Higher Voltage: Fool Your Alternator?

Hello again,

For my old Hyundai i bought a brand new battery and brand new alternator and i found that the alternator would not keep the battery charged correctly at 14v. It had to be a little higher than that. I had to bring the battery in the house now and then and change it with a 10 amp battery charger or 10 amp power supply and set the voltage much higher than 14v. I think it was 14.7v or something like that. That way it would hold the more normal voltage over a month or two when used in the car again and with short trips with the car.
Yeah with short trips the battery doesnt get as well charged either if the charge voltage is too low.

My newer Chev Silverado 1/2T pickup is too smart for its own good. The Alternator Voltage Regulator is controlled by one of the many on-board computers. It shuts off the alternator during start. A few seconds after start, the VR is set to ~14.8V, but the exact voltage seems to be a function of engine compartment temperature. After about 10min of running, the voltage drops to about 13.8V, and it stays near there for the duration of the trip. During the trip, as the engine compartment gets hotter, the VR voltage is further reduced.

I'm sure that the algorithm is published somewhere....

Some of the older members here will remember the 2 or 3 bobbin Regulator with a DC generator. This did 3 functions: Voltage limit, current limit and reverse current cutout. Temperature compensation was provided by the bi-metal leaf spring on the voltage bobbin's armature. Hence the higher terminal voltage on a cold startup. The current limit would be in operation until the battery started to come up, (otherwise the commutator would leak smoke and molten solder) After about 30 mins, when the under bonnet temperature rose to normal operating, the volts settled back down to about 13.8. This was an effective way of bulk charging and using the lower voltage setting (determined by the bimetal spring) to float charge (if the vehicle was running long enough) Note that the excitation of the generator's field was in a sense: PWM, by the action of the contacts in the voltage limiter, switching the field current on and off. (Some regulators had changeover contacts and 1 or 2 resistors to give a smoother control). This resulted in a somewhat sawtooth waveform (and commutator ripple). If one connected an oscilloscope to the battery, the peak voltage would be in the order of 15+ volts, Your analogue meter reads RMS so it would read about 13 or 14v. Batteries actually liked this PWM method of charging, this is why we used to get 5+ years out of a battery and never had any sulphation. And we all survived on 15 -20 amps of total charging current. I am not aware of any temperature compensation in these modern cars with alternators that output as much (or more current) than my welding machine....!!! Early alternator systems had external electro-mechanical regulators with temperature sensitive bi-metal leaf springs. These regulators were voltage limiting only as alternators were self current limited by virtue of the stator core saturating, reverse current was taken care of by those hefty rectifier diodes acting as a one way valve. MikeM: note your comments about temperature compensation, about time the vehicle manufacturers did this.

I think the VR124's are adjustable a little. It's something about the import cars and the small batteries they are using in them. I had a Nissan that charged at 15 volts. Probably some battery management circuit! It's important to match battery type to the manufactures recommended battery. Too much battery will cause undercharging if the alternator is not high enough current for it. I have run into this with my VW's.
Here is what we are working on now. but it's for a 6 volt generator. I can crank the voltage output up to 10 volts with it. I don't see any reason the same circuit with 12 volt values wouldn't work with an alternator.
Automotive 6 Volt Generator Transistor Voltage Regulator.

Sorry about off-topic, but speaking of higher alternator voltages, my car's alternator broke once. It decided to output 18V to battery. It was interesting situation...activated all heaters and other loads to keep it down to near 12V so battery doesn't pop.

I have seen 12 volt VW Generators light a 60 Watt 125vac incandescent bulb.
The the 18volt alternator, yes power the armature fully! Maybe a short did it. Could have been the Alternator Voltage Regulator?

Yes, If I recall correctly, there was short in diode bridge.

will help the battery store charge better

Click to expand...

What's "better" mean? The actual energy density of the battery as a package measured in either in Watts per Kilogram or Watts per cubic meter will depend entirely on the physical manufactured item. Subjective useful longevity (given any magical process) will be entirely that - that is: a totally thrashed battery may in fact (for you) do it's job of cranking an engine to life years after it "should" have been replaced - until that day where you need a little extra and don't have it.

In a perfect world, your alternator would put out a constant voltage, the current dropping to near zero around 14.2 volts. That's what lead acid likes. But in the real world, I doubt the average vehicle owner would notice a difference in useful service life.

Batteries tend to be replaced under extreme circumstances: Those minus forty mornings, or when the radio kills the reserve capacity in ten minutes three times in a row and the owner gets cranky about having to jump start.

Sure, you can shorten a lead acid primary battery's life easily enough .. just deep cycle it to death a few score times - but IMO there';s not much you can do to extend it's life, and trying to is a waste of time for the average POS automotive off the shelf offering.