I wrote earlier at some length about battery capacity requirements for K bikes.

This can be briefly summarised as:

• The battery is only used briefly to start the engine, thereafter sitting quietly being charged and then doing nothing.
  
• If all is going well, quite a small Ah (Amp hour) rating is all that is needed for that brief task.
  
• Extra capacity can be useful if you have a starting or other issue that means you need to use the starter a lot more than normal, or if you need to run electrical devices while the engine isn't running. Extreme cold will also increase requirements.
  
• The starting current requirement, usually described as the CCA (Cold Cranking Amps), isn't specified anywhere that we know of, but Rick G infers that the 75A rating of the starter relay gives a clue.
  

Hence the title of this thread - my original K, henceforth known as Big Red, had a battery that decided to no longer carry out its function. (Dead cell, I think. Attempts to charge made no difference, regardless of the charger.)

Being the cheapskate that I sometimes am, I bought a battery that is somewhat smaller than standard or recommended.
It is only 12Ah, about half the standard rating, with a CCA value of 200 Amps.



I had to insert a spacer so that the battery clamp would do its job, but otherwise, all good.

As it hadn't been run for a while, I gave it a few brief touches of the starter button to run the fuel pump and pressurise the fuel system, gave it the first stage of 'choke' and pressed start.
Result, instant fire up.

So far, a half capacity battery has done all that is required. We'll see if my faith is justified or misplaced. 

Bill

If you are in a warm climate, using the K regularly it will work without any problem.

Battery is only needed for starting the engine but there is a slight parasitic drain in that the clock stays live with ignition off. Once running the alternator provides power and also replenishes the battery. If its left for longish periods the drop may be enough to affect starting.

For cold winter climate the alternator may be slower to replenish the battery in that you will have lights and heated grips using some of the output. Also more power is needed for starting on a cold morning.

So my K100RT has a 32Ah battery in it with 300CCA. From my local lawnmower shop. Works perfectly and never lets me down. My K100LT also got a similar battery.

I don't know what reference outside temperature is assumed when the required CCA rating is calculated, but given the lead acid battery technology of the time it would have been calculated by BMW most likely for the colder climates they were marketing in.

agree olaf

for me up here travelling home in peak hour has the fan running for 5  minutes at a time at idle at every stop light on the way to and from work  ...when the battery isn't being replenished at idle revs .

with lights on this becomes a real pain with an undersized battery ...have been embarrassed a couple of times as the battery drops volts .at a stall event ...very hard to get a restart straight away ...usually have to wait till it recovers somewhat ..
cheapskate batteries just don't work well for me

cheers

The only way to measure a battery is on CCA for bike or car application in these modern times. Olaf chooses to use some old 1930s technology lawnmower battery which he claims is 300 CCA, where he could use a much better modern Yuasa that weighs a quarter of what he has with about the same starting power, but it costs €10 more to buy that battery.
Olaf also has a very expensive set of alloy tent pegs on board to save weight, go figure.

I remembered CCA was specified at a pretty cold temperature, but checked it out.

CCA: "The rating refers to the number of amps a 12-volt battery can deliver at 0°F for 30 seconds while maintaining a voltage of at least 7.2 volts." 

Even in Goulburn in winter, that's asking more than would be required. If it was that cold, I wouldn't be riding. I'm not sure I would be as lucky or skillful as smithy on black ice.

By contrast, my boat motor (Evinrude e-tec, 90HP) demands 640CCA above freezing and 800CCA below freezing.  

Putting that battery in is a cheapskate experiment, knowing I have other bikes available if I have an issue. 

Bill

indian036 wrote:

• ... the 75A rating of the starter relay gives a clue.
  

But only a clue.  75A is the continuous rating with a resistive load.  The motor is an inductive load, which would lead to a lower rating, but the short term amp rating could be several times the continuous rating.  Which is to say that it could likely handle a much higher rated starter motor.  The referenced ad below for a replacement K bike starter says that it is rated 1kW at 270 amps.


https://www.euromotoelectrics.com/product-p/bmw-strk007r.htm

Another piece of the puzzle, thanks beemerphile.   They don't often give full information, so I didn't realise that rating was only for a resistive load.

