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Running with a smaller Battery, less Amps

Discussion in 'Technical and Troubleshooting Torque' started by rollingroad, Nov 16, 2009.

  1. G'day everyone

    I've recently acquired a '56 BMW R50 from an uncle. It has been pulling a sidecar which is now not with the bike. To help the chair along, at some stage the engine was upgraded to a '71 R75/5. In the past the battery for this setup was carried in the attached sidecar.

    I will run the bike as a solo, so bought a sealed battery to suit the R75 engine. This battery is smaller than an original 1971 spec battery… but doesn't fit in the available space between the engine and the rear mudguard. Frame too small, engine too big! So. I rang a battery specialist and he will sell me a Deka battery which will fit the available space.

    The R75/5 battery should be 12v and 18Ah, this new one will be 12v but only 8Ah. The Cranking amps appear to be in an acceptable region (around 300), so I should be able to start the bike, but will I cook an 8 amp battery by running it in the bike? Don't really fancy acid burns. Advice appreciated - thanks.
  2. The bike regulator will prevent the battery from overcharging. With a smaller capacity you may have problems with cold weather riding if you do lots of short trips and need to restart the bike often.
  3. What about getting pannier bags and mounting the larger battery in there?

    This is an R60, but them bags be pretty stylish
  4. Well, I'm not an expert but. . .
    The 8 Ah battery just holds less charge. As Chris says the regulator will prevent overcharging. Basically, when the battery is full it will have a voltage the same as the regulated output as the alternator ( I assume a '71 has an alternator, and not a generaor) of 14.xx Volts, so the battery wont be charged any more.

    If the starter actually pulled 300 Amps, and you cranked it for a minute to start the bike on a cold day, you will have used 300 x 1/60 of an hour = 5 Ah of charge. You wont get a second shot at trying to start the thing! Fortunately the actual current drawn by a starter motor, after the initial effort to turn it over, is more like 30 to 50 Amps, I believe. At 30 Amps, that is only 30 x 1/60 = 0.5 Ah, so you can have a few goes at it.

    One issue with using a smaller battery is that even though it can start the bike and still have good charge left in it when the battery starts out full, a few short trips, a long time in stationary traffic or just a few starts on cold days could see the battery charge depleted more than a standard size battery would be. Batteries don't like being run to very low charge levels repeatedly. Therefore, you may see a shorter life than with a standard size battery.

    But for me, if it was going to fit and make using the bike as it is possible, I'd go ahead with the small battery anyway, and be prepared to connect it to a battery tender regularly to keep it in good shape.

    Enjoy the old bike. Fix the brakes as well, or upgrade them!
  5. I'll see what specs I can dig up, but there's a saying in airhead Beemer circles: "when in doubt, it's always the battery". These things have a BIG starter motor - usually more than 0.5 horsepower. The boxer engine is a bear to turn over cold.

    Stay tuned!
  6. Thanks for your quick replies, much appreciated everyone. So the regulator will keep the small battery from cooking, thanks Chris.

    I'll hang onto the bigger battery for a while, see how things go, DragonCypher's tip of maybe hiding it in a hard pannier is a good 'un too.

    And RoderickGI, you're right the bike has an alternator not generator. For now I'll get the smaller battery, keep the trickle charger handy and see how it goes. The restrictions of smaller capacity battery fits the expected cruisy life of this bike. If I get stuck there's always the kickstart…

    As for the brakes - we're talking cable-pulled drums front and rear - no hydraulics at all. How did they cope with a sidecar? I don't know, but I guess it explains why the lever is coming almost back to the bars. DragonCypher's pic shows a double disc front end which I hadn't seen before on this sort of bike, so who knows. Once I get this bike to go, getting it to stop will be high on the list!
  7. Took me about four goes to get a battery from Battery World to fit my R100RT.

    The battery which they had listed as for the bike was no longer made and they kept trying the equivalent. The batteries looked as near as dammit identical until you went to put them in and then wouldn't go in the frame by literally a mm. We fixed it in the end.

    So maybe shop around for a different battery brand of the same capacity which is slightly smaller?
  8. Maybe you can wear a back pack with a battery in it, just have the wires poking out and make a socket leading to the starter motor!

    Perfect, not only is this an awesome solution but nobody can start your bike without you and the backpack battery!!!

    Hold your applause, I'm here to help i need no reward.
  9. As the battery wears out and the motor goes out of tune, you will have trouble starting. You will find you need to keep the carbies cleaned and balanced a little more often and replace the battery a little more often.
  10. Learn to use the kickstart - I know the one on an R75/5 is a pain in the neck being awkardly placed on the left but it does work...

    They also push start pretty easily too. :)

  11. My brother in law put a set of Yamaha TZ750 drums on his R75/5. Same vintage and brilliant brakes. He got them cheap because all the guys racing them had changed over to discs. How you'll go doing anything with an R50front end I don't know but there are still some out there - I saw a set of TZ250 drum brakes for sale a month or so back for a couple of hundred dollars.

    Don't forget - you don't have the R75 brakes - you've got the 18 years older R50 brakes and that's why they'll be pretty ordinary.

    You don't say where you're from, but if you are in Victoria join the BMWMCCVIC - not only will you get all sorts of good advice but when the need arises you can get to service days and get the use of any special tools etc.
  12. I've bought the 12v8Ah battery from Battery Stop. Next I'll make up a battery carrier for it and see how we go.

    Thanks everyone for the suggestions. TonyE, top tip on the TZ drums, thankyou. I'm in Melbourne, so joining the Beemers MCC sounds like a very good idea so I can work through the issues with this bike and learn about these engines… this is going to be my "apprenticeship" boxer I think.

    And Zealt, top suggestion! **I am Buzz Lightyear** =D
  13. If you go with the small battery, I'd suggest turning the motor over on the kickstart a couple of times for the first start on a cold day. Another trick I've seen at the Alpine is to light a small fire under each cylinder to warm it up. Just make sure the carbs aren't leaking!