Over voltage

G-Man · Post by **G-Man** » Sun Nov 13, 2011 3:01 pm

I remember a while back when the rectifier 'went' on my CB400F, I was getting AC at the battery and that caused it to die.

That may be where the 15+ volts is coming from here. The meter might be saying 15v but it's not DC so it's not charging the battery. The lights, of course, don't care about ac.

Just a thought....

G

Snakeoil · Post by **Snakeoil** » Sun Nov 13, 2011 6:37 pm

Interesting thought, G-man. He said he put a new rectifier in the bike. Not sure if it is an original type or modern silicone.

I suppose you could test across the input side of the rectifier with the DMM set on DC and see what you get to confirm your suspicion.

Yeah, I'm sure the battery would be totally confused with AC being fed into it. Charge, discharge, charge, discharge....

regards,
Rob

highskyflyn · Post by **highskyflyn** » Mon Nov 14, 2011 3:10 pm

First of all, thanks for everyone responding, I have learned a lot of great information and will definitely come in handy. Thanks to wilf for reassuring me thats theres really no way a stator could be causing over voltage.

Next I feel a little dumb but this entire time of pulling readings from my multimeter, I used it on my car to see what reading I was getting (knowing for sure that my car isn't having any issues) and realized that my multimeter is reading 3 - 3 1/2 volts higher that it should be.

So all the reading I was getting in the beginning of this conversation was pretty much normal.

Although I figured out the over voltage issue I believe that theres still a problem with the electrical system because I'm still losing voltage and ending up with a dead battery.

Im going to go back and check every connection, verify wires are connected correctly, and make sure that everything is grounded properly.

Something that can be learned from this entire post is that if someone has an electrical issue and is using a multimeter, they should probably check the meter first that it's working properly. haha

Thanks everyone

Snakeoil · Post by **Snakeoil** » Mon Nov 14, 2011 3:53 pm

We all make mistakes or have equipment foil our efforts. The only people that never happens to are those who do nothing.

For what it is worth, Harbor Freight sells a really nice DMM for $20 when they are on sale and with another 20% coupon, it's a steal. I still have my Fluke from my field engineering days and the HF meter is as good if not better. Handles more amps for sure and also has a audible feature for continuity checks. You can literally "ring out" the wires with this one.

I always test a meter on a known source if I don't like the reading I"m seeing. Been burned by this before with faulty meters back in the Simpson days.

So, you may simply have a failure to charge situation going on here. That makes much more sense to me. Since you replace the rectifier, I'd start there. Make sure you have AC into it and DC out. DC should vary with engine speed so that's how you know you are not simply reading battery voltage. No change in charging voltage probably means not charging.

I learned a lot from the discussion as well so even if this turns out to be some simple problem like a bad connector, the thread was valuable.

regards,
Rob

Wilf · Post by **Wilf** » Mon Nov 14, 2011 5:29 pm

highskyflyn,

I'm glad to hear you've eliminated a major part of the puzzle, your meter! As Rob says, we've all been there, and I've been there more than once when smoke and/or explosive noises accompanied my "Aha" moments.

You said earlier that you had replaced the rectifier with a new one from Oregon cycles. I googled that name and came up blank. Can you provide a bit more detail on that, as I'm wondering still if the rectifier isn't involved in the problem.

......

Rob,

Ah, the Simpson days! I still have 3 of those beautiful instruments and a swack of salvaged meters on my shelf--love them!

Wilf

Snakeoil · Post by **Snakeoil** » Tue Nov 15, 2011 10:41 am

I need to beat this a little bit more. I tend to do my best thinking when I'm in the shower, lying in bed first thing in the AM or checking the dog for ticks. For some reason, when I woke up this morning, all I knew about alternator design and electrical circuits seems to replay in my alleged brain.

So, Wilf, not to argue, but I think your theory is flawed as was my interpretation. Here's why.

The output of an alternator is as you described, a function of the magnetic field strength, the number of turns in the stator coils and the speed of the field (rotor) passing the coils.

The key here is what each of those components produces or maybe governs is a better term.

