All this talk about regulators... thought I'd ask this. Current charging setup is a remanned NAPA nippondenso alternator and echlin vr38 regulator.

When I first start my car at beginning of trip voltage is in the low 14's with lights on. At the end of my commute some 1 hour later, its low to mid 13's, no changes other then driving car from point a to b. Any ideas? Only thing I can think of is engine compartment heat.

What you are observing is normal. The voltage regulators are temperature compensated. Hot batteries need less voltage than cold batteries to push charging current through them.

Here is the voltage chart from a 1979 Dodge Truck service manual. Same chart is published in the car manuals too. Cold batteries are more resistant to charging so they need more voltage to push charging current through them.

Is there any way, perhaps a set point adjustable regulator, to get it to output a steady voltage? For EFI, the difference between system voltage of 14.2 and 13.3 is significant when it comes to fuel delivery @ idle

I thought I heard of adjustable constant voltage regulators for oru system. ANy ideas of part# or application?

All EFI cars have temperature-compensating voltage regulation. Even the olde-tyme electromechanical regulators on our olde-tyme cars are temperature-compensated (yes, even the ones with a setpoint adjustor screw). Besides, temperature-compensated voltage is a good thing, not a bad thing. You definitely want it. If you're homebuilding an EFI system which must have constant voltage regardless of temperature in order not to distort your fuel maps, put the EFI system on its own regulator.

MP sold (maybe still sells) regulators incorrectly labelled as "constant output", which doesn't mean anything. They're standard, ordinary, off-the-shelf regulators in powdercoat blue.

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Too many people who were born on third base actually believe they've hit a triple.

Well the megasquirt compensates for varying battery voltage by adjusting pulse width. I think this may be why idle when cold is different then idle warm, or at least one of the reasons

Don't mean to harp Dan, just curious, but why do you say temp. compensated regulation is a good thing? I'm not saying its bad, I just wonder why it would be good. Car batteries should be happy at 14.2v regardless of the temp I would imagine, regardless of how much current you need to push through them to charge it.

I really don't care what the set output is, I mean, 13.5, 14.2, etc, just as long as its the same cold or warm.

Not sure what you mean by put the efi on its own regulator... as in seperate battery/alternator/regulator just for the efi? I don't want to go that far.

Nope, that's just it; they're not. Battery internal resistance changes significantly with temperature, such that a higher voltage is required to successfully charge a cold battery than is required to charge a hot one. So if you always used the hot charge voltage, you'd not charge the battery sufficiently or efficiently under cooler conditions. On the other hand, if you always used the cold charge voltage, you'd cook the battery and severely shorten its life (not to mention venting Hydrogen and fizzing acid under the hood) under warmer conditions. Temperature-compensated charging voltage, therefore, was an early addition to vehicle charging systems. It's been with us for well over 50 years.
There are devices you can buy that will feed a constant 12.8v (or whatever) to your load, regardless of the charging voltage being 13.2 or 14.2 or 15.1.
Oh, heck no! Just a box in the feed to the EFI computer!

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Too many people who were born on third base actually believe they've hit a triple.

FYI: J.C.Whitney does sell an inexpensive adjustable non-temperature-compensated floating-field Mopar style regulator. I've used it; the control is on the back side (!), but gives you a range +/- over a volt or so.

It seems to work fine and stay where it's set. I temporarially grounded it and set the voltage at the battery to be 14.2V at warm idle, lights off (my normal use).

I then replaced it with a Mopar Performance reproduction one; it also seems to work fine.

I know the one; little trimpot on the back to adjust the setpoint. It is temperature-compensated—at least all the ones I bought and used were, before I stopped buying from JCW some years ago.

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Too many people who were born on third base actually believe they've hit a triple.

Are gell cells as sensitive to the higher charging voltages at higher temps? The datasheet for the optima yellow cell I have states alternator charging voltage should be 13.65-15.0 but doesn't specify temperature. Maybe these newer technology batteries (that have lower internal resistance) aren't as sensitive.

As to the seperate regulators... constant 12.8v would be nice, but can it handle 10+A loads? I would have to put the fuel pump and injectors on it (megasquirt itself draws minimal current) and it will also have to live with the oscillating load of the injectors.

Gel cells don't like the higher voltages since you can't top them off with water if necessary............

Voltage should not affect the fuel pressure (even though the fuel pump output and pressure will vary) since you should have a fuel pressure regulator after the fuel pump..............

And with constant fuel pressure, the injectors shouldn't have a problem......
The pulse width of the injector (which is controlled by the ECU) should stay the same.... (I don't think the open/close time varies that much with voltage variations if the voltage is above some minimum.... anybody have specs on this?????????)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

And if there is a variation in the injector due to voltage, the ECU should be able to either shorten/lengthen the pulse to account for it..................

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Gell cells are sealed, as long as you don't heat them up past the point of the valves opening optima datasheet says for rapid charging up to 15.6v can be used @ unlimited current as long as battery temps are kept below 125F.

Good point about the regulator, I never thought of it that way. I'll start to look elsewhere about my idle vs temp variations. It just seemed wierd looking at voltage @ beginning of drive then seeing it dip almost 1v later at end.

The Optima is not a gel cell. It is an absorbed-lead-acid battery. Very different animal.
Yeah, the regulator I'm thinking of will handle 10A, but...why would you need to put the fuel pump on it? Doesn't make sense to me. That's what the fuel pressure regulator's for! And the injectors, they don't need constant 12.8v, either. The pulse width is determined by the computer, and the injector will open and close on time whether the voltage it sees is 12.8 or 13.2 or 14.

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Too many people who were born on third base actually believe they've hit a triple.

[quote="SlantSixDan The pulse width is determined by the computer, and the injector will open and close on time whether the voltage it sees is 12.8 or 13.2 or 14.[/quote]

Thats not entirely true, pulse width is set by the computer of course, but injector open and closing times are different with variance due to voltage. Lets say pulse width is 5ms. If computer fires the injector for 5ms, it will actually be open for say 7ms (1ms open, 1ms close) perhaps at 12v, but at 14v the open/close times may be 0.5ms. These figures aren't exact and only meant for illustration so please take them as such.

Thanks for the input, as mentioned, after more thought the varying voltage shouldn't be the cause of my issues. I'll have to look into how MS compensates for voltage variances some more.

What Dan was getting at is the computer will "see" the result of injector pulse, and adjust it accordingly. If the injector was to go for 5ms and went for 7ms the computer will "see" to much fuel as picked up from a oxygen sensor and try a smaller pulse to lean out the mixture, and do the reverse if it's too lean due to shorter injector pulse.