A 0.3V bus voltage difference wiill have a big impact on the battery life, if it is 28.3V. In theory the corect voltage is temperature dependent but in reality nobody in light GA does that, and 28.0V seems about right. And 27.7V would probably not top off the battery properly, although I reckon it will be fine… just take a bit longer.
Whether setting both to 28.0V will achieve equal load sharing, I don’t know. It would depend on the output impedance of the two sources.
I have 12v bus.
Left gen gives 14,4v right 14,1v.
When both connected bus voltage is 14,4v – right alt is on “idle” as it gives only 14,1v I guess.
14.4 or 2.4 v per cell and a lead acid battery will gas a little and is likely to shorten the life if kept at that voltage for long periods and not refilled which with a sealed battery can’t be done. Ideally it should be temp compensated.
Ted wrote:
14.4 or 2.4 v per cell and a lead acid battery will gas a little and is likely to shorten the life if kept at that voltage
Yes. I think I will need to reduce the voltage to 14,1v on the left one.
One could determine if two alternators can be paralleled and load-share. The voltage regulators all have a similar gain.
I did some tests on common voltage regs (e.g. Lamar) and found they are a simple 200Hz PWM whose duty cycle varies from 100% at 27.7V or less, to 0% at 28.3V or more. The control loop is just proportional between these two and presumably they found this delivers loop stability across a range of alternators. This comment from the designer of the B&C product line:
Alternators are exceedingly forgiving for regulation dynamics. Regulator gains on the order of 30-50 are common across the spectrum of alternator regulator products. This corresponds with your observation ( 28 / 28.3-27.7 ) = 46.6
Souds quite complicated :)
I don’t know what kind of regulators I have. They come from ’70s.
Comparing bigger planes – you have usually AC 400hz generators and they cannot be paralleled.
They supply separate busses. One bus can be powered by only one AC source because there is no synchronization of geretators. It would be too complicated.
Of course with DC – this is another story.
How well multiple DC sources will current share depends on their output impedance (how much the output voltage varies with the load) and how well matched it is to start with. A google search like “paralleling voltage regulators” digs out the basic theory.
My above-mentioned tests show that the reg works over a range of about 0.6V, and the rectifier diode drop (two diode drops actually, for a 3 phase rectifier) will anyway vary from about 1V to 1.6V (so the open-loop output impedance is significant), so two alternators should load share fairly well.
If the regs are 1970s they are possibly mechanical “vibrating leaf” types which turn the field current on/off really crudely – although a fully electronic reg was possible to make back in the mid/late 1960s.
The later regs are mostly the Lamar types and they have an adjustment potentiometer. It is best adjusted on the bench because you don’t want to be doing it with the engine running and the avionics powered from it, and then the pot breaks off and the reg drives the alternator to 50V 
AC generators need to be accurately phase-synced. I don’t know if big jets do that; I doubt it. I know they do rpm-sync engines to minimise vibration but that’s not the same thing.
Peter wrote:
AC generators need to be accurately phase-synced. I don’t know if big jets do that; I doubt it.
You are correct. Jets don’t sync generators. One bus – one source.
If one fails – the bus is then connected to the other gen.
tmo wrote:
I’ve read that Continental (I have a TSIO-360 in my Mooney) engines are prone to shearing the alternator coupling
Yes, that’s the same drive coupling I referred to for the front alternator 520’s (I’d forgotten it was used on some 360’s, I’ve been away from the details for a while). Yes, misinstallation, and misaligned gears could cause a broken driveshaft. Otherwise, for simple alternator torque [over]load, that coupling will allow a slip. The red silicone doughnut is friction fit in the outer case. Once it slips once through alternator drive overload, engine oil finds its way in, and the coupling will slip at a much lower torque, eventually becoming useless, and not repairable. The couplings are expensive. Our company worked for over a year to make a “new and improved” one, and gave up.
So I go really easy on loading alternators on those engines. Happily, more efficient avionics, and LED lights, make that much easier than it was when those engines were first developed.
