How can I tell if my battery is bad vs. bad charging system?

[zagoren]

Hello-

I have a 1986 R80/RT. I have one of those BMW Exide(sp?) 12V 19A batteries. Bought it last year. I've left the lights on a few times since then. I'm not sure what my next step should be. Here's the blow-by-blow.

1. Drove 20 miles last week on the highway to destination. Left lights on. Roll started it. Drove 30 miles back on highway.
2. Next day wouldn't start. Jumped it with another bike. Turned it off and the Amps dropped to about 12.4
3. Removed the battery and trickle-charged it up all night.
4. Next day drove 60 miles on highway. (bike started right up).
5. Today I tried to start (It's like 59 degrees Farenheit out...warm fall day) and it cranked about 15 times without starting...eventually losing momentum and stopaped cranking.

Im sure i can just pull the battery again...trickle charge it and start it tomorrow. But how do you diagnose if its the battery vs. the generator?

Thanks!

[Duane Ausherman]

Check the charge voltage when it is running. Did you try Google? I just put "bmw motorcycle charging battery" into Google to see what comes up. I got a commercial for a dealer selling chargers as first. Second was http://w6rec.com/duane/bmw/battery/ which is my article on the subject. It deals into far more detail than you want to know. Go down 3/4 and you will find the answer in detail.

Just about everything that you could want to know is already on the Internet, waiting for you to read at your leisure. Nobody needs to type it once more.

[zagoren]

Actually, a friend came over when this happened and "checked the charge voltage" when it was running. He's an auto mechanic. He wasn't able to diagnose the problem (I know!)

I've done the googling. I've read that article. I've also asked mechanics. Haven't gotten an answer. Perhaps I'm not technically adept enough to translate that article into a successful outcome.

Not all of us have garages or tools. Some of us are city people.

[daz]

Actually, a friend came over when this happened and "checked the charge voltage" when it was running. He's an auto mechanic. He wasn't able to diagnose the problem (I know!)

I've done the googling. I've read that article. I've also asked mechanics. Haven't gotten an answer. Perhaps I'm not technically adept enough to translate that article into a successful outcome.

Not all of us have garages or tools. Some of us are city people.

I think I have my facts straight on this. If not, someone will be happy to correct me:
Your volt meter tells the voltage of your battery. But it does not tell anything about the
condition of your battery. Just because it reads 12 plus volts does not mean your battery
is in good shape. It does not mean your battery can start your bike.
Think of that full charge as a QUANTITY rather than a VALUE. One battery holds a
certain charge. (Cold Cranking Amps). It’s brother who is older no longer can hold that
much charge. The older battery simply has less capacity. However, fully charged they
both read over12 volts. The 12 volts is the nominal difference in electrical potential. 12
volts does not tell capacity. 12 volts is a nominal value. The capacity or CCA (if you
must) is the quantity of charge.
It takes longer to fully charge the larger CCA battery. Take two batteries, same make,
model, and statistics. One is new. One is old and nearly worn out. The old battery takes
less time to charge because it holds less charge. 12 volts is nominal not
QUANTITATIVE. Each battery reads 12 plus volts but the newer battery can crank and
keep the lights bright longer.

[Kurt in S.A.]

Load test the battery. Various auto parts stores can do that. But you do an initial load test on the bike. As mentioned above, reading the voltage statically is only the surface charge. While reading the voltage, hit the start button. What does the voltage dip to? If it's well below 11v or even below 10v, the battery is suspect. That is the load test...you just asked the battery to respond to a load to start the bike.

Kurt in S.A.

[SteveD]

Bargain. ..just in case.
http://www.etx16l.com/

[Duane Ausherman]

daz, very good explanation. I get this warm feeling when I see that someone has an understanding.