My V-Strom Headlight Modification

The Problems

• The Strom has a weak alternator - you can only draw about 75-100 watts of additional power before the charging system can't keep up and you start draining the battery.  Since there are two low beams (and two high beams), if you could turn off one of the headlights you'd get another 55 watts of power available.
• With a bunch of accessories, it would be nice to know when the alternator is managing to keep up with the load, and when it isn't.
• The Strom's low beams are pretty good.  The high beams are OK, but they leave the road pretty dark.  Combining the two results in a much better beam pattern, at the expense of a bunch of power consumption.  If you balance the headlight switch right between the lo and hi positions, you can manage to get both to come on at once, but it's tricky and can't be good for the switch.
• By the time power gets to your headlights, it's passed through two switches, a fuse, and a lot of wiring.  That adds a fair amount of resistance, lowering the voltage that the bulbs see and thus the light output, and wasting some of your precious power on keeping your wiring harness warm.  With my Strom stock, I measured the voltage at the headlight bulbs at 11.5 volts, while the battery was at about 13.5 volts.

My Solution

I decided to add a three-way toggle switch to the left handlebar controls, as seen at right (in the photo the switch is in the middle position).  The three positions are:

• STUN: in the top position, with the headlight switch on high beam, both low beam bulbs and both high beams bulbs are lit.  With the headlight switch on low beam, just the low beam bulbs are lit.
• SQUINT: in the middle position, with the headlight switch on low beam, just the left low beam bulb is lit.  The high beam position works as normal.
• STOCK: in the bottom position, both low and high beams work normally.

I thought about adding a OFF position (non-US bikes already have this), but four-position toggle switches are rather hard to come by, and I didn't want to add another switch.  Plus, I don't see all that much need to turn the headlights off.

All of this headlight trickery works by switching relays on and off on a relay board I added to control the lights.  This means that my switch doesn't have to handle high current, and also avoids the resistance problem described above.

In order to monitor the state of the charging system, I added a digital voltmeter, as seen to the right.  With the bike off, it reads about 12.1 - 12.5 volts, depending on the state of the battery.  With the bike running, above 12.5 volts means the battery is charging, below 12.5 means the battery is discharging.  The charging system will bring the voltage up to 14.0 volts, if it can.

With both low beams on, the voltmeter reads about 13.5 volts at idle, and 14.0 volts at anything higher than idle.  With my Gerbing heated jacket and gloves turned on full (the jacket should draw 77 watts and the gloves 22 watts, for a total draw of 99 watts) and both low beams on the voltmeter reads about 12.5 volts - just breaking even; with just one low beam turned on, the voltmeter reads about 13.5 volts - healthy; with both low and high beams on, the voltmeter reads 11.5 volts - beware.

The Details

Here are images of the top and bottom of the relay board:

The board is a little proto-board from Radio Shack, measuring about 1.75" x 1.75".  I don't know if Radio Shack still sells them; I found it in a pile of junk at work.  The relays are made by Potter & Brumfield, model # T77S1D10-12. They are speced at 10A @ 30VDC with 100,000 cycles, so they should last a good long time.  There are two diodes (nestled between the two rightmost relays in the left image above) which make the SQUINT mode work.

When I get a chance, I will add a schematic and part list here.

The voltmeter is made by Martel Electronics, part # QM100V.  It cost $44 from Digi-Key.  It's nice for this application because it's water resistant, requires a single 1/4" hole to mount, and only requires two wires hooked up (power and ground) - many other panel-mount voltmeters require +5V in addition to the voltage to be measured.

I used standard automotive insulated blade connectors to splice into the wiring for each headlight - the relay board intercepts each headlight wire, uses it to drive a relay, and the output of the relay drives the headlight.  One nice thing about using male and female blade connectors is that if the board dies or a future owner wants to remove it, one can just reconnect the blade connectors to bypass the relay board.  To the right is an image of connectors for one headlight.

The relay board gets power by a separate fused wire that runs direct from the battery, for minimal voltage drop.  In the image to the right, it's the wire with the yellow fuse holder.  The other stuff supplies power to other accessories.

I wrapped the relay board in a giant piece of heat-shrink tubing with some material hanging off the end.  The flat metal in the third image is just used to get a nice flat tab off the end.  Then I zip-tied the flat portion to a frame tube - the excess heat-shrink material will cusion the relay board from vibration.

Regrets

If I did it again, there are a few minor things I'd change:

• Put the voltmeter on the left side of the fairing rather than the right.  The throttle cables block my view of the meter while riding.
• Use a different toggle switch.  This one has a very light feel, so it's a little finicky to get to the middle position with gloves on.  Plus, the lever sticks out too far, so I often bump it and change its state without noticing.

That's about it, actually.  STUN mode gives glorious light for night riding.  I use SQUINT mode with heated gear on my morning commute to work, and the extra charging makes up for my commute home (which is in the dark, usually with the heated gear on full and liberal use of STUN).  The voltmeter is fun to watch.  While starting the bike the battery voltage dips down to about 8V.  Wow!

sethml@ofb.net