rpm

[Vegasrandall]

what engine speed do you try to maintain to keep the electrics happy?

[montmil]

You do tend to get less alternator charging activity below 4K rpm. Lots of city riding will eventually drain the battery to the point that on a Friday morning when you're running late, the bike will refuse to wind up.

I keep a Battery Tender Jr plugged into at least one of the BMWs most all the time. Sometimes I forget but that's just one advantage of having two R65s. [smiley=thumbsup.gif]

Monte

[suecanada]

4000 and over as Monte says. Unless you get an upgrade kit to produce more than 280 watts from the alternator. Motorrad Electrik kit or Euromotoelectrics kit. But the bike seems happier at 4 and over whatever route ya take. I did run a battery down years ago because I didn't know this and had 1/2 city driving on my 45 mile commute.

[Vegasrandall]

I love it when someone says you have been riding like a wuss and need to go faster,it's like your doctor saying more red meat!

[Barry]

It's difficult for me to exceed 3500 rpm on my daily commute except very briefly in the lower gears and not a good idea anyway on a cold engine. My generator light is never on even at idle so although it may not be charging much it is charging. In the absence of a longer run at the weekend I give the battery a short top up charge and never have any trouble at all. I wonder if that's because the points ignition bikes draw only half the current of the later electronic system. I suppose even a running headlight will take a toll. I use a 20w halogen bulb in place of the pilot light which is surprisingly bright and draws 3 amps less than than the head light. If I had LED rear lights that would save a bit more especially the brake light which I use a lot at junctions.

I confess I don't understand concerns about the airhead charging system which in its day was better than just about anything else around.  Issues should only arise with excessive load from heated grips/clothing etc and only then would I consider alternator upgrades.

[Ed Miller]

"I confess I don't understand concerns about the airhead charging system which in its day was better than just about anything else around.  Issues should only arise with excessive load from heated grips/clothing etc and only then would I consider alternator upgrades."

Now picture me coming to BMWs with only my old Triumph for comparison, with it's 120 watt alternator!

[Barry]

Now picture me coming to BMWs with only my old Triumph for comparison, with it's 120 watt alternator!

Exactly.

I remember you could upgrade the alternator on my T140V from 120W to a massive 190W but I never needed to do it !
We didn't have electronic ignition or run daytime lights back in 1975 though.... but you did in the US ?

[Bob_Roller]

I think the main culprit, is the lack of charging at low rpm's .

My first bike was a '79 Yahama XS400 vertical twin .

I rode this bike for two years before getting the '81 R65 .

In those two years, I had no issues with keeping the battery charged, it was a 12N12A type battery, not a large battery at all .

After getting the R65, I had low charge problems with the battery within 3 weeks of riding the bike the same way that I had done with the XS400 .

I think a different regulator that would give you more charging system output at lower rpm's would have gone a long way in solving this problem .

[Barry]

I suppose the XS400 would have been revving a little higher and maybe didn't have electronic ignition. Come to think of it my R45 is always going to be revving several hundred RPM higher than an R65  which must help with charging.

Coming back to the original question Snowbum has some specific RPM figures:-
 http://bmwmotorcycletech.info/electricalhints.htm
"Any stock airhead electrical system, from the /5  180 watt system, to the last 280 watt system, will maintain the battery in a charged condition, from something like 2200 rpm....to certainly not over 3500 rpm....ONCE the battery is fully charged.   The later model airheads have some slightly higher power usages, and almost always will require 2800 minimum.  Adding an electric vest will raise the rpm required a bit....perhaps a FEW hundred rpm.  Continuous riding below 2800 is not good for the engine anyway, and may well not allow the battery to be recharged from, perhaps, sitting at a stop signal in city commuting.    Any really appreciable electrical load may well required 4000-5000 rpm, continuously.     On a practical basis, considering sometimes poorly maintained electrical systems, old components, etc., don't plan on over 80% of the rated watts output of the alternator"

More  comments here

http://bmwmotorcycletech.info/altcapability.htm

and he did some tests on the after market upgrades

http://bmwmotorcycletech.info/AftrMrktAlt.htm