How to measure the Back-Up Battery Voltage

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Hi there,

I am using an external RTC ic connected to a micro. The RTC is been supplied by 2 separated power supplies via 2 schottky diodes. The 2 power supplies are a non-rechargable Lithium battery 3V and a 5V dc regulator.

The micro is been supplied by the 5V potential only.

In power fail mode there is no power supply for the micro, but the RTC ic is been supplied by the Lithium battery.

I would like to measure and monitor the Lithium battery Voltage level using an ADC input. This can be done in normal operation mode (VRef = Vcc). But, in power fail mode and while there is no power at the Vcc pin, there must be no power at any micro's pin, in order to protect the micro from an unexpected latch-up condition.

What are your ideas about solving this problem (for Lithium battery voltage measurement), following the simplest way?

Michael.

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I'm thinking there could be a micro-power analogue switch device out there somewhere but I'm not familiar with any.

The idea is that you power this switch from the 3V battery and use the 5V source to switch it such that, when 5V is removed, the switch output goes Hi-Z. When 5V is present, the 3V is passed through.

I tend to think it will be too much of a drain on the battery. Maybe a MOSFET could do the trick.

Steve

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Something like this simple circuit might work.

The threshold voltage must be less than the difference between 5 volts and the battery voltage. If a CR2032, for example, you need a threshold voltage less than 2 volts. Transistors with 1 volt threshold are easy to find so this should not be a problem.

Choose a resistor value close to the ADC input impedance.

There will still be some leakage current when 'off'. How much depends on the transistor used, but should be on the order of a micro amp.

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Thank you guys,

This is what I was thinking off too.

I am wondering if there is another (different) solution.

Regards,

Michael.

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Michael,

A very, very simple solution would be to place a relay across your 5volt power supply. When the 5 volts is present, the relay is activated and its contacts connect your lithium battery to the ADC input. When the 5 volts is not present, the battery is not connected to the ADC.

If you are concerned about the power used by the relay you could use the techniques discussed in

https://www.avrfreaks.net/index.p...

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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Thank you Ross.

Best regards,

Michael.

User of:
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