1971 BMW R/75 battery issue

[bigvic54]

I've never seen anything like this before. Maybe someone has.

I just bought a 1971 BMW R/75. It came without a battery so I bought a new one for installation. I added the acid, per instructions, and put it on a battery tender overnight.

Today I checked the battery and it showed 13.5 volts. I installed it, left it in for a few hours, and checked it again. It showed 3.5 volts. I reattached the battery tender, left it for about 5 hours and it didn't charge. The tender shows that there is a fault.

The bike has no lights, accessories or anything else connected. There would appear to be nothing to drain the battery.

Any idea why the battery would drain?

Thank you for your ideas.

 

[kbasa]

I've never seen anything like this before. Maybe someone has.

I just bought a 1971 BMW R/75. It came without a battery so I bought a new one for installation. I added the acid, per instructions, and put it on a battery tender overnight.

Today I checked the battery and it showed 13.5 volts. I installed it, left it in for a few hours, and checked it again. It showed 3.5 volts. I reattached the battery tender, left it for about 5 hours and it didn't charge. The tender shows that there is a fault.

The bike has no lights, accessories or anything else connected. There would appear to be nothing to drain the battery.

Any idea why the battery would drain?

Thank you for your ideas.

When you charged it the second time, was the battery fully disconnected from the bike? If so, you may have a faulty battery.

 

[RPGR90s]

I've never seen anything like this before. Maybe someone has.

I just bought a 1971 BMW R/75. It came without a battery so I bought a new one for installation. I added the acid, per instructions, and put it on a battery tender overnight.

Today I checked the battery and it showed 13.5 volts. I installed it, left it in for a few hours, and checked it again. It showed 3.5 volts. I reattached the battery tender, left it for about 5 hours and it didn't charge. The tender shows that there is a fault.

The bike has no lights, accessories or anything else connected. There would appear to be nothing to drain the battery.

Any idea why the battery would drain?

Thank you for your ideas.

Battery make and country of origin? I worked on a /5 once that the owner had installed a new Chinese battery. Look about fifteen minutes to diagnose that the battery was crap via a load test. Owner got a warranty replacement for the same battery and yes, it had the same problem. He finally sprung for an Odyssey .
Have your battery load tested.

Lesson: Don't buy Chinese crap.

 

[stevenrankin]

quick test

An unofficial quick test on non electronic stuff equipped vehicles like your bike, to test for a parasitic drain on the battery is to remove the negative cable. Gently touch it to the negative post and look for a tiny spark. If there is a spark, you have a drain.

If you have a multimeter, you can use the amperage test to check with flow. However results depend on you meter, you may need a meter that reads milliamps.

If you don't have parasitic drain, then you have a crap battery. Good luck, St.

 

[Omega Man]

My Redneck engineering degree years ago developed a battery test that may be applicable in your diagnosis. As you mentioned that you filled the battery with acid, that means the electrolyte is accessible.

Remove the caps or strip that covers the electrolyte.

Using a VOM (volt, ohm meter) set to 12volts and being careful, insert the positive probe into the electrolyte closest to the positive battery post and then the negative probe into the next "hole" and look for a reading. Continuing this process, lift the 2 probes out now inserting the positive where the negative just was (keeping the positive always in line) look for a reading. Continue this "jump" to the next hole using the last hole and watch for the readings.

It really doesn't matter on the actual combined battery voltage as what you are looking for is one of these hole "jumps" to not read any voltage indicating a plate separation. Full battery voltage can be seen if a battery suffers from plate separation by the VOM detecting the voltage through the electrolyte which is the voltage but the battery will have no depth or ability to actually function as a battery.

Of course, as you have just had the VOM probes in battery acid, a through flush and wash off is needed.

OM

 

[stevenrankin]

Old times

Oh man, I haven't done that test in years, LOL. I used to have a battery tester with acid proof probes for that exact test, it got "donated" to a friend's son starting up in the mechanic trade. LOL, I bet it is sitting on the shelf collecting dust as there are not very many batteries to use it on anymore.

The other thing you can test with a voltmeter is voltage drop. If the battery is reading a full charge of 13.5 volts and you turn the key to the start position and the voltage plummets to 9 volts or less, you have a crap battery.

Personally I buy the Odyssey batteries now and avoid the cheaper brands. The reason being my local airhead shop went from four or five battery warranty problems per year to zero when they started persuading owners to install the Odyssey brand. I am sure there are other quality batteries beside Odyssey that are just as good.

I have had very good life and miles out of my current battery as well as the battery in my gone RS. St.

 

[nrpetersen]

Battery Tender logic looks for about a +8 V on the connected terminals before they will even turn ON. This is incorporated in them to compensate for a dumb reversed charging cable connection. Parasitic drain may have drained the battery to something below +8 V and therefore it logically refused to ever turn on.

Obviously the dry charged battery was nearly empty when you got it, or there is a substantial parasitic drain someplace.

 

[stevenrankin]

Drain

Re Reading the original post.

Assuming the new battery is good, what could be draining it. As I wrote, check for a drain first using the spark or multimeter.

