) pins, respectively.

With the basic electric concepts covered, the next step is to discuss the modular multilayer electric stack required for powering the GPS. This will be addressed in the next issue.

Text: Yuval Naveh
Photography: Yuval Naveh, Twisted Throttle, and Simon Eugster and André Karwath (Wikimedia Commons)


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