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Re: Voltage at start
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 21 Jun 2002 13:52:06 Share Post by Email
In Reply to: Re: Voltage at start, BobS, Fri, 21 Jun 2002 13:28:18
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I don't have specific knowledge of how the EDU does its stuff - I only know what I've seen from observation.

I happen to think the voltage display on the EDU may be one of the worst devices on the car. This is because so many folks fixate on tenths of a volt here or there, as if they had some sort of deep meaning and implications. The voltage at any time is affected by a bunch of variables, and it's being measured by a relatively inaccurate device.

Unless the EDU is reading way high (over 14.7) or way low (less than 12.5) when the engine is running, I try and avoid looking at the voltage. The starting voltage is somewhat useful for determining battery condition, but only by looking at trends over many starts. From one start to the next isn't good enough - there's too much noise in the data.

I use the EDU starting voltage for a hint of battery condition, but the big one is to listen for how fast the engine turns over. That worked before we had EDUs on cars, and surprisingly, it still works today. If the engine cranks really slow, I don't need the EDU to tell me the battery is tired.

That said, I have no trouble giving an opinion on how the EDU works:

I'm pretty sure there is no dedicated 'select low' circuitry - I traced out the EDU circuitry once a few years ago, and I don't remember seeing anything like that. I also assume that the A/D is a relatively low sample rate - maybe a few tens of hertz, maybe a hundred. No good data to back that up, just assumption.

The EDU updates the display only about every 0.5 hz or so, but I assume that is made up of a few averaged samples.

Again, I don't know exactly how Saab designed the EDU for determining the startup voltage, but I know how I'd do it. I'm pretty sure the EDU only gets a signal when the key is switched to ON - it doesn't have engine RPM. I would enable the logic when the car was switched on, and then average a few samples over a relatively short period - 0.1 or 0.2 seconds. Store the first period value. If the next period value is less, replace the first value with it. If it's greater, leave the first value alone. Leave that as the 'lowest' value.
The EDU gets wheel speed (for mileage calculations) so it would switch out of 'lowest' mode when it saw the wheels move.

I wouldn't try to get low voltages over a shorter period than about 0.1 or 0.2 seconds. After all, the idea is to get an idea of the health of the battery, not determine the absolute lowest voltage from one nanosecond to the next. That doesn't give battery health, that tells you if one armature coil on the starter motor is pulling more current than the other. Battery capacity has a pretty long time constant, and you'd want to measure over a period along somewhat the same time frame.



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