The strength of the field determines the voltage for a given RPM. If the field were an electro magnet instead of a permanent magnet, you could vary the voltage by varying the current thru the field and hence it's strength. Another way to vary the voltage is to vary the speed at which the coils cut thru the magnetic field. Since we have a permanent magnet rotor, our field strength is fixed so for a given RPM, voltage is fixed.

The current is a function of the number of turns in the stator coils and the load. More turns means more current capacity. This is where my memory starts to fail me. I'm not sure if speed can also effect the current. The coils are a fixed resistance so if voltage goes up, I suspect that the current must also go up due to Ohm's Law. But for a fixed speed, the current should be determined purely by the load on the circuit.

So, let's take our charging system scenario and Wilf's theory that there is a high resistance connection between the stator and the battery. Although the stator side of the rectifier is an AC circuit, each coil acts like a DC circuit. So we can set the whole AC/DC thing aside for this discussion.

If there was a high resistance between the stator and the rectifier this is what I believe would happen.

The stator puts out a fixed amount of power in watts. Watts are Volts x Amps. We know volts are fixed by the magnetic field and Amps are determined by the load, both for a given rotor speed. So Volts and Amps are fixed for a given load and a given rpm. If we introduce a resistance between the stator and the rectifier, which is a series circuit, the rules governing series circuits tell us that the current is shared equally amongst the loads on the circuit while the voltage varies. So, the new resistance introduced into the circuit would create a voltage drop acrossed it and the voltage at the input to the rectifier would therefore be lower. That in turn would mean the DC coming out of the rectifier would also be proportionatly lower.

So, in my humble opinion there is no way that the charging voltage could go up at the battery terminals if resistance was added to the circuit between the stator and the battery.

I guess I kinda knew this, but was not confident in it because I had not thought it through. It's been a looooong time since I've even thought about this kinda stuff. I did have a line in one of my replies that said the 15 volts at the battery terminals did not make sense given the other info provided. But I deleted it. We now know that it was not 15 volts, but normal charging voltage or perhaps just battery voltage. I personally think it was just battery voltage.

So, if you did have a bad/high resistance connection between stator and the battery, I would expect to see no voltage rise at the battery terminals with the engine running. If you check current to the battery, I would expect no current flow into the battery. It would probably show a reverse polarity reading meaning current is flowing out of the battery to the accessories. And since the accessories connect to the battery, I would expect to see the battery discharge until it ran out of sufficient power to operate the bike. So, that fits with an early comment made by HSF that the bike quit on him.

I realize that I'm coming to the same conclusion here, but this supports the suspicion that you are not charging the battery when you are running. Measuring battery voltage alone is not an indicator of battery condition. That's why your discharged battery drops to 6V when you hit the starter button. It cannot serve the load.

Recheck your rectifier connections. Make sure they are good solid connections and are correct. To Wilf's point, check stator output at the input terminals to the rectifier. The manual shows how to test the stator output (dynamo in the manual) using an OEM tester. I suspect, and others more familiar with testing the stator can confirm this, that the numbers given in the manual for the listed RPM ranges are what you should also see at the input to the rectifier.

Sorry for the long email, but I would like to think that this may take some of the mystery out of charging systems. I know it did for me.

If there is someone reading this with a solid eletrical background and sees errors in thinking or incorrect statements, please correct me. I'm not trying to act like an expert here. I'm just thinking thru what I think is happening and putting it in words here to share and have others more knowledgeable on the topic confirm, critique, correct, etc.. It think it will be helpful to all in the long run.

regards,
Rob

Wilf · Post by **Wilf** » Tue Nov 15, 2011 11:56 am

Rob,

I think your reasoning is correct. What I was trying to suggest was that any increase in load resistance results in an increase in stator voltage. I didn't get around to describing where that increase would show up based on where the increased resistance was in the circuit. I was trying to show the importance of good, clean connections throughout the charging circuit (from stator wires through to chassis grounds) but along the way I introduced some confusion, which you have fortunately pointed out.

With respect to the AC generator, the number of turns in the stator coils determines the stator output voltage, not the current. You could take a 6 volt stator and convert it to 12 volts by rewinding it with twice the number of turns but in order to fit all those turns into the same space the wire size would have to be reduced, thereby doubling the coil resistance. Power output remains the same but the increase in voltage is offset by a decrease in current.

Wilf