CR vs BR batteries for VBAT RTC


#1

I’m looking to use a coin cell battery on my photon’s VBAT input. (It’s important for my application that the clock keep running when the device is unplugged or loses connection to the cloud.)

I’ve done some reading on BR vs CR batteries, but it’s still not clear to me which would be more appropriate for the photon’s RTC.

I’m not doing anything special: mostly stored and operated at room temperature and don’t expect it to sit on the shelf for more than a few months.

Any advice?


#2

Hi @chap

I think a CR battery will work great and is about half the cost of a BR battery.


#3

Does VBAT only draw power when there is no power supplied to the VIN pin?
How long does a typical BR battery last? I read that it can work for 7 years.

I’m in doubt if I should create a charging circuit for a Li-ION coin cell battery or just leave that out.


#4

@kasper yes, VBAT only draws when there’s no VIN.

It was important for me to keep the RTC going as long as possible so I ended up going with the rechargeable coin cells, the ML line and have it recharge when VIN returns.

There’s a recommended charging circuit in one of their pdfs that makes it pretty straightforward:


#5

Thanks, that seems a doable circuit. Which ML battery did you end up using? Which capacity mAh will do? Did you test/calculate the time it will keep running?


#6

I’m using a ML-1220 mostly because of the size.

I measured ~3 uA average draw on VBAT.

Here’s a battery calculators I found helpful for quick estimates:
http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-battery-life


#7

@chap how is your ML-1220 setup going?

What kind of regulator did you use to get the 3.2v? Can you use the 3.3v Regulator that’s on the P1s?


#8

@wesner0019 the setup worked well.

In my quick testing it seemed to power the clock for several days, although it’s been so long I don’t remember my actual measurements.

I used the Photon’s 3V3 pin. I never tried this with a P1.


#9

Thanks!