Battery check

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ljbeng
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Battery check

Post by ljbeng » Tue Feb 08, 2005 1:00 pm

I need a battery test circuit with as few components as possible. I just want to test 2AA batteries in series and turn on an led if they are < 2.5V.

Ron H
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Re: Battery check

Post by Ron H » Tue Feb 08, 2005 1:58 pm

Do you want the entire circuit, including the LED, to be powered by the two AA batteries, or do you have a separate power source?

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philba
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Re: Battery check

Post by philba » Tue Feb 08, 2005 8:39 pm

wouldn't it be easier to turn on an LED if the voltage is above 2.5V? Then you could use the batteries themselves to power it.<p>then again, did the assignment specifically say LED on when < 2.5V?

ljbeng
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Re: Battery check

Post by ljbeng » Wed Feb 09, 2005 5:18 am

The main components are (2) AA batteries operating a pic micro (16f628). I don't want an led on when the batteries are new to conserve power. I do want it to come on when the batteries get low (even though this means faster battery drain). Possibly I could pulse the led on/off to conserve power. <p>My real problem is I can't think of a good way to have the a/d of the chip check the voltage of its own supply since the supply of the chip and Vref of the AD will always be at the same level.

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Externet
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Re: Battery check

Post by Externet » Wed Feb 09, 2005 7:10 am

Hi.
Do not use dividing resistors for Vref. Use a 2.5V reference voltage diode instead, or a precision Zener.
Miguel
- Abolish the deciBel ! -

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philba
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Re: Battery check

Post by philba » Wed Feb 09, 2005 7:52 am

Yes, it really helps to explain your circuit a little more.<p>I'd do it the other way around. Use Vcc as Vref and the 2.5V (or what ever) zener to limit the AD input. The logic is reversed. When the supply falls to 2.5V, your AD result will rise to the Vref value (or pretty close). When the batteries are fresh, it will be 2.5/3.2 of Vref. Of course, that only works if you actually have an ADC (check the 628 spec...)<p>And yes, I'd definitely pulse the LED as 2.5 is pretty much end of the line for 2 alkaline cells.

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jwax
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Re: Battery check

Post by jwax » Wed Feb 09, 2005 8:26 am

Maybe a Maxim http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2363?
Or, hack a cheap smoke detector for its' low battery voltage beep/flash chip?

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Dave Dixon
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Re: Battery check

Post by Dave Dixon » Wed Feb 09, 2005 11:24 am

I agree with JWAX. Maxim has a ton of voltage monitoring devices that are super super small.
They are also pretty good about sampling.
Best of luck,
Dave
BTW - the above link didn't work for me.<p>[ February 09, 2005: Message edited by: Dave Dixon ]</p>

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philba
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Re: Battery check

Post by philba » Wed Feb 09, 2005 12:58 pm

it can be done with a zener, a couple of resistors and the 16F628. Think comparator...

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haklesup
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Re: Battery check

Post by haklesup » Wed Feb 09, 2005 1:37 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>I don't want an led on when the batteries are new to conserve power. <hr></blockquote><p>If you power a flasher circuit (<1Hz) and slightly underdrive the LED (less current than spec, less brightness but enough to see) you should notice only a slight loss of battery life. (see the thread on lighted hockey puck)<p>Problem with signaling with a constant-on LED when the battery gets low is that it will only work for a short time as the battery gets completely killed by the LED.<p>A low voltage monitoring IC will probably result in the lowest component count but you will need to spend time looking for the right one.<p>You could also consider using a seperate coin battrery to power the monitoring/LED circuit. This would simplify comparing the AA batt voltage to a reference and allow for "LED on when low" indication you want.<p>To do it with one battery choose a voltage reference IC or zener (less efficient) of sufficiently low voltage to serve as a reference and compare that to a resistor divided voltage proportional to the battery (use high values to avoid wated current).

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Re: Battery check

Post by L. Daniel Rosa » Wed Feb 09, 2005 1:44 pm

Briefly turn on an LED and read the voltage across it. Since you aren't calculating the voltage, you only need a threshhold value. Once the LED is above about 72% (I don't know whether you have an 8 or 10 bit ADC on that chip, and I'm in a bit of a hurry), your supply voltage is low enough to light the warning. For reaading purposes, you only need to power up the LED for a few uS- it may be visable in the dark but not distracting.

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jwax
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Re: Battery check

Post by jwax » Wed Feb 09, 2005 4:45 pm

Sorry about that bad link.
Here's a neat little solution though:
http://www.ncws.com/rcrock/lowbat3.htm

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