motorcycle electrics jerry skene

BMWBMW Square Route Rally - 2006

Motorcycle Electrics

By

Jerry Skene

2BMWBMW Square Route Rally - 2006

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

• 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)

• 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


Watts• 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• 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

• 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 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• Measured in mm2

or 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• 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 & 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 & 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• The most common addition is driving or fog

lights• Wire these directly to the battery through a fuse

and relay


Adding Fog Lights


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 • Even a small decrease in voltage to your headlight can

dramatically reduce its light output:

Voltage Output10.5V : 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 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

• Wire cutters

• Long-nose pliers

• Wire strippers


Tools & Supplies - tools

• Get a Digital Multimeter– < $8 at Allelectronics.com

• Good crimp tool is an excellent investment– (try Starkelectronics.com)

• Soldering iron or soldering gun for heavy gauge wire


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 ? ? ? ?

motorcycle electrics jerry skene

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