by Austin Jack Smith
As I understand it the voltage regulation system on almost all auto voltage regulation devises are what are called Constant Potential Voltage Regulation, which means the voltage output from the DC power source is controled by the voltage regulator by adjusting or changing the field current to the generator field windings. This is why you are advised not to run the generator without the regulator attached for more than a few moments, just long enough to measure the voltage output of the unregulated generator. Ref. the generator testing information in both Bentley and the Haynes manuals. The DF on the voltage regulator and generator stands for Dynamo Field, our English friends call the generator the Dynamo or we call the Dynamo the generator.
As the battery voltage comes up the amount of current being put in the battery is less, Voltage times Current (in amps) = watts or a unit of electrical work. so basically the generator puts out the same watts only different voltage and amps. Some of you old timers will remember when old Fords and others didn't have voltage regulators only reverse current relays (to keep from motorizing the generator when the car was shut off) in order to keep from overcharging the battery it was common to run with your headlights "on" on long trips. If that wasn't possible you could move the third brush in the generator and cut your voltage output down or up.
The voltage regulator used to be adjustable, but you had to have a very good voltmeter, a resistance load (to simulate the current demands of the battery and the vehicle) a very accurate ampmeter and a accurate thermometer. All of the instruments were necessary because temp. is also a function of the regulator. Today it would be foolish to try to adjust a regulator unless it was an emergancy, if the generator goes bad it is very likely that the regulator caused this, barring brush wear and othe mechanical wear factors. If your generator packs up change the reg as well.
A lead acid storage battery produces a fixed voltage per cell, (2.5 Volts) the state of charge makes very little difference until the battery goes completely dead. The only way to accurately measure a batteries state of charge is the specific gravity of the electrolyte (Sulpheric Acid Water Mixture) H2S04+H20 Under normal conditions at 59 degrees F. a fully charged battery will read 1.300 meaning their is enough free acid in the water to raise the weight of electrolyte to 1.3 times the weight of pure water. So a discharged battery will need quite a few amps of charge to bring its voltage up to 2.5 volts per cell, as the cell voltage increases the amp flow will decrease until the battery and gen voltage are the same and then the battery is on what is called "zero float".
Now trying to charge two batteries in parallel the batteries will try to equalize temselves out first ballancing the load until they both have equal voltage, which is very hard to obtain with two batteries of different size and age. On Submarines where I spent 11 years of my youth as an Electrician's mate we would do a special chargeing proceedure every 28 days called an "equalizing charge" the batteries consist of two batteries of 126 cells each and they were charged until all 252 cells were within a .01 volts of each other and the specific gravity corrected for temperture, of course was within .001 of each other. I hop[e you can see this is not a big problem but can have some consquences if not done right.
This is why a voltmeter is a better measurement of your electrical system than an ampmeter. Point to remember the physical size of a lead acis cell has no bearing on its voltage, but it does indicate the ability to store electrical chemical energy. A single 2.5 volt cell on a submarine holds 31 gallons of electrolyte and weighs over a ton, but only can deliver 2.5 VDC, but for a much longer time. The abiltiy to store electric energy is measure in Amp Hours, it is often advertised in how much it can give up in a certain time intervel. It takes 250-300 amps to stall the starter on a VW. So if you want to motor your engine over for a minute you will need about 450 Amp Hr. Battery. When a battery starts the end of its life cycle the plates that are in a battery gradually get covered up with a sulper compound called sulphate and it will charge very fast and discharge just as fast. This usally shows up in the winter when the oil is thicker and it is harder to start the engine. The additional amp Hrs. are just not there and the battery dies.
I have spent more time than I meant to and I'm sorry, but just remember
what has already been said in previous discussions. You can eletrically
isolate the two batteries from each other by useing large diodes or
electrical check valves, but they use a small voltage to function so
they will effect your charging rate. You can use electrical relays to
isolate your batteries, easly found in auto stores called starter
relays. What ever you do by having a voltmeter switchable to either or
both batteries or just two voltmeters cheaper and easier to measure the
state of your batteries you can see what you need to do.