Re: Battery level tester.

From: John Crighton (john_c_at_tpg.com.au)
Date: 09/26/04 

Date: Sun, 26 Sep 2004 22:39:51 GMT

On Sun, 26 Sep 2004 14:28:11 +0100, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

>I read in sci.electronics.design that John Crighton <john_c@tpg.com.au>
>wrote (in <41569542.19425106@News.individual.net>) about 'Battery level
>tester.', on Sun, 26 Sep 2004:
>>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.
>[snip]
>>>
>>Hello Colin,
>>I think I know what you are on about. You want some warning
>>that your battery is about to fail/run out of capacity. Is that it?
>>
>>I saw a little circuit that was used to test lead acid back up
>>batteries, that were on constant trickle charge. Every hour
>>a heavy load, lasting about a millisecond was connected
>>across the battery bank. An op amp was used as a comparator.
>>One input was connected to the battery using a suitable divider
>>and the other input was connected to a referece voltage.
>>
>>If the battery voltage drooped below a preset level when
>>the heavy current flowed in the load, the comparator's output
>>triggered a buzzer. The workers then had a warning that
>>one or more of the batteries in the bank was not performing
>>well and would be no good for back up power when required.
>
>All this is, is a load, which he already has, and a 'voltmeter' that has
>just a 'go and a 'no-go' mark. He's already got a better voltmeter than
>that.

Hello John W you have missed the point of my suggestion.
Colin doesn't have a high current load tester which in the blink
of an eye can check the fitness of the battery to continue its job.
It is a warning device not a voltmeter. Read what I wrote again.

>>
>>How does that help you Colin?
>>Well lets say that you made up a similar rapid discharge
>>tester with a load resistor, in series with a power mosfet
>>across the battery. Some 555 timer ICs could apply a similar
>>1 millisecond pulse to the mosfet every 10 or 15 minutes
>>(or whatever time period you like.)
>>The 50 amp or so load resistor can be made up of cheap
>>wire wound types of several watts, it doesn't have to be
>>super big, power wise, because it only acts as a load for a
>>millisecond. Trial and error will show you that the load resistor
>>can be surprisingly small. 1 millisecond every 10 minutes is a
>>low duty cycle.
>>
>
>There is absolutely no point in all this complication.

Yes there is. John W. you just haven't seen the point yet.
By zapping the battery with a quick 1 millisecond heavy
load, the battery's fitness to continue is checked.
There is nothing complicated about a resistor and a
high power mosfet, switched by a couple of 555s
supplying a 1msec pulse every 10 minutes?
If the triggerd buzzer doesn't sound, Colin will know
that the battery is still fit to continue.

>>
>>You mentioned a 10 amp load for your telescope setup,
>>so connect some old car headlights up to your battery
>>and time how long it takes your battery to discharge to
>> the point where the buzzer on the rapid discharge tester
>>starts to sound, meaning that the internal resistance of
>>the battery is rising, meaning the battery is not so good
>>anymore, nearing exhaustion.
>
>He can already do this with the loads he's got. He doesn't need another
>contrived test.

John W, How can he perform a high current test with
the loads that he has? You are saying this because you have
missed the point of the rapid discharge test. The load he has
is a light load. You can not do a quick high current test with a light
load, You are not understanding what I said. Read what I said
again. I am suggesting using a very heavy load directly across the
battery terminals for a very short period of time (1mSec) which will
check the fitness of the battery to continue.
You haven't read what I said, just pooh poohed without reading
carefully.

What the original poster "needs" is up to him. You sound a bit
like a school master dismissing peoples suggestions.
Let's hear your suggestion for testing the fitness of Colin's
battery to continue its job.

>>
>>The problem with this idea is that towards the end
>>of the batterys life, the one millisecond rapid discharge
>>tester would cause voltage glitches that could upset
>>your computer. You would have to run a number of trial
>>and error tests and adjust the thresh hold of the warning
>>signal to suit yourself.
>
>Yes. The glitching would create havoc.
>>
>>That is all a bit crude, I know, but it is cheap and
>>will give you a bit of an idea how much time you
>>can go on using your battery. You know roughly
>>how much time is left from your trial and error tests
>>with the car headlamp as a load.
>>
>It's all totally unnecessary.

Blooming Heck! That sounds a bit arrogant!
I guess most hobbies and pastimes are unnecessary :-)

>--
>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

Regards,
John Crighton
Sydney

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