Re: Battery level tester.

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

Date: Sun, 26 Sep 2004 15:11:30 -0500

On Sun, 26 Sep 2004 16:18:08 +0000 (UTC), "Colin Dawson"
<nospam@cjdawson.com> wrote:

>Let me put this another way. Say I've been using my battery for a while. I
>will consider it completely flat when is reaches 10v either under load or
>not. At the moment, my battery is reading 11.3v and is under load. My
>circurit, is showing 11.3v. When I turn off the load, the battery voltage
>immediatly jumps up to 11.7v. When I turn on the load it slowly returns to
>11.3v. One of the devices that I'm using is a laptop. When the hard drive
>is working, the current rapidly changes as the drive heads move across the
>disk and data is read/written. This turns the LEDS on the circuit into
>quite a good light show, which is really annoying and I want to stop it
>doing that. 

---
A really simple way to do that would be to partition your voltmeter so
that it only displays mutually exclusive LEDs which correspond to
three voltages; say, >11V, which would light a green LED, 11V to >10V,
which would light a yellow LED, and <=10V, which would light a red
LED.  Very easy to do with a single LM339, a voltage reference, and a
handful of resistors.  Interested?
---
 
>>>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.
>> ---
>
>hmm, I shouldn't have said that.  It's why I built the circuit in the first 
>place.  What I want is a steady reading, not one that makes the LED's look 
>like a reject from a bad SCI FI Film.
---
OK
---
>> 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?
>
>Yes, that's exactly what I'm trying to achieve.
---
Well, that's a little tougher than three LEDs, for all the reasons
Fred Bloggs enumerated, but if you wanted to try it what you'd need to
do would be to monitor the current coming out of the battery an the
battery voltage while keeping track of time in order to determine the
total energy delivered into the load.  Knowing how much energy was in
the battery to start with, all you'd have to do would be to
continuously subtract what was used from what was left after the last
subtraction and sound an alarm when you emptied the battery.  Of
course, to do it properly you'd also have to keep track of everything
else Fred mentioned, which may really be overkill for your
application, since all you really care about is when it's getting
close to shutdown, and the yellow LED will tell you that.     
-- 
John Fields

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