The document discusses motorcycle electrical systems, covering key concepts such as volts, amps, watts, and ohms, as well as their applications in motorcycles. It provides information on batteries and charging, common electrical accessories, and tools necessary for maintenance and modifications. Additionally, it highlights the importance of understanding voltage drops and proper wiring to enhance performance and reliability.

1. BMWBMW Square Route Rally - 2006
Motorcycle Electrics
By
Jerry Skene

2. 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

3. 3BMWBMW Square Route Rally - 2006
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)

4. 4BMWBMW Square Route Rally - 2006
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

5. 5BMWBMW Square Route Rally - 2006
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

6. 6BMWBMW Square Route Rally - 2006
Watts, cont’d
• BMW motorcycle alternators provide the
following electrical power:
/5 180W
R90s 238W
1000cc to `80 250W
recent R/K bikes 400 – 600 W
K bikes (pre-2005) 700W
K LT 840W

7. 7BMWBMW Square Route Rally - 2006
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.

8. 8BMWBMW Square Route Rally - 2006
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

9. 9BMWBMW Square Route Rally - 2006
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/V
or 80/12 = 6.6 Amps
(next larger common fuse size is 7.5A)

10. 10BMWBMW Square Route Rally - 2006
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

11. 11BMWBMW Square Route Rally - 2006
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

12. 12BMWBMW Square Route Rally - 2006
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.

13. 13BMWBMW Square Route Rally - 2006
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

14. 14BMWBMW Square Route Rally - 2006
Adding Lights & Other Accessories
• The most common addition is driving or fog
lights
• Wire these directly to the battery through a fuse
and relay

15. 15BMWBMW Square Route Rally - 2006
Adding Fog Lights

16. 16BMWBMW Square Route Rally - 2006
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

17. 17BMWBMW Square Route Rally - 2006
Brighter headlights
• Even a small decrease in voltage to your headlight can
dramatically reduce its light output:
Voltage Output
10.5V : 510 lumens
11.0V : 597 lumens
11.5V : 695 lumens
12.0V : 803 lumens
12.5V : 923 lumens
12.8V : 1000 lumens ←Rated output voltage
13.0V : 1054 lumens
13.5V : 1198 lumens
14.0V : 1356 lumens ←Rated life voltage
14.5V : 1528 lumens

18. 18BMWBMW Square Route Rally - 2006
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.

19. 19BMWBMW Square Route Rally - 2006
Tools & Supplies - tools
• Wire cutters
• Long-nose pliers
• Wire strippers

20. 20BMWBMW Square Route Rally - 2006
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

21. 21BMWBMW Square Route Rally - 2006
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

22. 22BMWBMW Square Route Rally - 2006
? ? ?
? Questions ?
? ? ?