Battery question

[marcmax]

At the risk of sounding completely ridiculous, what is the purpose of the battery on an R65? I know it powers the starter but if you bump start or kick start does it still need to be installed?

I have seen a number of BMW's cafe'd with a battery bungied where the air box used to be. Why can't it just be removed. If it needs a battery installed to energize the ignition does it have to be a big battery? How about a small battery relocated in a hidden location, say under the tail piece or where the tool tray resides.

[Bob_Roller]

On a lot of vehicles that are used for transportation purposes, the charging system does not provide enough power to satisfy the electrical system demand at low engine speeds .

You need an electrical 'reservoir' to store electrical energy when the charging system isn't capable of doing the job itself .

On earlier bikes, the ignition system was a magneto type, just like on small gas engines today .

Some bikes had an electrical  system work off of the magneto magnets, a very low output capability, that would just power a head light and tail/brake light .

[Barry]

On some bikes it is theoretically possible to replace a battery with a "battery eliminator" which is just a large capacitor but frankly on an airhead the alternator just wouldn't work at bump start speeds so there would be no spark.

You could use a much smaller capacity battery and that would still work if you abandon the electric start facility. A version of the original GS model came with kick start only and a smaller battery.

[marcmax]

I happen think that an airhead looks good as a cafe racer but I always thought it looked unfinished with a battery bungied to the top of the gearbox. I have been told that an airhead will not run without a battery even one stripped of all lighting requirements for racing, etc.

[Bob_Roller]

You need 12VDC excitation voltage to the alternator rotor to get the alternator to work .

You may be able to run without a battery, if you had generator, instead of an alternator .

A generator has residual magnetism due to permanent magnets, they have magnetic lines of flux, that when the armature rotates, the lines of magnetic flux, pass through a conductor like copper, will start to produce electrical flow without an external source of power to get things started, like an alternator needs .

[steve hawkins]

I kept my normal battery setup with my cafe Racer as I could not see the point of moving it, even though I have removed my airbox.

The current battery tray is anti-vib mounted, does not look half bad with the side panels removed and is fit for purpose.  Assuming you paint it of course...

But, I suppose the fly in the ointment is the battery itself - But I am sure it could be beautified if you felt that way inclined.  

At one point in the distant past, I considered, as a way to drop some weight, looking for a 4 speed kicker gearbox, so that I could lose the starter all together - but as mentioned above, you still need a battery.  And it was going to cost a fair bit to source and rebuild said gearbox - A cheaper way to go would be to replace the Bosch started with a Valeo as long as the teeth matched up.  A fair few lbs saved.

[Barry]

There are lots of big capacity twin cylinder bikes that use smaller batteries than ours. I reckon if a valeo or better still a Nippon Denso starter was used to reduce the cranking amps it would be possible to have the electric start work OK on a smaller battery - say 12 Ah. That would save quite a few lbs on starter and battery combined.

Even if the battery didn't last as long they are only half the price of larger batteries

[marcmax]

I think Steve's example of painting the battery box to match the rest of the frame looks great. I have always just been curious about all the people that feel the need to mount their battery on the gearbox. Maybe an anticipated shift in the center of gravity for handling? After all the battery is a pretty hefty chunk of lead.