Help interpreting voltage data

[Californio]

Hopefully this will be of assistance to others dealing with the alternator/aux battery charging problems. I'm running three 12 volt house batteries combined by a marine voltage-sensitive relay to the starter battery, wired with 8AWG including alternator to starter.

This morning I did a fairly systematic test at various points, under various loads. Here are the more interesting data I found:

At rest, 12 hours + with no start, alternator reads 12.56, aux batteries 12.64 (they are disconnected from the starter battery below 12.8 volts) and cigarette lighter reads 12.63. That means there is just a tiny drop between the aux batteries and the cigarette lighter, making it potentially a useful place to measure voltage.

At idle, 14.53 at alt, 14.44 everywhere else. Again, no drop at the cig lighter.

Idling w/hi beams (SA set up) 14.36 at alt, 14.13-14.17 everywhere else.

Idling w/hi beams and AC: 14.14 at alt, 13.78 at aux batteries. BUT: after waiting a bit, the voltage starts to fall. In 3 minutes, 13.72 at aux. battery.

Idling w/hi beams, AC, Dometic: 13.36 at alt, 13.20 at aux. But now voltage is falling more: after 3 minutes, 12.81 at aux, 12.66 at cig lighter.

This tells me that at idle, the 90 Amp alt is not putting out enough current to adequately charge the batteries. This is why voltage is falling. Because there are four batteries interconnected, the voltage falls fairly slowly.

At 2000-2500 RPM, full load, alt voltage goes up to 13.82. At aux battery, 13.25.


The way I read this data, and please chime in if I'm wrong, is that at idle the alternator is unable to keep up with full load. It is able to keep up with charging the batteries alone. It gets to almost adequate voltage to run full load/charge batteries at 2500 rpm. However, if it's running at 13.25 +- at the aux battery, that isn't optimum for charging the Optima though it might work for the others.

I haven't been able to test it at highway speeds of 4000 rpm but would guess that the alt output would be marginally higher than at 2500.

Which tells me, all in all, that the 90 amp alternator is BARELY adequate for (a) charging four batteries (b) high beams (c) AC (d) Dometic.

Yes, this is an extreme test, but better to know now than in August in Nevada. On a practical level, one thing it tells me is don't drive with the lights on during the day, and run the Dometic on propane!

[syncrosimon]

With my stirling alternator regulator I have seen the standard Bosch 90 put out 70 amps at tick over, and 120 amps when revved.

The amount of amps that the alternator produces is proportional to the voltage of the system. Your voltages look high, thus not many amps as the alternator thinks that your batts are charged. This is the fundamental flaw of the alternator, it only reads the voltage.

http://www.thesamba.com/vw/forum/viewtopic.php?t=405940&postdays=0&postorder=asc&start=0

The alternators are well capable of producing the amps, it's just that because they are solely controlled by the system voltage it gets it wrong. Sticking on a bigger alternator will make no difference what-so-ever.

Simon
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[Californio]

So if I'm reading you right, the standard Bosch regulator will never allow the alternator to put out enough current to do what I am asking of it. Hence need for another, "smarter," regulator, but not alternator. Gotcha.

What if the voltage goes down? Will the standard regulator allow the alt to put out more current in that case? Thus increasing the voltage throughout the system back to what the regulator considers normal or adequate?

[crazyvwvanman]

You are testing wrong. The idle test is hardly applicable. Nobody drives for hours at idle with everything running full blast and high beams on. Your other data is pretty clear. At 2000-2500 you say the alternator voltage is 13.82 yet only 13.25 at the aux battery. Can't you see that you have a voltage drop there? That is a wiring limitation, not the alternator.

We generally drive at more like 4000. The alternator has a fan to cool itself so more rpms means more cooling. Expecting to abuse it at high load while making it spin too slow to cool itself is a fallacious methodology. Of course it's output drops as it heats up. Physics.

Mark

[Californio]

True, but I was testing at idle because I wanted a worst-case scenario. Obviously from a recharging standpoint it will get better from there.

There's about a .6 volt drop under extreme load between the alt and the final aux battery that would be caused by a combination of wire, connector, and relay loss along the way.

However, it seems like in a real-world sense, if I'm already running 8AWG wires with clean, soldered connectors and a 125 amp relay, there's isn't too much more to do about voltage loss.

Increasing wire size is hard at the starter battery end because of space limitations.

Maybe rewire with 6AWG wire beginning at the alt, remount relay in engine compartment, then to batteries? I always hated working in the starter battery compartment, afraid the + is going to short on something after too much twisting and turning over 20+ years...

[crazyvwvanman]

Now that you know what drop you are dealing with, use your test meter to track down where that .6 drop is. Measure under load at the alternator and on 1 side of the relay, then the other side. Then you will now how much drop is in the wiring before the relay and how much if any the relay has. Then compare the out side of the relay to the aux battery post to see how much drop is in that wire.

It is often an advantage to run a big wire from the alternator just for the aux battery, through a relay. The more current through a wire the bigger the voltage drop. Splitting off the aux charging current will let you get a more consistent voltage at the aux battery, no matter how many dash loads are on.

Once all the wiring drops are dealt with you can consider an adjustable voltage regulator for the alternator. About $25 and you can set it so the aux batteries get a more optimal charging voltage.

Mark

[tencentlife]

You need to start digging on Ohm's Law to understand what Mark keeps trying to tell you.
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[GWTWTLW]

So until today, my voltmeter was wired into the key in relay in the fuse box. Over the past couple of weeks I've noticed that my voltage has been fluxuating between 12 and 13 volts. My yandina has also been clicking on and off a lot lately. The aux battery side of the yandina is connected to the existing red wire that originally went to the fridge relay. I'm guessing that runs through the panel somewhere and is dropping below 13.2 and clicking off.

I was figuring I was going to be putting an alternator on today. However, I'm getting 13.9 at the alternator and about 13.8 at both batteries at idle. That seems acceptable to me. I did notice that the positive terminal on the alternator gets pretty hot. Is that normal?

I also moved the lead for my voltmeter directly to the battery and it is pretty stable at about 13.8. A question though: it's no longer switched with the ignition. I'm assuming that is ok? I wouldn't think it would wear down the battery. Any thoughts? Anyone use a switch or a relay? Thx!!
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