Hi all, very sorry about this as I'm sure I'm going over old ground but I can't find a straight answer. Will a 51913 battery (original fitment to lots of BMWs, apparently including the K75) fit and be secured in the battery tray of an 8v K100 with no additional modifications? Do I not even need a different "top"?

My old 53030 is nearly dead and the ABS never sets, which is bad when the MOT is coming up.. Plus 51913s are a bit cheaper, much lighter and much easier to fit through the hole in the frame! Removing and fitting a 53030 is like wrestling a bear, as I'm sure you all know.

Thanks all!

ABS is very sensitive to battery voltage. Also to sensor clearances. Get those right then get a loan of an ABS brain to see if you can get it going.

Tosi in Japan does a repair too.

I'm pretty sure it's just the battery, it's very weak - slow to turn the motor over and needs charging if I leave the bike stood for 2 weeks or more. It has been going downhill for years..

I see you can get real proper gel 51913s on ebay for £50, I'd be happy with that. YTX12-BS's are £30 but you gotta pay the BMW tax, right?? Otherwise how could we all feel so good about ourselves..!

As far as I am aware, the ABS is not tested at the MOT. The bike would need to be ridden for it to finalise its self checks before the warning lights were extinguished anyway.

Cars are a different matter altogether.

Mike

First off, ABS is sensitive to battery voltage as Olaf mentioned.  I have had a couple low voltage ABS faults when starting a bike with an old battery after an extended period of inactivity.  I just check the code and reset it.  No big deal.

As far as the battery size, there is no difference in the running load(lights, heated grips, engine management, etc) between the K75 and the K100.  The starter motors are interchangeable, the only difference is that the k100 has one more cylinder, and I'm not sure that makes that much difference since the starter is only pushing one compression stroke at a time.

Since the only difference is the number of cylinders, as long as you have adequate cold cranking amps the size of the battery will only be a problem if you have a problem getting the engine to start and you have to crank it for an extended time.  The smaller battery has about 10% less amp hours so it will run out of power sooner.

Right now, I have the smaller battery in my 1988 K100RS, and it has no problem with starting the bike and running the electrical system.  My experience says that you could go with the smaller battery with no problem.  Just make sure that all the ground and power connections for the starter are kept nice and clean to make sure there is minimal power loss when starting.

I wish I'd never mentioned the ABS, it's not really my question  

I guess the real question is whether K75 and 8v K100 battery trays are the same. When I ask if a smaller battery will be okay, I mean physically smaller - will the clamp even hold it in place with the original screws, will the cables reach to it if it sits slightly further away, that kind of thing. I have no doubt a lower capacity battery can be used if a mostly dead lead acid 53030 can still spin it round (and never manage to put out enough voltage to set the ABS even after a run or straight off the charger!).

I found a few references to Motobatts and YTX20HL-BS that people have fitted over the years, but there is surprisingly little about the smaller battery that the tray seems to be made for and standard fitment for the K75, BMW's usual 51913. I need to buy one for a dead R1100GS I was recently given (yes really!) so I guess I have nothing to lose by buying one and trying to fit it. But it'd be nice if someone could confirm it'll work before I wrestle the big sloshing 53030 out of there!

Ha ha ABS......a dodgy ABS relay can create a drain too....

I have had from 19 to 32Ah batteries in my Ks.

The battery tray and brackets are the same for K100 and K75. Follow this link and then click the 'Part Number' for '#12 Battery hold down'. You'll see the compatibilities with different models.
https://www.realoem.com/bmw/enUS/showparts?id=0504-EUR-11-1988-K589-BMW-K_100_RT_84_0504,0505,0514_&diagId=61_2716

Don't know about fitting the smaller sized battery. I guess you could always pad it out with something. Like sit it on a piece of foam so it sits higher against the top bracket.

I think a minimum of 20Ah is best, but as someone mentioned above the crucial rating is the CCA. The battery spec for early K100s was 29Ah and 360cca. A bit of over kill I think. This was later revised down quite a bit for the K1100s but can't really see any reduced power requirements with that model. But with a smaller battery regular maintenance and checking of the electrical system is important.

Yeah, had that problem with my ABSII until I installed a new battery.

I've never noticed a difference in mounting different size batteries.  Off the top of my head, the difference between the two batteries used on K bikes is the thickness.  The height and length are close enough to ignore. 

The narrow battery may get the terminals a bit close to the top clamp.  It's not a problem for me though because i have sheet of rubber cut from an inner tube to insulate the terminals from the clamp, it fits under the clamp and covers the top of the battery.

Thanks all!

Holister - You're right, everything does seem to be shared between the K100 8v and the K75.

.75 - Checking the dimensions on Yuasa's website, the 53030 and 51913 are the same 186mm long and 171mm high, just the 53030 is 130mm wide and the 51913 is 82mm. I had thought about whether the terminals would be close to the top bar, but forgotten again when it came time to post on here.. I'm 34 you know, age creeps up on all of us. I do have some lino off cuts I could use as insulation, so not a big worry.

I plan to fit one of these, the 20Ah and 95 CCA should be plenty.. If not, it can go in the R1100GS. Hopefully the weird vertical screw terminals won't be too much of an issue either.

20Ah is fine but 95 CCA troubles me.