I'd expect that it wouldn't run at its full rating under normal circumstances, so the actual starting current is likely somewhere in between.

Trying out the new battery, as I said, gave no starting problem, but it was also about 27C, (about 80F) so it would be capable of delivering more than its 200CCA given it wasn't at all Cold.

Bill

BobT wrote:The only way to measure a battery is on CCA for bike or car application in these modern times. Olaf chooses to use some old 1930s technology lawnmower battery which he claims is 300 CCA, where he could use a much better modern Yuasa that weighs a quarter of what he has with about the same starting power, but it costs €10 more to buy that battery.
Olaf also has a very expensive set of alloy tent pegs on board to save weight, go figure.

Olaf also has a nice Odyssey battery and a €10 [including postage] set of alloy tent pegs.....He's happy. His bike sounds happy too, speaks a different language now with a new Remus exhaust. No more cracked headers...

As for the lawnmower battery in the RT, well it was a Saturday morning and I needed a battery and the lawnmower shop was cheap as chips....and its never given a moments bother.

Olaf we are talking about you, there is no need for you to post on this thread!   I will inspect your battery and tent pegs when you arrive.

BobT gave me such grief about my brick sized batteries on my last trip to France that I bought an Odyssey battery while I was camped at St Leon.

Trouble was it came with a K1100LT attached to it.......

No idea what sort of CCA it has but I suppose I could look at the receipt. But given out cold winter weather and that it works every day I guess the CCA 'is adequate'. But so is the lawnmower battery that started the K100RT first turn after 6 weeks. I suppose its adequate too.

Yep, you will be inspecting it in person.

If I am at home when you are here! We might be bombing around Europe on the three Ducatis.

All this hot battery talk!

Building my minimalist 85 K100RS, looking for the smallest battery possible.   Found this at 4.25"x1.25"x3.75" and 12oz weight (lithium).  Says it will start Big V8 trucks over 10 times in a row before a recharge - made for 1300cc high comp race bikes.

https://www.amazon.com/dp/B00VM7O6NC/_encoding=UTF8?coliid=I2Z84DPYZ1UB17&colid=1USI5INJCZ9GN&psc=0

If I use it permanently on the bike, will the factory alternator charge it?  My K is a toy only - no long distance rides or stuck in heavy traffic for long.  I don't mind having to "plug it in" to a battery maintainer when it's parked in the garage.  Just curious if I could get by on a day of riding with it.

BobT wrote:If I am at home when you are here! We might be bombing around Europe on the three Ducatis.

The Three Brexits attack Europe..!

boostd4 wrote:All this hot battery talk!

Building my minimalist 85 K100RS, looking for the smallest battery possible.   Found this at 4.25"x1.25"x3.75" and 12oz weight (lithium).  Says it will start Big V8 trucks over 10 times in a row before a recharge - made for 1300cc high comp race bikes.

https://www.amazon.com/dp/B00VM7O6NC/_encoding=UTF8?coliid=I2Z84DPYZ1UB17&colid=1USI5INJCZ9GN&psc=0

If I use it permanently on the bike, will the factory alternator charge it?  My K is a toy only - no long distance rides or stuck in heavy traffic for long.  I don't mind having to "plug it in" to a battery maintainer when it's parked in the garage.  Just curious if I could get by on a day of riding with it.

If it starts the bike then it will charge off the bike. I had a smaller lithium, weighed in at 600 grams on one of my Ducatis and had no problem with that starting the high compression big twin.

92KK 84WW Olaf wrote:

BobT wrote:If I am at home when you are here! We might be bombing around Europe on the three Ducatis.

The Three Brexits attack Europe..!

I think that you are confused. Jude did not vote for brexit, Trish did, and I was not allowed a vote despite being a UK tax payer. 
I don't attack Europe, I live here.