Re: Battery level tester.

From: John Fields (jfields_at_austininstruments.com)
Date: 09/26/04 

Date: Sun, 26 Sep 2004 07:45:12 -0500

On Sun, 26 Sep 2004 09:22:59 +0000 (UTC), "Colin Dawson"
<nospam@cjdawson.com> wrote:

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

---
I don't think so.  What's happened is that you have fixed in your mind
what you want to hear and you don't understand what you've being told
so you've concluded that since it doesn't sound like what you want to
hear it must be bogus. 
---
>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)
---
If you increase the number of devices demanding current from the
battery it won't "overrate" the rest of the stuff, all it'll do is
reduce the amount of time until the battery voltage drops to some
arbitrary point.
---
>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.
---
Yes, it will.  There is a voltage below which current shouldn't be
taken from the battery, and once the battery voltage decays to that
point it should either be disconnected or recharged.
---
  
>What I want is a "Battery Level Meter".   Just because I start pulling 10A 
>from my battery doesn't mean that it's capacity suddenly drops, as a 
>VoltMeter shows.
---
Yes, it does.  If you look at the discharge curves for _any_ battery
you'll find that as the rate of discharge increases (as more current
is drawn from it) the smaller its capacity becomes.  The capacity of
most batteries (C) is rated in Ampere Hours, but full capacity can
only be achieved if some fraction of the one hour rate of current is
drawn, ususally C/10 or C/20 for lead-acid batteries.  That means that
if you have a fully charged 100AH battery rated for C/10 and you draw
10 amperes from it, its voltage will decay to the cutoff point (say
10V for a 12V lead-acid battery) in 10 hours.  However, if you take
100A from it its voltage will decay to 10V in substantially less than
1 hour.  Also, since the battery's internal impedance will cause its
voltage to fall more and more as more and more current is drawn from
it, that will futher shorten the time until it reaches cutoff.
---
    
>I don't care what the Voltage of the battery is.
---
Well, you should, and that's precisely why I said that you want to
hear what you want to hear,  not what's at variance with what you
believe.
---
 
>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.
---
LOSE its alignment...
Take a look at your drive's specifications and I'm sure you'll find
that there is a VOLTAGE which is called out below which the drive
won't be guaranteed to remain in alignment.  For that purpose, you
need to monitor the voltage at the _drive_, not at the battery, since
if you measure the voltage at the battery the drop in the cable going
to the drive won't be accounted for. 
---
>(at this point Colin has thrown his teddy out of the cot)
>
>Get the point now?
---
That you're frustrated because of your ignorance _and_ spoiled _and_
petulant?  It's starting to sink in...
---
>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?
---
Yowzah boss!!!
         +-------------------+
        /                    |
    A--+                   [FUSE]
       |                     |
   [AMMETER]                 +----------+
       |                     |          |
       +-----------+      [DRIVE]  [VOLTMETER]
       |           |         |          | 
   [BATTERY]  [VOLTMETER]    +----------+
       |           |         |
    B--+-----------+         |
        \                    | 
         +-------------------+
       
Everything else connects (just like the drive with its own set of
wires and its own fuse) to points A and B. That is, directly to the
ammeter and the battery.
But... That's still only going to give you voltage readings and
current readings, so you'll still have to disconnect loads depending
on battery voltage.
Or maybe you want something to let you know how much charge is still
in the battery or how much time you've got left until it goes flat?  A
battery "gas gauge" kind of thingy?
Well, boss, if that's what yuh wants, just ax fo' it an' ahm sho' some
of us ol' niggers'll jump at the chance to serve yuh.
   
-- 
John Fields
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