6v battery indicator

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rebeltaz
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6v battery indicator

Post by rebeltaz » Wed Dec 26, 2007 2:01 pm

I built a 4 D-cell battery pack to run my digital camera, but I am now using a 6v 7.2Ah lead-acid battery. I am trying to find a simple schematic using LEDs (maybe red-yellow-green?) to display the battery charge status when I press a momentary pushbutton. Any ideas? Thanks....


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Derek Tombrello
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terri
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Post by terri » Thu Dec 27, 2007 4:55 am

Battery status with a load or without? (Open circuit or running the camera.)

How big a load?

What voltage cutoff points are you looking at for "status?" (At what point will the camera quit?)

Is there any way to connect directly to the internal battery connections in order to use the camera's battery status indicator? (Rather than the wall wart plug.)
terri wd0edw

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MrAl
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Post by MrAl » Thu Dec 27, 2007 9:18 am

Hi there Derek,

There are a number of approaches to this problem, the simplest
is probably just a four comparator chip like the LM339 and a
voltage reference diode (both very cheap).
The diode sets the reference voltage so you can power this with
the battery to be tested. The LM339 compares the battery voltage
to the voltage of the reference diode and trips on three different
levels: low, med, and high. The three levels are set using standard
1/4 watt resistors (also cheap).
The cost is low and complexity very simple, and you can set the
voltage levels to whatever is required for your battery.
For example, 5.5 and up could be 'high' (green LED),
while 5.0 to 5.5 'med' (yellow and green LEDs lit),
4.5 to 5.0 (yellow LED on) 'med low',
and anything lower than 4.5 would be 'low' (red LED).

You can also use a microcontroller if you are into that.

You can also make the thing very low power by using low power
devices and using only high brightness LEDs and power the LEDs
with only a small amout of current (like 100ua) so you dont need
a pushbutton, and/or power the LEDs on for only a part of the
time, like say 2 seconds per minute, which would reduce average
power by even more (1/30).
Some uC's use very low power and that coupled with high brightness
LEDs driven at low current (even 1ma ok) and enabled for only
2 seconds out of every minute would be very good. You'd have time
to read the status and still keep drain power very low without having
to press a button.
BTW, even a drain of 400ua on a 2 ampere hour lead acid cell
would take 7 months to drain it. A drain of 200ua over a year,
which is most likely less than the self discharge rate.
LEDs vs Bulbs, LEDs are winning.

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rebeltaz
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Post by rebeltaz » Thu Dec 27, 2007 10:30 am

terri wrote:Battery status with a load or without? (Open circuit or running the camera.)

How big a load?

What voltage cutoff points are you looking at for "status?" (At what point will the camera quit?)

Is there any way to connect directly to the internal battery connections in order to use the camera's battery status indicator? (Rather than the wall wart plug.)
What I am looking to monitor is battery voltage without a load. Actually, even plugged into the AC socket on the camera, the battery indicator seems to work. The main reason for this is a quick "should I charge the battery or not" indicator.

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rebeltaz
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Post by rebeltaz » Thu Dec 27, 2007 10:32 am

MrAl wrote:Hi there Derek,

There are a number of approaches to this problem, the simplest
is probably just a four comparator chip like the LM339 and a
voltage reference diode (both very cheap).
Thank you.. I will see what I can do.

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Post by terri » Thu Dec 27, 2007 1:33 pm

Well, you still have to know your setpoints, even if it's going to measure open circuits. I don't know why people automatically go to integrated circuits to solve a simple problem like this.

All you need is a voltage divider with three taps, with the LED limiter resistances figured into the taps.

Oh, and a pushbutton.

Jeeze.

Keep it simple.

'Bye, now.
terri wd0edw

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rebeltaz
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Post by rebeltaz » Thu Dec 27, 2007 2:05 pm

terri wrote:Well, you still have to know your setpoints, even if it's going to measure open circuits. I don't know why people automatically go to integrated circuits to solve a simple problem like this.

All you need is a voltage divider with three taps, with the LED limiter resistances figured into the taps.

Oh, and a pushbutton.

Jeeze.

Keep it simple.

'Bye, now.
I am the first to advocate simplicity using discreet components over microcontroller ICs. Believe me. I've even written an editorial to Nuts & Volts on their choice of PIC chips (which I enjoy using) when a simple transistor-resistor-capacitor setup would work just as well, if not better. My past designs will attest to that fact.

When it comes to this, I assume that you mean to use a resistor with a high enough value that the LED will only light above a certain voltage level. While that will work, I think that it would be easier to ascertain levels when there is a definite on-off state of each LED instead of the varying brightness levels that would occur with only a divider network.

Thank you for your time and advise, but this is one application where I think even I would prefer an IC (but not a microcontroller)...