Given the age of the bike, look very closely for issues with wiring, rubbed insulation, places were a wire may be shorting. Also, IF the previous owner did ANY wire repair or rewiring for any reason, consider this a possible source of Previous Owner's disease.

Factory wiring is reliable unless aged or messed with. I don't have a wiring schematic for your bike so I can't really research any possible issues in the factory system. Good luck, St.

 

[bigvic54]

Battery make and country of origin? I worked on a /5 once that the owner had installed a new Chinese battery. Look about fifteen minutes to diagnose that the battery was crap via a load test. Owner got a warranty replacement for the same battery and yes, it had the same problem. He finally sprung for an Odyssey .
Have your battery load tested.

Lesson: Don't buy Chinese crap.

The battery is Die Hard, which I think was made in the US.

 

[bigvic54]

Battery Tender logic looks for about a +8 V on the connected terminals before they will even turn ON. This is incorporated in them to compensate for a dumb reversed charging cable connection. Parasitic drain may have drained the battery to something below +8 V and therefore it logically refused to ever turn on.

Obviously the dry charged battery was nearly empty when you got it, or there is a substantial parasitic drain someplace.

The battery wasn't empty when installed. I charged it with the Battery Tender over night and it was at 13.5 when installed.

 

[bigvic54]

Re Reading the original post.

Assuming the new battery is good, what could be draining it. As I wrote, check for a drain first using the spark or multimeter.

Given the age of the bike, look very closely for issues with wiring, rubbed insulation, places were a wire may be shorting. Also, IF the previous owner did ANY wire repair or rewiring for any reason, consider this a possible source of Previous Owner's disease.

Factory wiring is reliable unless aged or messed with. I don't have a wiring schematic for your bike so I can't really research any possible issues in the factory system. Good luck, St.

I absolutely agree with your thoughts. The wiring is a **** show and I'm checking every wire.

 

[bigvic54]

When you charged it the second time, was the battery fully disconnected from the bike? If so, you may have a faulty battery.

When the battery is connected to the bike it wont charge. When I remove the battery it will. Additionally, when the key is out it's fine. When its in it isn't.

 

[Omega Man]

When the battery is connected to the bike it wont charge. When I remove the battery it will. Additionally, when the key is out it's fine. When its in it isn't.

That piece of information makes me think that there is a short in the key system. If the key switch can be eliminated (unplugged?), that sounds like the next test………To clarify, it’s just the key in and not switched to any position?

OM

 

[stevenrankin]

Before

Okay I am going to jump on the sales pitch. Before you start really digging into the problem, it is a real good idea to have a wiring schematic on hand. Yeah, Clymers and Haynes have them but, the best on the market is the ones from EME.

Looking for an electrical issue other than a bare wire, loose connection or broken wire without a good schematic can be real frustrating.

Just for my information, your ignition switch uses the "nail" style key? I am NOT familiar with their inner workings however I do know they can be problematic due to wear. I will quiz my buddy who is more familiar and see if something comes up. St.

 

[kbasa]

That piece of information makes me think that there is a short in the key system. If the key switch can be eliminated (unplugged?), that sounds like the next test………To clarify, it’s just the key in and not switched to any position?

OM

Exactly what I noticed.

 

[bigvic54]

Okay I am going to jump on the sales pitch. Before you start really digging into the problem, it is a real good idea to have a wiring schematic on hand. Yeah, Clymers and Haynes have them but, the best on the market is the ones from EME.

Looking for an electrical issue other than a bare wire, loose connection or broken wire without a good schematic can be real frustrating.

Just for my information, your ignition switch uses the "nail" style key? I am NOT familiar with their inner workings however I do know they can be problematic due to wear. I will quiz my buddy who is more familiar and see if something comes up. St.

I have a schematic, thanks.

 

[44006]

you say "I just bought a 1971 BMW R/75. It came without a battery so I bought a new one for installation."

I do not know your level of experience with /5 so forgive me for the following:

Stock /5 has one large diameter wire from neg post directly to ground at transmission vent.

Stock /5 has another large diameter wire from pos post running forward out of sight to starter motor.

Stock /5 has another smaller wire (black on outside red inside) from pos post going into harness near battery.

I have seen folks connect the smaller black wire to the neg post instead of pos post and this of course kills the battery.

 

[FELAW]

I've never seen anything like this before. Maybe someone has.

I just bought a 1971 BMW R/75. It came without a battery so I bought a new one for installation. I added the acid, per instructions, and put it on a battery tender overnight.

Today I checked the battery and it showed 13.5 volts. I installed it, left it in for a few hours, and checked it again. It showed 3.5 volts. I reattached the battery tender, left it for about 5 hours and it didn't charge. The tender shows that there is a fault.

The bike has no lights, accessories or anything else connected. There would appear to be nothing to drain the battery.

Any idea why the battery would drain?

Thank you for your ideas.

I found the parasitic loss was in the ignition switch on the headlight. Despite rebuilding the switch, the old push-in key and the switch plates inside the headlight shell seemed to be the culprits. I put a toggle switch on the main lead coming from the battery and switch it off and take the key out when not riding.