Re: Battery level tester.

From: John Woodgate (jmw_at_jmwa.demon.contraspam.yuk)
Date: 09/26/04 

Date: Sun, 26 Sep 2004 11:53:19 +0100

I read in sci.electronics.design that Colin Dawson <nospam@cjdawson.com>
wrote (in <cj61pj$eur$1@hercules.btinternet.com>) about 'Battery level
tester.', on Sun, 26 Sep 2004:

>I think everyone here has managed to completly miss the point of what
>I'm asking.

No, I'm afraid that you are asking a question that doesn't make a lot of
sense in the context in which you pose it.
>
>
>There is enough voltage in my setup to power everything that I want to
>power. The cables are thick enough to power everything that I want to
>power, and then some. (I can easily double the number of devices
>without ricking overrating the stuff that I've used. I've done it)

The cable 'rating' may be based on temperature rise, not on voltage
drop. A few volts off a 230 V supply are negligible, but not off 12 V.

The behaviour of your meter, as you describe it, indicates without doubt
that you have voltage-drop problems, either in the cables themselves or
in the terminations.

What size(s) are the conductors in the cables?
>
>At the moment my "Battery Monitor" is actually a "VoltMeter". I don't
>want a VoltMeter connected to the battery, as is doesn't tell me when
>it's time to start thinking about recharging the battery.

Well, it WOULD. Not the best possible solution, but it would. The fact
that the voltage falls is enough to tell you that you need to charge.
Since you have a deep-cycling battery, using the voltage as an indicator
is unlikely to shorten the life of the battery through over-discharge.

The situation is different for 'float' batteries that are intended not
to be deeply-discharged. For them, using the voltage as an indicator is
OK if the 're-charge' point is set at a high enough voltage, but the
required voltage is temperature-dependent, so it's not easy to implement
such a device. In your case, the simple voltmeter solution is an
acceptable solution.
>
>What I want is a "Battery Level Meter".

What do you understnd by 'level' in this context? It has a lot of
different meanings in electronics.

> Just because I start pulling
>10A from my battery doesn't mean that it's capacity suddenly drops, as a
>VoltMeter shows.

That is true. It's about the only thing you have written here that IS
true.

> I don't care what the Voltage of the battery is. I
>WANT TO KNOW WHEN I NEET TO START TURNING OFF DEVICES BECAUSE THE DAMMED
>BATTERY IS ALMOST FLAT AND MY TELESCOPE IS ABOUT TO LOOSE IT'S
>ALIGNMENT. (at this point Colin has thrown his teddy out of the cot)

You CAN tell that with a voltmeter. There are more complicated
possibilities, notably an integrating ampere-hour meter, but although
that would be desirable, it isn't necessary.

Your telescope loses alignment **precisely because it receives a voltage
that is too low**. Your simple voltmeter will tell you when that
condition is being approached.
>
>Get the point now?

I would mention that we don't mind ignorance on this NG, and we (many of
us) will work hard to dispel it, but we do not like arrogance.
>
>Won't anyone give me a straight answer on how the hell to build an
>Ammeter circuit, so that I can get the "BATTERY LEVEL MONITOR" to give a
>correct reading?

You CAN'T convert your voltmeter circuit directly into an integrating
ampere-hour meter, if that's what you mean by 'battery level meter' and
'battery level monitor'. You could pull it to pieces and use some of the
parts, but an integrating ampere-hour meter is not a trivial project for
you to build.

But there really isn't any need; your voltmeter will do what you want if
you improve the cables. 

-- 
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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