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Low Battery Circuit

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nc2bexpress

nc2bexpress

Electrical
May 15, 2007
8
US
Hello, have a circuit question for you guys!
I'm trying to incorporate a low battery indicator to an existing circuit and looking for some help/advice.
Unfortunately the circuit itself is already a low voltage circuit, (x2) 1.5V button batteries.
Additionally, the circuit has to be as minimal in components as possible to help conserve the final size of the final circuit.
I know the conventional way would be to lower the Duty Cycle and frequency but how could I implement this?

Any designers willing to help!?

Thanks!
 
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Oh, heh. Sorry, just realized that was suppose to be 4 buttons. Even that isn't much of a help though.
Anyways.
Thats kind of the conclusion that I came down to. Thats kind of why I'm in this design issue.
Any low-voltage alternatives that might come in mind?
 
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Well.. only if you can have limits on the indication. Like the indicator only shows low battery for 3 seconds after turn ON. Or out of the box, only shows a Good Battery Indication on start-up.

Keith Cress
Flamin Systems, Inc.-
 
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Just a gripe. I've yet to really find a low battery circuit that doesn't tick me off, one way or another.

My digital camera has a circuit that shuts down the camera when the battery is "too low," even though there's no plausible reason that the camera can't work down to 4V, but NOOOOOO, put in rechargeable NiMH batteries, and the instant the voltage drops down to 5V, the camera turns itself off. Dangit, I HATE THAT!!!

TTFN

FAQ731-376


 
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Hmm, battery indication on start-up.... I need to swim that around the noodle. Although its counter intuitive for the low battery, its a darn good way of showing that the battery is good. Solves the problem by showing the opposite.

I liked the idea of the flashing light, lowered-frequency/timer deal. By limiting it to a handful of flashes when the device turns on would be a great solution.
How could I go about making the light to flash (lets say)5 times and shut off afterwards?

BTW, thanks for the quick responses and I couldn't agree more IRstuff!
 
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One of my gripes is low battery indicators that cause an additional drain on the battery. (And keep in mind that the additional circuitry that detects the low battery could significantly reduce your battery life as well.)

Are there safety or reliability issues, either with the circuit/batteries or the application? Looks like you're not using rechargables, so there's less concern there. What are the consequences if the device just quit working? Do you really need a low battery indicator?

What are you really trying to accomplish? How much advance warning are you trying to give? If I'm miles away from anywhere and my widget suddenly tells me it has 15 minutes of life left, that might not help me. Then like Keith said, a battery good indicator is much more useful, tell me before I set out if I can expect the thing to work for another hour/day/week/whatever.

I heard of a company that had a low battery indicator built into a conventional flashlight (batteries, incadescent bulb, switch). An LED would turn on to tell the user that their flashlight was getting dim. Brilliant! If your normal operation gradually and safely degrades, (light dims, wireless range slowly reduces) that could be the indicator right there. Also, you could modify the operation intentionally. It all depends on what your device is. Is there some parameter about the normally operating circuit you can change to indicate low battery?
 
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Look at consumer products. Some of them are pretty well thought out. But most such products running on 4 button cells do not have any such indication.

 
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I think the decision really hinges on the criticality of the outputs of the circuit.

Obviously, the camera guys feel that it's better that you blame the low battery circuit for not getting a picture, rather that to think you got the picture and then find out later that you didn't.

TTFN

FAQ731-376


 
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This is actually going to be a small safety device. So the conundrum I'm in is the need to warn of impending power loss at the sacrifice of losing more power! This is true for all battery indicators. Ironic

JimKirk:
I've debated the idea of the fading 'Good' light but I can't help shrug off the concept that it will accelerate the drain of battery life in an attempt to indicate a dying battery. More Irony, yet very useful. The big trade-off is really 'will the user accept a shortened battery life in order to know when the battery will fail'.

All great ideas though.
 
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If it's a safety device, then H*LL yes!

In fact, this is one case where you want there to be AMPLE warning so that the user has time to schedule the battery change when the safety device can be placed off-line.

TTFN

FAQ731-376


 
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Look at Smoke Alarms.

Run for a year on 9v battery. Very safety critical. Low battery warning system nailed 110% perfectly.

Battery gets low, start beeping once in a while. Do this without taking any significant power.

 
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There is definitely a need for the extra circuit.

VE1BLL: Good point. Don't know if I'll be able to untangle the main circuit from the low battery circuit but I'll take a look at it! Thanks
 
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(Look closely at the theft alarm on your car. Short bursts of very short flashes at long intervals)

Put it one of them tiny PICs, have it wake up and measure the battery voltage every couple of minutes, and do nothing if the voltage is okay.

If the voltage is low, it should flash an LED, say in bursts of short fhashes every 10 secs, with the number of flashes in a burst increasing as the voltage goes down... or something. Mock one up and screw around with the flash patterns.



Mike Halloran
Pembroke Pines, FL, USA
 
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A flashing LED isn't going to wake anyone up at 1am.

Depending on exactly what this mysterious critical device is, it might require a beeper. Or at least design the entire system so that the LED will keep flashing for a few days to give the humans a fair chance to notice. That might be difficult because flashing once every minute can be easily missed too...



 
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The safety device itself will actually be passive until the user interacts with it. Thus saving power when its not being used.
Thats kind of why I like Keith's idea of giving the user an indication upon 'turning on' the device. That way, hopefully, they will take note of the fact that the device is on its way to dying.
I did think about using a PIC device to help monitor the circuit but I was hoping to keep the device as simple as possible. Nonetheless, its still an viable option.

Thanks again for this great brainstorming session! Hopefully one day I'll be able to help offer good advice on this forum! Happy to be a part of this site!
 
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I would still recommend the PIC device - a 7 pinner. They run on extremely low current. Like the battery rots before they consume all the energy. Hope there's one that has A2D, as I haven't looked closely.

With a PIC you can easily tailor the "annunciation" be it light or sound. I'll cleanly handle the 'warn on power up'. Essentially the PIC on its own cold boot does the check. You can even have it switch on a load resistor to actually 'test' the batteries at "running load". This gives a much more solid test.

My $50 P.O.S. electronic rat trap turns on a bright green LED for 3 seconds just when you turn it on, to show that it has sufficient charge to zap a rat,(so they say).

Once your device is running, if it has no obvious sign it is running, (like a transmitter), the PIC can wake and see the actual load's drop in battery voltage and alarm yet again with blinks or peeps to tell the victim bobbing in the water that they can now give up all hope.

Keith Cress
Flamin Systems, Inc.-
 
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"I would still recommend the PIC device - a 7 pinner."
7? , Now I'm really tryin' to figure out which one?
 
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