Not enough POWER CAPTAIN! I think...

Hey all,

Firstly I know this subject can be found with a google search, but the solution given is fit all electrics and test to see if battery is draining, if so turn some stuff off.

Doesn't help me much, basically I have a 2013 F800gs apart from heated grips, GPS (to be installed), HID Hi/Lo beam (35w each). That is all the juice I am using so far, I don't use heated clothing and won't be in the future, outdoor adventure junkie so I layer if its cold.

What I am currently undertaking is fitting some AUX lights, got BMW bracket ordered now just working out which ones. I have decided on Clearwater (know its expensive) but I ride a lot to work and at night. I am looking at their lights and their Wattage rating. I can fit the larger ERICA/Krista lights but the erica's are 60W each on high. So this brings me to my question (finally I KNOW!!!)

If I have grips, GPS, High beam and ericas on full (120W for Erica's) is that tooo much for the F800gs 400W alternator?

Could go Kristas at 70W high beam combined but if I can want as much light as I can pack into a single unit so the ERICA's are really what I am after....

Cheers for any input.


Got told was 150W to run the bike not sure how accurate that is.

Assuming your 150w figure for running the bike is correct you should have no problem. That's a big assumption though, there's a lot going on in a modern fuel injected bike that uses power! Without an accurate figure it's always going to be guesswork.

Do you have something else that is about the same power requirement? Like a car headlight - even one attached to a car? Then you can temporarily hook that up to your bike and measure the battery voltage with the load.

The alternative it simply to travel slower on your standard lights - the extra lights just mean you can travel faster (when they are on).

What about your fuel pump? Surely that draws significant current?

I've been told, several times, that these days, if the battery is a bit suss it is almost certain that it wont have the threshold power to start the engine & run the EFI.
So, your calculations need to be accurate.

Isn't it something like Watts = volts x amps? Total wattage is simply the sum of the wattage of each device? Nothing else to it?

Yep.

When the EFI fuel pump runs flat out it is about 5 amps so 60 watts. But if you have a fuel pump controller it won't run flat out - it will run at part power to suit the engine demands.

But none of that will tell you if it will work on your bike.. run a voltmeter and you 'll know - no calculations, no errors.

That's true.
When you travel slower your lights last longer.

PS: The above comment may not actually help the OP.

Short answer is no, you won't have a problem.
If the alt is as you state a 400w unit, it is almost certainly designed to have some decent accessories, including lights, fitted.

Tis a simple equation - look up ohms law.

http://www.ohmslawcalculator.com/ohms_law_calculator.php

No need for Ohms law.
Just make sure the total load doesn't exceed the capacity of the alternator.
Both are expressed as watts.

yeah some things have current draw expressed as amps, so the equation is handy if you want to add it all up.

i think the op will be right. worst can happen is a flat battery if the alternator cant keep up

went spotlighting one night on the farm with a 100w spottie bodged in to the 4 wheeler. didnt know the bike couldnt keep up till i rode it into a ditch, stalled it. wouldnt pull start, so it was a long walk back to the farmhouse

Just add up ALL the devices using power
Headlight 55
Tail-light ? 15
Instrument lights 5
Etc etc etc

Add them up and they should come safely under the 400watts delivered.
Also remember they may claim 400 but really might be 380 for example.

If your sums add up to anything greater than 350 i would be cautious and maybe give it to a BMW dealer to wire it up.

Hey all thanks for the reply.

Agreed should be a very simple calculation. But have struggled to find anywhere that gives up any information on how much the bike needs power wise just to run. Fuel, Canbus system etc.. .....

Hi/Low beam 35W HID each so 70 Watts.
Grips 35W (so Im told)
GPS (not sure how much they draw when plugged in.)
AUX Lights

Lights + Grips + AUX Lights + GPS

If I knew how much juice was left from the alternator (even BMW doesn't seem to know, which is handy. or unable to tell me.... ..) would be an easy decission....

Have three options lights wise as far as power consumption goes, prefer to be able to ride hours on end without draining the battery.

Option 1: 48 Watts for pair Darlas
Option 2: 70 Watts for pair Kristas
Option 3: 120 Watts for pair of ERICAS

If I went Option 3 like I want: 120 + 35 + 70 + GPS (unknown) subtract from 400..

Good old maths states 175 Watts left for bike system and GPS. If I went with Option 2. I'd have 225 watts left.

All depends on how much juice the Bike needs for normal operation. Not going to be having AUX lights on for ages with bike idling. Perhaps easier to Go Option 1 or 2 and have Juice to spare. Not knowing bike specs on this sucks balls though... :huh

gps will be fuck all. mine is on a 1a fuse. 10w max

Alternator is variable output , depending on RPM of the motor driving it too. (400W at high revs).

not variable, the voltage regulator dumps unwanted juice as heat :d

Settle......

Found this, best bit of Info I have found so far.....

http://mistergizmo.com/bmw/f800electrical.pdf

Comments...

352 Watts at 3000rpm.
Erica Lights 120 watts
Low Beam Baseline 147 watts (my low beam and High are 35W HID so save a bit of juice here..)
High Beam 56 watts
Grips 46 (on high)
Cooling Fan 64

= 433 Watts... Bit over me thinks (393 with HID conversion)

With Krista brings it down to 363 Watts (323 minus watts saved from my HID headlights). High and low beam now 35 watts instead of 55 watts so save 40 watts there.. Still food for though...

not sure about your 800 but on high beam are the low beam bulbs still on?

Yeah on High beam both Hi and Low beam bulbs are on. High beam on the F800 kinda illuminates above the Low beam line (for more distance) and fills in around the edges of the low beam a bit more.

Err, there's more to it than that.
A regulator doesn't simply dump unwanted energy as heat. It fundamentally regulates the voltage. (and maybe a bit of current. ) The alternator will produce a voltage that will vary with rpm. It's rectified and then the regulator smooths this out and applies a steady voltage to the battery regardless of rpm. (almost ) The power capacity of the alternator will determine what current it can deliver and as the load increases the current demand increases. The voltage supplied to the battery remains constant. A good regulator circuit will generate little heat regardless of the load or rpm.