This is meant to be a guide for you to use so you can follow along with the full video to help make it easier to understand. (Full video at bottom of page)


Want to know how to add electrical accessories to your motorcycle without over drawing from your charging system? In other words, without killing the battery. If you answered yes, then this article is for you. (This could be a GPS, phone charger, heated gear, auxiliary lights and anything else that draws power from your motorcycle that isn’t always on when the bike is running.)


To begin, we will need to understand some basic electrical principles.


Section 1: Relationship of volts (v), amps (a) and watts (w).
On the most basic level, all we need to know about electricity for our purposes is that volts multiplied by amps equals watts. Or for short:

V x A = W

If we know the watts and need to find amps we divide like this:

W / V = A (watts divided by volts equals amps)

Motorcycles operate at around 12 volts so we can replace V with 12 like this:

12 x A = W or W / 12 = A


Example: My motorcycle’s charging output is 23A (23 amps) at idle so, to convert to watts do this:

12 x 23 = W or 12 x 23 = 276w


Section 2: Charging system basics

There are three components of a charging system. The last one is irrelevant for our purposes, but let’s not leave anything out:

1) Battery – Starts the bike and stores power. Will also compensate for a lack in output from the alternator. This is what will kill your battery if you exceed the output of the alternator for too long.
2) Alternator – Charges the battery and powers components. For example the headlight gets power from the alternator.
3) Regulator/Rectifier – Rectifier converts VAC (alternating current) from the alternator to VDC (direct current) for the battery and other components. The regulator will keep the alternator from over charging.

Note: The output of the alternator is directly related to the engine speed or RPM. At idle the output will be lower and therefore cannot support as much of a load. As the RPM increases, the load capacity also increases. This is important to know if you are knowingly exceeding the output of the alternator.  (See diagram below.)


Section 3: How to find the available charging output for accessories

STEP 1) For this first step we will need to know somethings about the motorcycle in question. Some information will be found in the manual, some with Google, and some may need to be guessed at based on normal or average numbers for similar motorcycles.


Let’s use my motorcycle as an example, a 2015 BMW R1200GS.

The total charging output can be found in the owners manual or service manual. It will most likely be a range starting at the output at idle and then the output at a certain RPM. Mine looks like this:

Total charging output at idle: 23 amps

Total charging output at 6,000 RPM: 35 amps

We can split the difference based on normal riding conditions, for me that is about 5,000 RPM. We will call it the middle number and say 29 amps to be on the safe side. This is an underestimation. 

Now we need to use our formula (VxA=W) to convert this number to watts as this is what information most accessory manufacturers will give you.

V x A = W or 12 x 29 = 348 (Let’s round that up to 350 watts)

It’s also a good idea to convert the output range to watts. So that looks like this:

Range of output in amps: 23 amps to 35 amps (idle to 6,000 RPM)

In watts that looks like this: 276 watts to 420 watts

This is good to have for future reference.

STEP 2) Find the spare output for accessories by working backwards from the total output and then subtracting the loads that are always on when the bike is on. We know the total output is 350 watts, so we will be subtracting from that.



The common loads that are constantly on are:

  • Headlight
  • License Plate
  • Tail Light
  • Instrument Cluster
  • Computer
  • Fuel Pump
  • Cooling Fan
  • Ignition




The typical loads for these items can vary but they look something like this:

  • Headlight: 55 watts
  • License Plate: 5 watts
  • Tail Light: 10 watts
  • Instrument Cluster: 10 watts
  • Computer: 25 watts
  • Fuel Pump: 60 watts
  • Cooling Fan: 50 watts
  • Ignition: 25 watts


This gives us a total of: 240 watts. This is what is always pulling from the alternator.

Let’s subtract that from our total like this:

350W (Total output) less 240W (Constant power draw)

350-240=110W (Cushion for accessories)

Now you know how much wattage you can add without causing charging issues.


Now let’s look at some common accessories you might want to add.

  • GPS: 5 watts
  • Auxiliary Lights: 50 watts
  • Heated Gear (per piece): 85 watts
  • Phone Charger: 3 watts
  • Heated Grips: 50 watts
  • Radar Detector: 2 watts

This gives us a total of 195 watts. This is above our cushion so if we have all of these accessories running for too long, we will drain the battery. It’s best to limit it to two big items like the heated gear and heated grips, and skip the auxiliary lights. Or some combination like that.

If you want to increase the wattage cushion try these tricks:

1) Convert light bulbs to LED to save wattage
2) Add a circuit to turn off your LO beams when you turn on your HI beams, if your motorcycle doesn’t already have this
3) Change your fuel filter as this can double the load of the fuel pump if its dirty

Note: Ah or amp hour rating of a battery: How long a battery will last when being drawn from at a specific rate. This is important to know if you are knowingly overloading the charging system so you don’t go too far with it.


Section 3: Where to get power

 Avoid tapping into a CANBUS system. This will set off fault codes. 

The risks of hooking directly to your battery

1) It becomes messy if you are adding multiple wires. This can cause shorts and can cause the battery terminals to come loose.
2) There is nothing stopping the accessory from draining the battery when the bike is off.

Instead, use a relay (it’s just a switch) to make sure the accessory only has power when the ignition is on.

If you are adding multiple wires to the relay consider using a distribution block to get all your wires connected to one place. This will be downstream of the relay so everything turns off with your ignition.


Section 4:  Running wires on your motorcycle

There are some common causes of failure and malfunction when running wires that can easily be avoided. They are:
1) Heat. Poor insulation (cheap wire), and too close to hot parts (exhaust).
2) Tension (poor wire routing choice).
3) Loose connections.
4) Rust and oxidation.
5) Moisture, keep it in a dry spot.
6) Moving Parts. Even your seat, which seems stationary actually moves when you get on and off the bike. Handlebars are another problem area.
7) Cheap connectors should be avoided.
8) Pulling connectors apart by the wire instead of by the connector. This will pull the wire out of the connector.


PRO TIP: Make sure to test everything before you tighten bolts, replace panels and even before zip tying wires. This way you don’t end up doing twice the work if something isn’t right. 


Full video (not live until 6/15/18 at 5:30am):




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