DATALOGGER SHIELD: RTC BATTERY LIFE VERY SHORT

[NikoJob]

Hi there!

I'm facing an issue with datalogger shield (ADA 1141), more specifically with the RTC battery life.
https://www.adafruit.com/product/1141

Everything runs well, and the PCF8523 does it job nicely, but I came to realise that in less than one week not being powered, date information was lost (i.e. RTC appeared as not initialized anymore).

Then, I soldered a cable on the battery holder to monitor the battery voltage via one analog pin of my UNO, and while it is about 2.8V when the battery is new, it does drop to less than 2.5V after not being power for 12hours .

Theoretically, the PCF8523 will need about 150nA so that the CR2020 battery life should be in excess of 5years. I repeated the experiment with other battery brands, but with little success. Battery is drained within a week, at best.

Is this a known issue, or did I miss something ? Many thanks for your suggestions!

Nicolas

[jps2000]

You are discharging the battery via the analog pin of the uno.
There is a protection diode in the controller between the analog pin and the VCC of the controller that conducts when the controller is not powered.
You need to measure just the battery current in off with a multimeter

[NikoJob]

Well, reading the answer I now feel a bit stupid ... thanks a lot for the explanation!

My take from this is: I wanted to monitor the battery condition (say, every 24h), and send a warning to the user. This is not possible, at least directly.

Next question is: Is there a workaround or should I simply abandon the idea? Or maybe, switch to another product, like here:

https://www.adafruit.com/product/3013

This breakout features a "VBAT" pin, but will I run in the same problem? My lack of basic electronics knowledge is really limiting, please excuse the possibly dumb question.

[jps2000]

You can use a little reed relay to switch the battery voltage to the analog pin of the uno just for the measurement.
There are also electronic switches but they need more careful design not to run into similar problems.
other RTC will not help

[NikoJob]

... Reed relay ? .. Never heard of that before... Let's see...

Found this:
https://www.pickeringrelay.com/introduc ... cs-part-1/

Wow! Looks like a switch, normally open with 10^10 Ohm minimal open circuit reference, less than 2€/unit. I should be able to fit that onto the datalogger prototyping surface without trouble.

A very, very big thank you, sir !

[jps2000]

You may think I suggested such a relay just because it is small.
That is only one part of the story. The other is it has a gold contact in protection gas environment an hence can switch very little currents. Standard relays may not switch properly in such an application. They need some current over the contact to switch.
I do not know if you know how to connect a relay to a controller?
Look into the data sheet for the coil resistance. Your GPIO Pin needs to handle that current if you do not have an additional transistor. You may however also use 2 GPIO Pins in parallel.
Next, switching a coil causes at the ON to OFF transistion a negative voltage peak across the coil. So you have to connect a small diode (1N4148) across the coil such that the cathode faces VCC and the anode the GPIO Pin.
Some relays also ask for correct + and - polarity
--> Study the appropriate data sheet