I've got a 1986 K100 RT. I purchased it from one very careful owner this Summer past. The bike had only 16000 miles on the clock when I bought it and since then - mainly in the month of August - I've put nearly 4000 extra miles on the clock. The bike has had little use since early September and in that time the battery has run flat. I tried to charge the battery so I could ride the two hundred miles from London to my parents in the North West of England for Christmas. The battery didn't fully charge so wouldn't start the bike. I made do with a push start from a neighbour and rode North. At journeys end I put the battery on charge and it achieved full charge (14 V) by morning. As soon as I reconnected the battery the starter motor started to turn over and, drawing massive current, immediately flattened the battery. This happened even with the kill switch on.
Any suggestions on the cause of the problem and a solution would be gratefully received
Philip

This is a very easy one.

This is a common problem with the K bikes. Because the voltage of your battery was low, the intensity drawn by the starter (amps) increased exponentially until the battery went flat. As a result, the contacts of the starter relay burnt and are lightly welded together.
As a temporary fix, you can open the starter relay and clean the contacts with a piece of very fine sand paper (600 grits).

Hi,
As Bert says relay sticking is a common problem when trying to start with a flat battery or suspect battery. Sometimes a sharp whack on the side of the relay box will dislodge the relay contacts apart again.

Sadly you are almost certainly in for giving the bike a Christmas present - a new battery. The flooded cell type are not too expensive, the agm (absorbed glass mat) are a bit more expensive, there have been a lot of negative reports about the genuine BMW battery longevity, my dealer parts guy says apart from being eye wateringly expensive they have not been reliable over the last year or so. YUASA have a good reputation and last well if looked after.

Can I suggest you also splash out and get an Optimate III SP or Accumate V charger and permanently connect it through the bike electrical socket above the coils whilst the bike is not being ridden, most models have one of the sockets, the plugs are available from your BMW dealer or others. I have tried the Hella plugs they are OK but the genuine BMW plugs although double the price are much better made.

Either of the above chargers will keep the battery in top condition by putting it through a complex charging process which will ensure that the battery is ready to go and either will warn you if the battery is not holding charge and either can be left permanently connected whilst the bike is standing which ordinary car battery chargers cannot as ordinary car battery chargers left on for any long period will damage the battery.

As a very outside chance the Optimate III SP can sometimes recover sulphated batteries (which your old one almost certainly is) so may be the better choice to buy and have a go recovering the old one prior to buying a new battery.

You probably already know the following but don't ever disconnect the battery after starting, if the revs rise much above tickover even a momentary disconnection of the battery can cause expensive electronic damage to components due to over voltage. All alternators without a battery present to force them to self regulate can output high voltage (I have seen 120 volts out of a car alternator which was being used as a high voltage generator) that sort of voltage on Motronic and ABS brains will fry them dead as a doornail in the blink of an eye but as I said at the start you probably already knew that sorry for banging on.
Regards,
K-BIKE

Thanks for the advice and recommendations. The Optimate looks like a must buy.
Philip

Just fitted a new relay and battery. For those who don't know, the starter relay is found in a compartment directly under the tank. The two prongs on the back of the tank that fit into gromets on the frame pull out fairly easily freeing it. The clips described in my Haynes manual were absent.
The relay is secured by two self-tapping screws and is easy to remove and replace.
Philip

Hi Philip, just be aware there are additional causes to the fault you describe and while the new battery and relay may be the solution in your case, in my case it was bad contacts in the ignition switch.

In another case high resistance in the clutch switch.

I also upgraded the earth lead and put a 100AMP kill switch in series just in case it re-occurred as there is no way of killing the starter relay if it locks on short of disconnecting the battery.

Regards,

Phil