Crafstman Lawn Tractor Reads 35VAC Across Battery

[N4HMR]

I'm rebuilding a 94-95 LT4000 lawn tractor. Having so much fun I can hardly stand it!

Today I decided to check out the charging system - and let me say up front it is working. Maybe a bit overtime. With the throttle about half-way, I measure about 15.3 VDC on the battery posts. If I speed up the motor, it goes a bit higher.

Then I decided to check for proper diode operation (make sure it isn't shorted) so switched to the AC volts position. With the red positive meter lead on the + post and black ground lead on the - post, I read right at 32-33VAC. I was NOT expecting this reading!

Reversed the meter leads and read 0VAC. Got two more meters and tested with them - got the same results.

I then shut the motor down and unplugged the alternator output connector to check the diode - reads good. Conducts in one direction, shows open in the other. See attached Schematic.

Note: I was using cheapo Harbor Freight DVMs and an older Radio Shack analog meter. In order to see the forward bias on the diode, I had to reverse the meter leads between the analog meter and the DVMs. Obviously a difference in the circuitry of the meters.

So, why would I read 32-33VAC during the initial test and 0 when I reverse the leads?

Note: This motor has two alternator windings producing AC voltage - one to charge the battery and the other to power the headlights.

 

[(*steve*)]

You have a cheap multimeter. . Don't expect the AC range to give you anything sensible if there is any DC present.

If the AC reading is 2.83 times the DC reading, then you're just reading the DC.

 

[N4HMR]

You have a cheap multimeter. . Don't expect the AC range to give you anything sensible if there is any DC present.

If the AC reading is 2.83 times the DC reading, then you're just reading the DC.

OK, I can agree with the cheap meter part of your answer. However, I see about 2X the VDC reading for the VAC result, not 2.83X.

I do have a 4 channel Iwatsu SS-5711C 100Mhz scope. I think that with the AC selection it isolates any DC. Should I expect that to yield a more reliable reading?

You didn't comment on why I would see any AC on the + post if the diode was functioning properly. I can only assume at the moment that it is.

 

[Colin Mitchell]

The alternator is NOT just a simple AC generator.
It has a feedback system that produces the magnetic field and this is adjustable by the feedback current so that the generator does not produce too much current when working at high RPM.
For this reason you cannot measure any of the voltages and you can only measure the current if you have a very low resistance shunt for measuring the current.
Even 0.1v or 0.2v across the shunt will change the charging current enormously as the regulator is trying to detect the absolute peak voltage of the battery so that the charging current is reduced considerably.
There are only two things that alert you to faulty charging.
If the battery gradually gets less charge and can just start the mower, either the charging current is too low or the battery is starting to fail.
If the battery is drying out, the charging current is too high and the battery is gassing.

What you will be reading and seeing on a CRO is absolutely meaningless unless you know how to interpret the readings.
It is the CURRENT that is the important factor and all the readings are purely voltage.
You have to know the EXACT charging voltage of the battery you are using as there are 2 or 3 different types.

 

[N4HMR]

Thx for the replies. Finally got tired of screwing with these meters and put my 4-channel Iwatsu to work - got some sensible readings then.

At engine idle after 1-2 min of 'recharge', scope showed about 15.3 VDC and approx .1 - .2 VDC ripple. Ripple was about what I would expect. At full throttle, scope showed right at 15.8 VDC with a very slight increase in ripple voltage - but not much.

The scope was showing maybe a .2 volt less difference than the meters. The clock reference on the scope is a .6 volt 1Khz signal, and it was right on the money on that scale. No matter, .2V ain't gonna shake any trees in this case.

My only concern now is the high charging voltage - cooking the battery. This is a Briggs engine, so called their Tech Support line for info on this but they decided to take the week off, so it'll be next week before I'll find out if this is typical for this engine or not. Not sure what I could do about it other than maybe add a sizable wire wound resistor in series to drop a couple of volts.

Any other thoughts?

 

[Colin Mitchell]

As I said before, what you are reading is totally irrelevant.
The only thing that is important is the current.
And this is a very difficult thing to measure.
0.2v difference is VERY IMPORTANT because the battery is very much like a zener diode.
As you increase the voltage to it, the current increases enormously.
The only way to test the generator is to make a glass battery and look at the gassing.
The signal entering the battery is AC and even if you add a very low-ohm resistor and feel the temperature, the voltage-drop across the resistor will decrease the current by an unknown quantity.
You don't know the impedance of the generator and if it has a voltage-sensing circuit.
To get a terminal voltage of 15.8v indicates one of the cells may be failing as this is certainly high for most batteries. You may have a "sealed battery" that has a high gassing voltage.
The whole thing is much more complex than taking a reading.
If the battery is drying out it is being overcharged. If it is failing to start the mower it has a faulty cell.

 

[(*steve*)]

What about using a clamp on DC current meter Colin?

 

[Colin Mitchell]

"What about using a clamp on DC current meter"

It is not DC current.

 

[(*steve*)]

OK, what about a DC clamp meter that can correctly read a signal with up to a couple of kHz AC on top of it?

 

[Colin Mitchell]

It is not what we call "AC."
It could be PWM or any shape and no meter will interpret it.
The only way to get a reading is to have a very low-value shunt 050 and a thermal detector embedded and have it calibrated.
That's why you cannot simply change the battery for another brand or another type.
It may have a different "gassing voltage."

 

[(*steve*)]

I guess Colin has just undiscovered flux gate magnetometers. Or perhaps because they're so new Colin's not aware of them.

I'm sure he'll come up with another reason he's right though. He always is.

 

[Colin Mitchell]

"I guess Colin has just undiscovered flux gate magnetometers. Or perhaps because they're so new Colin's not aware of them.

I'm sure he'll come up with another reason he's right though. He always is."


Don't you understand: The current 5 minutes ago may be charging the battery. The current is now "gassing the battery."

I have been answering my students for the past 50 years.

 

[(*steve*)]

So now the story changes from

voltage measurements are not enough:

It is the CURRENT that is the important factor and all the readings are purely voltage.

you can only measure the current if you have a very low resistance shunt for measuring the current.

to

It's really hard to measure the current:

The only thing that is important is the current.
And this is a very difficult thing to measure.

...and it's AC

The signal entering the battery is AC

to

it's not DC and it's not AC:

It is not DC current.

It is not what we call "AC."

to

you can't even measure it:

It could be PWM or any shape and no meter will interpret it.

not even with a low value shunt:

even if you add a very low-ohm resistor and feel the temperature, the voltage-drop across the resistor will decrease the current by an unknown quantity.

to

You can only measure it one way (with a low value shunt):

The only way to get a reading is to have a very low-value shunt 050 and a thermal detector embedded and have it calibrated.

to

measuring the current is no use

The current 5 minutes ago may be charging the battery. The current is now "gassing the battery."

and from the fact that there are several types of batteries (and that once again voltage is important):

You have to know the EXACT charging voltage of the battery you are using as there are 2 or 3 different types.

to

all batteries are so different so you must replace with the same brand as well as type:

That's why you cannot simply change the battery for another brand or another type.

It's a damn shame that in the last 50 years that nobody has figured out how to charge a lead acid battery other than by direct inspection of the plates.

I have been answering my students for the past 50 years.

Should I read your answers Biblically (where the latter statements have precedence), or Koranically (where earlier statements have precedence)?

 

[Colin Mitchell]

Steve, this discussion is going right over your head.
You are completely missing the complexities of the issue.