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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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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
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