Would very low battery cause low charging voltage?

[mr49]

Hello, new visitor here.

Just acquired a 92 R100, and friend introduced me to this forum. Recently, when I was poking around the charging system, I notice that if the battery is almost out of juice (less than 12.3V without load), the charging voltage becomes very low. But once I recharged the battery, the charging voltage went up to normal.

From my very little understanding of airhead charging system, I don't quite understand why the charging voltage would be influenced by the battery's own voltage. Can experts shed some light on this?

Thanks.

[Duane Ausherman]

To measure without a load means almost nothing. You don't know of the battery has a capacity of 100%, or 3% of original.

How did you measure the charge voltage and what did you get? You didn't tell us. I infer that you used a battery charger to charge the battery, is that right? You then know nothing about the battery charger on the bike. That is the one that counts.

You could easily have a charging system that works perfectly, but the battery has no capacity, so it won't turn it over.

[mr49]

Let me be a bit more specific.

One morning, my bike won't start. So I pulled out the multimeter and saw that the battery has only 12.2v when the key is off. That dropped to below 10v when I cranked the engine. I did manage to start the engine once at that voltage. While I was at it, I also checked the voltage at battery terminals when rev engine to 4000 rpm. I got around 13.3v.

Then I put the battery on charger. Once it's fully charged, I let it stabilize for 12 hours. At this point, I repeated above measurement again. Now the battery voltage seems all normal at 12.6v. And the charging voltage at 4000rpm looks good too, above 13.8v. Of course the engine starts right up.

What puzzles me is why the charging voltage so low at 4000rpm when the batter is extremely weak.

[Duane Ausherman]

I am not sure what a "weak" battery means. A brand new battery that has 100% capacity, but only charged up to 13 volts will appear to be weak. "Weak" has little meaning. I guess it means that the battery isn't doing what you want.

Do you run the bike while you have the voltmeter on the battery terminals? You should see the voltage rising and eventually stop increasing. When you reach that terminal voltage, that is the charge voltage.

Your charging system could easily be set too low. In that case it won't matter if the battery is new or old, it will be a "weak" battery.

I don't think that I am helping you at all.

[Major Softie]

I am not sure what a "weak" battery means. A brand new battery that has 100% capacity, but only charged up to 13 volts will appear to be weak. "Weak" has little meaning. I guess it means that the battery isn't doing what you want.

Do you run the bike while you have the voltmeter on the battery terminals? You should see the voltage rising and eventually stop increasing. When you reach that terminal voltage, that is the charge voltage.

Your charging system could easily be set too low. In that case it won't matter if the battery is new or old, it will be a "weak" battery.

I don't think that I am helping you at all.

BUT, you're doing a fine job of it.

[Krauser4valve]

If you are saying that the dc voltage across the battery terminals is low at 4k rpm with a flat battery, well, that is normal. As the amperage to the load increases, the voltage must drop unless you have a very large alternator, which would in turn rob more motive power from ye olde bike.

[Duane Ausherman]

If you are saying that the dc voltage across the battery terminals is low at 4k rpm with a flat battery, well, that is normal. As the amperage to the load increases, the voltage must drop unless you have a very large alternator, which would in turn rob more motive power from ye olde bike.

What??? Say that in another way please.

[Dave Backmarker]

Mr49, Your question above, "Why is the charging voltage so low at low rpm?" I memory suits, the Airhead alternators start creating current at 3500 rpm.

DaveM
1975 R90S
2000 R1100RT

[lrz]

Y'all been drinkin'...?

Mr 49- it could be time for a new battery. Have you checked alternator brushes, voltage regulator, clean & secure connections esp. ground? The usual suspects.

[hal]

If you are saying that the dc voltage across the battery terminals is low at 4k rpm with a flat battery, well, that is normal. As the amperage to the load increases, the voltage must drop unless you have a very large alternator, which would in turn rob more motive power from ye olde bike.


What??? Say that in another way please.

Now that made sence to me