Battery voltage monitor

[cak]

Back when I was having charging system problems caused by a faulty mechanical regulator, which then damaged my alternator (see the thread

73.5 charging light problem

Patton (John Sturgill) posted a note with a photo of a little voltage monitoring box (he calls it the BMA-1, Battery Monitor and Alarm). I pestered him about it for a while and he allowed as to how he was building them and would sell me one, but didn't want to get in trouble by using the Pelican bboard for self-promotion ... in fact, he deleted the post not long after.

I bought one (about $40, as I recall). It's a little thing, about 1x1x.75 inches, with two LEDs and two wires. If the applied voltage is <11.4V, the red LED comes on. If the applied voltage is >14.8V, the green LED comes on.

It took me a long time to figure out where to mount it to my satisfaction. I tried tucking it up in the ashtray, but couldn't always see the LEDs. Patton has his mounted at the lower outer corner of the dash/knee bolster, but I've got the VPC a/c installed, which puts that location well below eye level.

I finally ended up with it here:
The box is supplied with mounting ears, but I didn't want to drill any holes. I cut off the ears and attached this with contact cement, so it's completely reversible. I mounted it far enough left that it doesn't interfere with removing the steering column cowling.

The other consideration was where to pick up voltage to monitor! I wanted to get to the red/black on back of the gauges, but they are on the "other side" of the steel "firewall", and all the electrical connections go through that with round plugs. The cigarette lighter has the same proble.

I considered going after the light switch, but working in that area is a nuisance and already pretty full of wires.

I was about to attach it to the inline fuse holder for the radio, and decided to have another look behind the gauges. This time, I noticed that the ventilation control cables pierce the "firewall" via a grommet. Voila! A few minutes of fishing with a stiff wire, and I was able to run the BMA's wires to the back of the combination gauge, where I used piggyback connectors to grab red/bk and brown.

In use, the device is pretty neat. I was worried that I wouldn't be able to tell that it was working (or, more importantly, that it had failed) but that doesn't seem to be the case. The red LED flashes briefly at switch on and somewhat longer at switch off (as the alternator field collapses, I guess). The green LED flickers from time to time - a little at start as the engine is settling to idle (and the battery is getting max charge), and often when switching the lights off or between main/dipped.

It would be OK with me if the box was smaller and perhaps used a single two-color LED. The threshold for the red LED could be higher, I guess, to warn that the battery is too discharged to be properly charged by the alternator - I'm not certain where that threshold is, to be honest, but know there is one.

But now I'm picking nits.

Props to Patton for a cool little device. It's not as fancy as adding a voltage gauge to the bottom of the tach, but a whole lot cheaper and probably as effective. I certainly consider it cheap insurance, given how much damage a faulty regulator seems to do to these cars!

[cak]

I've been driving around with this gizmo for almost a month now, trying to get a sense of what is "normal". I was kind of surprised at how often the regulator seems to rattle on/off in a sort of square wave, rather than being smoother - but I think I hadn't thought very hard about how the thing must work. It makes a certain amount of sense - it's basically doing PWM on the output of the alternator. But I never really understood why it would swing all the way to low voltage (11.4v or less).

I've come to appreciate the value of two LEDs - since it's a fast-acting device, I can see this behavior when both LEDs flash. If it was a single bi-color LED, it probably wouldn't be as obvious. Very cool.

Today, it started acting differently - when I drove off in the morning, it was doing what I'd call "heavy charge" - very bright at both extremes. I was going to drive out to our cabin, about 4 hours, and considered turning around to get the appliance car ... but it settled down, and I pressed on.

All was good until the twisty bits, where it started up again. I'd been thinking about this while driving, of course - and connected it to something else I'd noticed, that going over a hard bump will sometimes cause both green/red to flash on.

I decided that there must be an intermittent short to ground somewhere. As I thought about it more, I decided that the likely candidate was the extra battery cable in the now-empty box (the previous owner had installed an Odyssey in the driver's side box); I couldn't remember just how it had been covered and secured.

When I stopped, I was pleased to find that the unused terminal is ty-wrapped in place, but only covered in tape. I'll fix that, but it's not the culprit.

So, look at the other box ... ah. It appears that the battery is not in quite tight enough, and has slid slightly, such that the positive post is very close to the hold-down strap! I'll just bet that was the problem... I moved the battery, and will think about what I can use to provide a bit more shim.

Amazing what you can learn from two LEDs and some thought. And what a potentially big payoff a $40 investment can have - this would have been a nasty fire if it had shorted hard!

[cak]

Well, that was interesting.

I just took a four hour drive across hill, dale and freeway, after having pushed the battery around so that there's no chance of the + terminal shorting to anything.

Other than the red LED coming on at start (expected) and the green flickering at higher RPMs (also expected), they never blinked once! Much more sensible, if not as entertaining.

So, as far as I can tell, this may be a "day one" (to me) issue. For all I know, the rapid fluttering is what caused my mechanical regulator to go out of whack - I can easily imaging that the points didn't like being slammed back and forth between the limits, rather than just a light chatter. (Maybe not - doesn't matter.)

Someone asked me why not just put in a voltmeter, and it's a reasonable question. I think that a voltage meter has three problems that the LED monitor doesn't:

1) unless you do the trick setup of putting it in the bottom of the tach (spendy), it takes up more space and will look more tacked on (or replace an instrument I like). Admittedly, this looks sort of tacked on, too - but not at all unlike some of the other things Porsche tacked on to the interior along the way.

2) most voltmeters are internally damped, and one with a needle will at least have the mechanical damping of moving the needle mass, so it will show the 'average', rather than the extremes, and the extremes are what tipped me off.

3) the electrical system is a good place for "idiot lights", because it's mostly working, and I want to be alerted when it's out of range. The bright LED catches your attention where a slight needle flicker might not.

Patton's little box does all that.

I'm just glad that I don't have more items in the car that care a lot about overvoltage - probably just the radio. Those of you with DME might think twice about something like this ...

(no, I'm not getting any kickbacks. I just think this is a sweet solution!)

Now, to figure out how to mount the Odyssey 925 better. I think I'll be fabricating a plate out of a cutting board...

[PCA7GGR]

Chris,

While reading this thread, I got to thinking, I've got something like that in my toolbox. I went out to the garage and found the gizmo pictured below. Its called a "Auto Electic Analzer" made by Micronta (#22-1635) and plugs into the cigarette lighter. With the the engine off and low beams on, it checks the battery and, once on the road for 15 minutes, it checks the alternator. I got it at the local RatShack (AKA Raido Shack) years ago.

[cak]

Sergio .. that's a neat gizmo to have in the toolbox. I'll have to look for one. (I always thought that Micronta was Radio Shack's house brand!)

[PCA7GGR]

Come to think about - I did get it at RatShack.

[moneymanager]

For a later version of this gadget, see my writeup on the early S list.
http://www.early911sregistry.org/forum/showthread.php?t=24094

[Newsboy]

Isn't 11.4 a little low for the red light. My car won't start at this level of discharge. I bought a digital battery monitor that plugs into the cigarette lighter. It gives you a digital readout when the car is off or while driving. It has two power ports, so you can plug in additional items. Cost around $20.00. I'll see if I can locate the website where I bought it and post the info.

[Newsboy]

Here it is. http://www.cetsolar.com/ I bought the Digital Volt meter And Socket Multiplier.
They also have one without the sockets. I tested it against my digital multimeter and they gave the same readout.

[RoninLB]

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