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Motorcycle Electrics. By Jerry Skene. Overview of Seminar. Volts, Amps, Watts & Ohms What they measure and how they are used (includes Ohm’s law) Batteries & charging Adding lights and other accessories Common tools & supplies. Volts.

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Motorcycle electrics

Motorcycle Electrics


Jerry Skene

Overview of seminar
Overview of Seminar

  • Volts, Amps, Watts & Ohms

    • What they measure and how they are used (includes Ohm’s law)

  • Batteries & charging

  • Adding lights and other accessories

  • Common tools & supplies


  • Measure of the electric potential between two conductors, or the ability to move an electric charge through a resistance

  • Motorcycles use a 12V DC (direct current) system

  • Measured with a voltmeter

  • ignition-off voltage is ~ 11.9 volts

  • operating voltage is ~13.8 volts

(lowest lethal voltage is ~ 60 volts)

Amp ampere
Amp (Ampere)

  • Measure of current flow through a circuit

  • Headlight bulb draws ~ 4.5A

  • Tail light bulb draws ~ 0.4A, or 400 milliAmps (mA)

  • Electrical symbol for current is I

Minimum lethal current through the

human body is 80 – 100 mA


  • Watt is a unit of electrical power (P)

  • Corresponds to the power developed in a circuit by a current of one ampere flowing through a potential difference of one volt: Power = Volts x Current (I)

  • a headlight connected to 12V Volts, drawing 5 Amps consumes 12 x 5 = 60 Watts

Watts cont d
Watts, cont’d

  • BMW motorcycle alternators provide the following electrical power:/5 180W R90s 238W1000cc to `80 250W recent R/K bikes 400 – 600 WK bikes (pre-2005) 700WK LT 840W

Motorcycle electrics
Ohms (Ω)

  • A Ohm (Ω) is a unit of electrical resistance

  • An ohm is the resistance that produces a potential difference of one volt when a current of one ampere is flowing through it.

  • A typical 60W high beam bulb has a resistance of 0.4 Ω.

  • 5’ of 18Ga wire has a resistance of 0.03 Ω.

Ohm s law
Ohm’s Law

  • Very useful law relating voltage, current and resistance

  • States that Voltage (V) across a circuit is equal to the current (I) times the resistance (R): V= IR or I=V/R or R=V/I

  • So… the voltage drop across a 5’ length of 18Ga wire (R=0.03Ω) carrying 5A of current is…. V=0.03 x 5 = 0.15V

Ohm s law1
Ohm’s Law

  • Use Ohm’s law to calculate the size of fuse needed to power an 80 watt heated vest:P = 80W V = 12V (P=V x I), so I = P/Vor 80/12 = 6.6 Amps (next larger common fuse size is 7.5A)

Wire gauges
Wire Gauges

  • Measured in mm2or gauge # American Wire Gauge)

  • Can use resistance table to determine total resistance of added wiring

Cross-sectional area Overload Typical Use current rating

0.5 mm² (20 AWG) 3 A instr., turn signals

0.75 mm² (18 AWG) 6 A headlight

1 mm² (17 AWG) 10 A h/l ground

6 mm² (10 AWG) 30 A battery

Batteries charging
Batteries & Charging

  • Motorcycle batteries are of the Lead Acid design

  • There are several variations of Lead Acid:

    • Wet Cell (flooded):serviceable, and maintenance free

    • Gel Cell

    • Absorbed Glass Mat (AGM)

    • Gel and AGM require less service than flooded Lead Acid

    • Gel Cell and some AGM batteries may require a lower charging rate

    • your motorcycle’s charging system usually dictates the type of battery you require

    • Gel cells have a lower recharge voltage, and can be damaged by too high a charge rate – best to use a Gel charger

Batteries charging1
Batteries & Charging

  • A fully charged battery should read 12.8-12.9 V

  • Sulfation occurs when the voltage drops below 12.4 V

  • Most premature battery failures are caused by sulfation buildup – a result of losing charge

  • Buy a trickle or smart charger, and use it frequently.

Batteries charging2
Batteries & Charging

  • Common causes of premature failure:

    • Battery sits too long between charges. As little as 24 hours in hot weather and several days in cooler weather.

    • Battery is stored without some type of energy input (self discharge).

    • "Deep cycling" an engine starting battery. Remember these batteries can't stand deep discharge.

    • Undercharging of a battery: charging a battery to 90% of capacity will allow the 10% of battery chemistry not reactivated to sulfate the battery

    • Heat of 100º F increases internal discharge. As temperatures increase so does internal discharge. A new fully charged battery left sitting 24 hours a day at 110º F for 30 days would most likely not start an engine.

    • Low electrolyte level - battery plates exposed to air will immediately sulfate.

    • Incorrect charging levels and settings. Most cheap battery chargers can do more harm than good. See the section on battery charging.

    • Cold weather is also hard on the battery. The chemistry does not make the same amount of energy as a warm battery. A deeply discharged battery can freeze solid in sub zero weather.

    • Parasitic drain is a load put on a battery with the key off. This can drain a battery in a few days

Adding lights other accessories
Adding Lights & Other Accessories

  • The most common addition is driving or fog lights

  • Wire these directly to the battery through a fuse and relay

Replacing your stock brake light with an led light
Replacing your stock brake light with an LED light

  • Recent BMW’s monitor brake light current

  • LED’s take much less current: 0.2A vs ~2A

  • You must wire a resistor in parallel with the LED light to fool the computer into thinking you have a stock brake light

  • Ohm’s law says R=V/I = 12/2 = 6Ω, can use 8Ω

  • Remember P = V X I, so P = 12 x 1.5 = 18 watts, so you need a 20W power resistor

Brighter headlights
Brighter headlights

  • Even a small decrease in voltage to your headlight can dramatically reduce its light output:

Voltage Output

10.5V : 510 lumens11.0V : 597 lumens11.5V : 695 lumens12.0V : 803 lumens12.5V : 923 lumens12.8V : 1000 lumens ←Rated output voltage13.0V : 1054 lumens13.5V : 1198 lumens14.0V : 1356 lumens ←Rated life voltage14.5V : 1528 lumens

Brighter headlights1
Brighter Headlights

  • Consider running heavier wires in parallel with your existing headlight wiring to minimize voltage drop.

  • BMW uses 0.75mm2 (18ga) wire for headlights, causing ~1.2V drop in voltage

  • Adding 12 ga wire in parallel to this will provide a significant increase in brightness.

Tools supplies tools
Tools & Supplies - tools

  • Wire cutters

  • Long-nose pliers

  • Wire strippers

Tools supplies tools1
Tools & Supplies - tools

  • Get a Digital Multimeter

    • < $8 at

  • Good crimp tool is an excellent investment

    • (try

  • Soldering iron or soldering gun for heavy gauge wire

Tools supplies supplies
Tools & Supplies - supplies

  • Tap connectors are common: be sure to use the correct size for the size of wire – too small can damage original wire, too large will not make a reliable connection

  • Heat-shrink tubing and a heat-gun will help make weatherproof connections

? ? ?? Questions ? ? ? ?