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Thread: Low battery voltage32 days old

  1. #1
    Junior Member Klr650's Avatar
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    Default Low battery voltage

    I'm new to battery powered stuff and and brown out etc. I have a small application in which the PIC spends most of its time asleep but wakes up when it detects movement. The circuit is powered from a small li-po battery which can be recharged via a DC socket. My concern is about the circuit operation if the battery doesn't get charged for whatever reason and falls below a certain point. Does the PIC operation become erratic or does it simply stop? Will it resume normal operation once the battery charger is connected?

    So, Ideally I'd like it to stop working when the battery voltage falls below a certain value but as soon as a correct supply is applied perform a reset and start running normally. I'd like to achive this using no additional external components - is this possible?

    Thank you.

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    Prolific Poster towlerg's Avatar
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    Default Re: Low battery voltage

    You certainly cannot assume anything about PIC operation as it approaches Vdd low.

    If the PIC you are using has Brown out detect that might work or alternatively many PICs have a built in voltage reference eg 16F690.
    George

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    Prolific Poster joesaliba's Avatar
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    Default Re: Low battery voltage

    I think that the best approach will be to send the PIC to sleep if the voltage falls below a certain point. Then, wake up the PIC, check voltage; if voltage above that certain point continue operation, if still below, go to sleep again.

    The only question is? how often do you want the PIC to wake up and check what is the voltage?

    If you check the datasheet for a particular PIC you find at what voltage a PIC can work.

    For example, an 18F26K22 operates from 2.3v to 5.5v. I used one of these with a LiPo battery and a LDO regulator @3.0v and works very good. You can also use an LF part such as 18LF26K22 which operates from 1.8v to 3.6v and use a lower regulator.
    Regards

    Joseph

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    Junior Member Klr650's Avatar
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    Default Re: Low battery voltage

    Thanks for that. Because of size issues I'm using a 12f675 so I could use its ADC to monitor battery voltage.

    Trouble is that at certain times the pic could be woken up many times in quick succession (via a tilt switch) although I suppose I could disable that if the voltage falls below a certain level.

    Interesting!

  5. #5
    Fanatical Contributor Tim's Avatar
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    Default Re: Low battery voltage

    In a product I designed I used the comparator and Vref in the pic as I could not use the LV detect pin. It generated an interrupt when the voltage dropped below your threshold setting.

    You are in a difficult position if the power could come back up by itself as you can be caught in limbo land

    Again with my application I did the following but bear in mind that I had no batteries

    1 Turned off power hungry IO
    2 Turn on the supercap
    3 Save all that was important
    4 Check the line voltage using an ADC to check if was coming back up again
    5 Sit in a loop doing nothing until the MCLR reset the pic

    I understand that it will not be of much use to you in your application but may be an inspiration to make you think of a solution.
    Tim

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    Junior Member petemilligan's Avatar
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    Default Re: Low battery voltage


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    Junior Member Klr650's Avatar
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    Default Re: Low battery voltage

    Thanks for your replies - I've got a circuit up and running that looks at the battery voltage and goes to sleep if it falls below 3.4V.

    However, I have read that these batteries have a built in switch that disconnects if the battery falls to 3V? If this is true then I don't need to monitor the battery myself. I have noticed that the battery does disconnect if I try to overcharge it so there is some electronics inside. Today I'm going to discharge one to find out.

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    Junior Member Klr650's Avatar
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    Default Re: Low battery voltage

    This morning I have spent a couple of hours of my life monitoring the discharge of one of my 100mAH batteries! Attached is a screenshot of the data. I made up a 50 Ohm load then took measurements.

    As you can see, the battery voltage falls off rapidly just after it reaches 3.5V and it seems that there is no cutout operating which is a shame!

    lipodis.jpg
    Last edited by joesaliba; 20th November 2017 at 12:39.

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