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evahle
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Re: 6v battery indicator

Post by evahle » Thu Dec 27, 2007 2:39 pm

rebeltaz wrote:I built a 4 D-cell battery pack to run my digital camera, but I am now using a 6v 7.2Ah lead-acid battery. I am trying to find a simple schematic using LEDs (maybe red-yellow-green?) to display the battery charge status when I press a momentary pushbutton. Any ideas? Thanks....


---
Derek Tombrello
www.ShelbyTVService.com

Hi Derek. Myself, I'd use the LM3914 chip. You'd only need 5 resistors for setting any voltage points to turn on the leds. You can have regular LEDs (up to ten) or can use a BAR display. Just google the LM3914 to get the datasheet. Also this IC doesn't require resistors to the LEDs. Hope this helps.

evahle

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rebeltaz
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Re: 6v battery indicator

Post by rebeltaz » Thu Dec 27, 2007 6:29 pm

evahle wrote: Hi Derek. Myself, I'd use the LM3914 chip. You'd only need 5 resistors for setting any voltage points to turn on the leds. You can have regular LEDs (up to ten) or can use a BAR display. Just google the LM3914 to get the datasheet. Also this IC doesn't require resistors to the LEDs. Hope this helps.

evahle
I checked out the datasheet for that chip. That looks like a pretty simple solution to my problem! I actually thought that I had one of those laying around, but I can't seem to find it. I'll order one next order I place.

I appreciate your reply...

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Post by terri » Thu Dec 27, 2007 10:26 pm

Derek said:

"I built a 4 D-cell battery pack to run my digital
camera, but I am now using a 6v 7.2Ah lead-acid
battery. I am trying to find a simple schematic
using LEDs (maybe red-yellow-green?) to display
the battery charge status when I press a momentary
pushbutton. Any ideas? Thanks...."

I apologize. I did not see in the specifications
where it states that if one LED is lit, the others
must be out.

Can you point it out to me?

You have, basically, three status levels of interest:

6.8 Volts -a recently charged battery consisting of
three 2V lead-acid cells.

6.3 Volts -the "resting Voltage" of a 3-celled lead-acid
battery some time after charging, but not yet having had
a load on it.

Indeterminate lower Voltage depending on where the
camera quits.

Actual Voltage points mat be set near or at these exact
voltages depending on need of the camera user,
temperature, etc.

If all three LEDs are lit, the battery is in state 1.

If two LEDs are lit, the battery is in state 2

If no LEDs are lit, the battery will not run the camera
and needs recharging.

The specifications, as set out by the OP and as I
understand them, can be simply met by a resistive
divider network with an LED across each divider
resistor, with resistance values appropriately chosen.

Oh, and an NO pushbutton switch.

Oh, and one of the specifications was "simplicity."
terri wd0edw

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ADDENDUM

Post by terri » Thu Dec 27, 2007 10:32 pm

(Inadvertently omitted)

If one LED is lit, the camera will run, but for a time
depending on where the user wanted the setpoint set.
terri wd0edw

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rebeltaz
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Post by rebeltaz » Thu Dec 27, 2007 10:39 pm

terri wrote: Oh, and one of the specifications was "simplicity."
Yeah. Great. Thanks.

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MrAl
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Post by MrAl » Fri Dec 28, 2007 9:06 am

Hi again,


I automatically go with the most 'stable' design i can think of at the time.
This is why i suggested a comparator ic and a voltage reference diode.
If the temperature doesnt vary that much or you are willing to put
up with a little more variation, you can use zeners or even forward
biased diodes. With even more variation, skip the ic and just use
zeners, forward biased diodes, and the LED voltage itself to get the
turn on points you desire.
In the long run though, the cost of a LM339 and a voltage reference
diode ic and a few quarter watt resistors isnt that much anyway and
it's going to make a very accurate and repeatable circuit. The real
cost is a little more wiring than with a zener, resistor, and LED alone.
LEDs vs Bulbs, LEDs are winning.

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LM3914 Circuit

Post by Hello » Mon Jan 21, 2008 1:31 pm

http://www.uoguelph.ca/~antoon/circ/batmon12.htm

Here's a link to a 12V Lead-Acid Monitor using the LM3914, which easily could be adapted to your application.
The LM3914 has the feature of a handy internal voltage reference, and is simple to apply. Also, in addition to the fact that the indicator LED's require no series current limiting resistors, the outputs can be wire-OR'ed when Pin 9 is left open (DOT mode), so that various input signal ranges can be combined to light a single LED.

Also, the outputs can, of course, be used to drive light loads other than LED's, such as driver transistors and SCR's.
One small note is that the low end of the range corresponds to Pins 1, 18, 17, proceeding upward to Pin 10.

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