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Old 03-06-2012, 09:29 AM   #13
SpaceManSpiff
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Joined: Mar 2007
Location: Tucson, Arizona
Oddometer: 1,075
older thread, I know...but with the qoute by frztl in white for legibility

"Hi folks.
I can see this thread has been hanging around for a while without a lot of empirical data and some of the folks I like over at F800 riders forum (those that don't HATE BMW) have been asking electrical questions as well.

I used to be a licensed industrial electrician and an automotive service manager for Volvo for many years. I used a Fluke 867B hand held graphical multimeter (most accurate hand held in the world) and a Fluke 112 (accurate enough for some things) as well as a snap-on ac/dc amp clamp to make these measurements on a 2009 F800GS

When you turn your engine off but leave the key on current draw on the battery is as follows:

Headlight switched to low beam, kill switch in KILL position: 1.74 amps

Headlight switched to low beam, kill switch in RUN position: 1.81 amps

with headlight switched to high beam: 5.67 amps

Notes: Measured with Fluke 867B current shunt, extremely accurate. Side stand makes no difference in current draw, only kill switch.


When you turn your bike off with the key the current draw is as follows:
830 milliamps for 3.5 seconds, 170 milliamps for next 3 seconds, 88 milliamps for next 54 seconds, 1.316 milliamps thereafter

Notes: Measured with Fluke 867B current shunt, extremely accurate. the spool down is just computer and CAN-bus crap, 1.316 milliamps is a very low constant draw to run the clock and save volatile memory!


When the bike is running, headlight on low beam, battery fully charged, amp shunt between a battery post and connector to factor out residual charging you come to this.

Bike running, low beam headlight, nothing else on thats switchable: 10.8 amps
Bike running, high beam headlight, nothing else on thats switchable: 14.9 amps

Heated grips are hard to quantify. I am going to list the maximum draw when the grips are first turned on but note that as they heat up they begin cycling on and off once per second with progressively less on time as the grips reach maximum temperature. Sitting in traffic on a 50F day the grips will draw less on high then they will on high while going 80 MPH on a 20F day but the absolute maximums are:

Heated grips on high, grips cold: 3.38 amps
Heated grips on low, grips cold: 2.04 amps

Note: Once again, the above is a worst case draw, as the grips heat up this typically falls to half or lower. If you are riding through Wyoming or Siberia during a cold winter day, this maximum draw might be continues, if it is above freezing this draw certainly wont be this high. Note also, the CAN-bus will shut down the heated grips if it needs the capacity to charge the battery. This has happened to me while idling with the high beams on and heated gear running. Measurement taken as above but also factoring out running current. Pretty accurate, could have been more accurate but didn't feel like tearing into the wire harness.


The cooling fan is certainly not a constant draw unless you are idling and the ambient temperature is well over 100F, or your radiator is blocked by crud :) When it is on...

Cooling fan draw: 4.7 amps

Measurement made with ac/dc current clamp clamped around cooling fan wire which was a BITCH to get to with my current clamp but readings are very accurate.


All the above is useless unless we know what the alternator is capable of. I measured the alternator when it was as hot as I could get it from idling the bike for long periods until the fan started cycling on and off. If you are riding in anything but the hottest weather the output will be slightly higher.

I used a variable carbon load and applied increasing load until the system voltage fell to 13.6 volts at various engine RPM's. I picked this voltage because the voltage regulator is sure to be full off at this point yet any AGM battery will not be discharging at this voltage unless the temperature is truly extreme. I could discuss why I didn't use 12.8 or 14.2 but that is for a battery or general electrical thread.

1,280 RPM (idle) 17.7 amps at 13.6 volts
3,000 RPM 25.9 amps at 13.6 volts
4,500-7,500 RPM 27.8 amps at 13.6 volts

What does this mean??? For those of you that use terms like "electromotive force and intensity of current" Do the math, if there is some figure you really desire, ask away.

For mere mortals: If you like adding options, you are in luck, this is not a massive touring alternator but you sure didn't buy a KLR.

What loads you can safely add depends on how you are going to use it and how you use the bike.

If you plan to walk out to the garage and eye your bike as it idles for hours on end, not very much! Without any switchable loads on you have just 6.9 amps to play with, that translates to roughly 98 watts, depending on how the manufacture rated it's product as watt draw at say 14.2 volts will typically translate into fewer watts at 13.6 volts.

Screaming down the highway stopping only when you need gas you can safely add 140 watts of accessories, even with the high beams and heated grips on high the whole way. Actually you can add a bit more because as system starts to get dragged down to, say, 13.6 volts the battery will still be fine but all of our accessories will draw about 5% less then stated.

Is it bad for the charging system to heavily load it? Not even slightly bad. A PM alternator is at full load the whole time the bike is running. As apposed to how a car alternator works, a PM voltage regulator sinks excess current to ground to regulate the voltage so it is actually the same to the alternator and easier on the voltage regulator to be under more load.

Is it bad to draw the system below 13.6 volts? Yes, even at idle it is, only bad for the battery and only a little bad as long as it is above 13.6 the majority of the time.

I could go on forever about the characteristics of AGM batteries and what kills them but that too is for a different thread.

Hope this helps, feel free to ask about anything you don't understand. I like this forum because it does not spend endless hours trashing BMW and will check back often.

Oh, some perspective. A 2008 KLR650 has a 360 watt alternator, older ones had just 192 watts. A 2008 KTM 990 adventure has 450 watts. Most other similar sized bikes hover around this area so BMW is in about the middle of the road on alternator capacity.

That said, neither the KTM's or Kawasaki's alternators start to produce 90% 0f their rated output until above 5,000 RPM. BMW's alternator produces 90% of rated output at just 2,700 RPM. BMW placed a very good alternator in this bike by comparison."
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