1929 Buick Charging Target Voltage

[raydurr]

Hello. Does anyone know what normal charging system maximum voltage should be? This would be warmed up, with engine at a fast idle and no accessories on. Currently my system shows 20+ amperes and 8.6 volts at a warm fast idle. I have not removed the cutout cover yet as I avoid disturbing anything electrical that is old and working correctly.

[EmTee]

That sounds a little high to me.  I'd probably shoot for about 7.5 Volts.  It depends, though on how frequently you drive at night.  It might take a higher voltage  to offset the lighting load.  What does the ammeter say with the lights on at fast idle?

[Bloo]

Voltage is unregulated. Once the battery is charged nothing changes, the generator keeps pumping current in and tries to destroy the battery. At 8.6v, it has probably already done a bit of damage to the battery. I'd check the water for sure. It has probably boiled quite a bit off. If the battery is hot wait for it to cool down.

 

Current is regulated though, and 20 amps is WAY to much for a system like that. Try 8 or 9 amps. The cutout does not regulate anything. If it is disconnecting the generator when you stop the engine, it is working as designed. The current is set by moving the third brush in the generator. It is the only thing you can set.

 

Edited April 22, 2023 by Bloo (see edit history)

[raydurr]

This is shown in the 1929 Buick Service Manual .

 

[Bloo]

That tells you how to set the cutout. It's still just a switch and has nothing to do with why the current and voltage are way too high. What cutouts do is look for a voltage to "pull in". This is when the cutout is cut out (and the generator is therefore disconnected) because the engine is off or turning too slow.

 

Remember that the generator is disconnected. When it starts generating, and the cutout reaches the pull in voltage, the 6.7-7.5 volts mentioned, it pulls in. This connects the generator to the rest of the car, so the generator can supply current to the car and also charge the battery.

 

There is a problem though. What happens when you shut the engine off? The generator terminal on the cutout is still connected to the car (and the battery). There's probably enough voltage to keep the cutout pulled in. It takes less voltage to hold a cutout in than to pull one in. The battery will try to spin the generator like a motor, but it will be stopped by the engine. The generator might burn up. Even if it doesn't, it will draw current while trying to spin and run the battery dead. To deal with this there is a second winding on the cutout. When current flows backwards, it will force the cutout open. This is the 2-1/2 Amps (or less) mentioned. Now that the cutout has been forced open, it will need to see 6.7-7.5 volts to pull in again when you restart the engine.

 

If the cutout pulls in with the engine running, and cuts back out with the engine off there really isn't anything to do to it. It is working. If you needed to set one up from scratch after repairing it, those are the specs you would use. 6.7-7.5 Volts pull in, and 2-1/2 Amps or less in reverse to cut out.

 

For the charging setting, look in the generator section of that book and see if you can find where they mention setting a "third brush". It is the only regulation you have, and it isn't very good. 20 amps is too much. The manual may have hints. Let us know what you find out.

[Oldtech]

A 29 should, I think, have one of those thermal switches that reduces the output when it is warmed up.  The output then will be higher until the gen warms up. but yes, 20 is way too high. As EMTEE says, 8 or 9 would be good . Manual should explain how to set 3rd brush.

[raydurr]

I ran across this instruction on the third brush adjustment. 
I will attempt to lower the voltage today. 

[Oldtech]

Careful here. They are setting the Maximum output. Cold.  Ideally it should just charge enough to be above zero when the lights are on and the motor warmed up good so the thermostat opens.

[Bloo]

Yeah, 19 Amps is a lot. It must have a thermal switch like @Oldtech mentioned. I've seen that on other GM cars of the era. The way that works is when the generator starts to overheat, the points open and cut back the charging rate. You can have the current set higher (cold) with one of those than with a plain third brush generator.

 

Whatever the warm charging rate is (amps) with the headlights off, the battery will have to absorb and dissipate as heat once it is charged. Voltage cannot be set as there is no voltage regulator. The higher the current is set, the more of a tendency there will be for the voltage to run away and get unreasonably high after the battery is fully charged.

 

[EmTee]

Keep an eye on the battery electrolyte level.  Frequently having to top-off the battery with distilled water means the charge rate is definitely too high.  What does the ammeter say with the lights on and engine warm at fast idle?

[edinmass]

 

Figure 40 shows a shunt......it should cut the output by 60 percent when the engine is warm.....say fifteen minutes after start up. If the shunt is missing, you need to install one. 

[raydurr]

Today I moved the third brush to its limit of travel in the lowest current position. With headlamps on, warmed up and at road speed the ammeter shows 5-6 amperes. Engine warm and at a fast idle with no accessories yields around 7.4 volts. The system shows a slight positive charge at hot idle.