I saw that 95CCA as well.  The Yuasa battery website actually lists the CCA for that battery as 180 amps.  It's a bit low, but usable if the weather isn't too cold.

So! The long and short of it is, it works. The longer version is, it works very well but I'd buy a 51913 with the big square terminals, not these little flat screw ones. The even longer version is basically the same but with pictures and commentary, so here goes..


Big sloshy batteries are bad, mmmkay. I had no idea this one was leaking from the top either, getting the crystals off the underside of the brace took a lot of screwdrivering and wire brushing. Yuck.

51913 on the left, 53030 on the right.

51913 on the top.. You know what this is.

The wonderful 51913 shaped indentation in the original battery tray!



The problem with the flat screw terminals is that the ancient cables on the bike are used to vertical terminals, and they have holes in them that are a bit big. I don't like this very much, in fact I think I'll relegate this battery to the R1100GS and get another with big square vertical terminals for the K. They're only a couple of quid more on ebay.


The positive cable isn't so bad.

One annoying issue that is probably just because I've used a cheap battery, is that it's not quite tall enough to reach the brace as the rubber thinger on the end that holds the coolant expansion tank still is quite tall and not squishy. I guess the 51913 also sits slightly lower in the cut out in the battery tray. Not sure how K75s get around this??

I suppose you could turn the brace over and put the rubber bung in the other way round, that would make it sit a little lower though you wouldn't have as much stabilisng effect from it. Myself, I did what any self-respecting backyard mechanic would do - packed it out wi' birra wood.

Proof that everything still fits:

The special bit of wood could have been thinner, things are quite tight now, but it does all go together.

As for the performance of the smaller battery, well it's absolutely fine - I turned it over on the starter without "choke" to test and it went round fast enough to actually start running, much to my surprise. It hasn't fixed my ABS though, I guess that's another issue for another thread.. Oh and the 51913 is SO much easier to fit, just lower it into place!

Hope this helps someone out there if they want to fit a smaller battery or find themselves in sudden need and can't get a 53030.

Dry kindling for the Dragon then, Snod?

Snod Blatter wrote:The problem with the flat screw terminals is that the ancient cables on the bike are used to vertical terminals, and they have holes in them that are a bit big. I don't like this very much, in fact I think I'll relegate this battery to the R1100GS and get another with big square vertical terminals for the K. They're only a couple of quid more on ebay.

Or, you could mount something like these.

You could use something ribbed under the battery  (I've used a mudflap for this) to raise it, then you could turn the bracket around again. It increases air circulation for the battery, and insulates it from a bit of heat.  You can also get plastic moulds just for this purpose.

Dai wrote:Dry kindling for the Dragon then, Snod?

Hmmmmmm! That Dragon was nearly 7 years ago now and I'm still not sure I'm over it enough to book myself in again! But I see they are still hopeful it will go ahead.. Their strict no refunds policy seems a bit, err, strict though? Encourages people to go even if they've got the plague.

Rather than spending a frankly hilarious amount on some little angled brackets I was thinking more like buying one of these for £2 more:

Still a 51913 but with the 53030 style terminals.

I'm not so inclined to put something under the battery to build it up when it fits the space in the original tray so nicely, but I guess you could. I'm surprised no one with a K75 has chimed in, do we not have 75ers here any more? Maybe they're as surprised as I was and it's just because my battery is a little small, no idea really.

Don't sweat the height, just do what I do and put a couple layers of rubber sheet(gasket material, or pieces of old inner tube) under the battery hold down.  Make them a bit wider than the top of the battery and they will protect the terminals from shorting out to tools and other metal bits.

Snod, my way of dealing with clamping different size batteries was to do away with the OEM clamp altogether.

On all of my bikes for many years I have used a webbing strap around the centre of the battery to provide the clamping action and have never had one come loose over some pretty poor Aussie roads both sealed and unsealed.

On my K75, K1100 and R80G/S I used the Motobatt 51814 (220CCA & 22AH) which fits into the recessed centre section on the battery tray on the K75/K100. I cut two slots in the base of the tray either side of the recess to feed the strap through as seen in the photo below.



Then you will ask, how do we hold down the coolant bottle?
My solution was to make an L shaped bracket from flat aluminium suitably drilled to hold the rubber block that sits on the coolant bottle and also drilled for the OEM hold down screw. This bracket fits down between the coolant bottle and the battery (after the battery is firmly clamped). On the lower end of the L bracket I have made a pad around the bracket from self amalgamating tape that rests against the side of the battery when the long screw is tightened to hold the coolant bottle. Hopefully the photos will explain better than my words.





This arrangement has been on the K75 for 50,000km so far with no issues.

Cheers

Brilliant!

It is indeed quite brilliant, haha. It does pose the question of whether a 51913 was ever officially supported in the K75 though, if such mods are needed.. The Motobatt 51814 is the same as a 51913, as I understand? Lots of people selling the 51913 for K75s but maybe they are all wrong..!

SO far it has only been my 16 valve K100RS bikes that had the larger batteries.  Both of my K75's had the smaller battery as well as my K100RS 8 valve.  There have been no problems with that battery on any of the bikes that had it. 

Just put a nice flap cut from a heavy inner tube or some 3mm rubber sheet to shim the top of the battery and protect the terminals and go for a ride.  Stop agonizing over it.

Oh sorry, I thought this place was for discussing (often banal) details of K ownership. My bad.