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PaxInstruments / t400-electronics

Electronics for the Pax Instruments T400 temperature datalogger
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Super capacitor as RTC backup #113

Closed charlespax closed 9 years ago

charlespax commented 9 years ago

There is potential for using a super capacitor as the RTC backup rather than a battery. This has been discussed in issue https://github.com/PaxInstruments/t400-electronics/issues/1.

charlespax commented 9 years ago

Sam's comment from issue https://github.com/PaxInstruments/t400-electronics/issues/1:

I found some engineer start using super-capacitor to replace RTC battery. It is rechargeable, small in size (dia. 3.8mm) but higher cost (around USD 0.7 @ 100pcs for 0.03F) http://www.mouser.com/ds/2/396/capacitor01_e-12220.pdf

Maxim has SUPER CAPACITOR CALCULATOR and estimate the Operating time from SuperCap. http://www.maximintegrated.com/en/design/tools/calculators/product-design/supercap.cfm If we use 0.03 Farad in DS3231 , estimate 99 hours backup time for RTC.

image

Example circuit for charging super-capacitor: http://industrial.panasonic.com/www-data/pdf/AAA4000/AAA4000PE17.pdf

Please see the discussion for RTC using super capacitor. https://forum.sparkfun.com/viewtopic.php?f=14&t=34059

charlespax commented 9 years ago

The 99 hours would give us about four days of RTC backup time. I feel like this isn't long enough.

The link to the charging circuit example is for Manganese Rechargeable Lithium Batteries (ML series), not super capacitors.

samchoy88 commented 9 years ago

How many days of backup time time would be long enough?

charlespax commented 9 years ago

I don't know. It would be nice if the date/time could set once and forgotten about, so a year or more would be good. What do we have wit the current battery?

I think the main use case would be to have the date/time stay programmed after the main battery runs down. For that, 99 hours would be great.

Is there a rechargeable battery option? The document you link to mentions a manganese rechargeable lithium battery that only needs a resistor and diode to charge.

charlespax commented 9 years ago

This is still a cool idea, but right now we're sticking with the battery. Moving to Electronics version :-)

charlespax commented 9 years ago

Will not do. Closing issue.

NuclearPhoenixx commented 6 years ago

The problem with rechargable batteries is that you need relatively complicated charging curcuits. For the super caps, you need a diode and a resistor, that's it basically. Also there are already super caps with 6.8mm diameter, but 0.3F(!) capacitance. This would give you 10 times as much backup time (41 days!) at less than double the diameter.