Re: Battery level tester.

From: Charles W. Johson Jr. (qrus19_at_mindsprUng.com)
Date: 09/26/04 

Date: Sun, 26 Sep 2004 17:13:47 GMT

"Colin Dawson" <nospam@cjdawson.com> wrote in message
news:cj6q40$1kl$1@titan.btinternet.com...
>
> "John Fields" <jfields@austininstruments.com> wrote in message
> news:ch9dl0950jntid20e0udompkfggakf3g3i@4ax.com...
>> 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.
>> ---
>
> Maybe your right. I have locked my mind on a possible solution and refuse
> to accept "No" for an answer. I'm a computer programmer by trade and
> find that other developers (and me sometimes) say that something cannot be
> done, only to find that with a little more thought and creativity, that
> there is a way (all-be-it complicated) to accomplish whatever is was.
>
> I do see electronics as being any different to a computer program, after
> all they're not that unrelated. A program is, to put it simplistically is
> basically a sequence of flipping switches. (ok ok, that's probably too
> simple)
>
>>
>>>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.
>> ---
>
> oops, sorry. I should have been talking Amperage, the leads that run from
> the battery can carry about 40Amps (Not sure the exact thickness, but it's
> the stuff that's used in model power boats to connect from the battery to
> speed controller. I think it's about 4 or 5 mm thick)
>
>>
>>>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.
>> ---
>
> OK, I need to get more litteral about this. At the moment, the circuit
> that I'm using measures a voltage range between 11v and 14v. I know from
> previous experiments that I want to recharge my battery when the voltage
> drops to about 11v with no load (other than the monitor circuit). Also I
> know that as I increase the load the voltage that the monitor reads is
> forced lower by .x of a volt depending on the load. The higher the load,
> the higher that this .x is. At the moment my circuit, when under high
> load, shows the 11V reading pretty quickly when the battery is under a
> high load. When I shut off the load, the voltage returns to a higher value
> and gives it's true reading, which match the chemical thingy in the
> battery (which I can't see in normal use as it's not illuminated, and shut
> in the boot of a car)
>
>>
>>>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.
>> ---
>>
>
> 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.
>
>>
>>>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.
>
> <snipped>
>>>(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...
>
> I asked for that.
>
>> ---
>>
>>>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?
>
> Yes, that's exactly what I'm trying to achieve.
>
>>
>> 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
>
> Colin Dawson
> www.cjdawson.com
>

Colin
    What you want is called an Integrator, basicly it'll be a lowpass
filter, to filter the spikes caused by powering on and off.

Charles