Supercap for RTC?

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potatotron

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Has anyone looked into using a supercapacitor to run the Teensy 3.x RTC instead of a coin cell?

I'm working on a design where physical access to the finished circuit will be a problem but it would be normally powered by mains AC at all times so I think the RTC backup would only need to be capable of running for a few hours to cover power outages etc.

Thanks in advance for any guidance.
 
A tiny rechargeable battery is an alternative to a supercap. Might not be easier though.

I purchased was an ML2430HZ1 thinking I could run a Teensy 3 from it, not realizing that although the battery stores plenty of charge, it is only rated for very low current draw. Also despite this being a "lithium-ion" battery, it's not the common chemistry we find in cell phone or laptop batteries, so the recharging profile isn't the same. In my research, I found a management IC for it, but it's sitting in a drawer waiting for me to learn more about how to design a proper circuit for it.

If you can make this approach work, you could have either months of time disconnected from the grid or an extremely small battery.
 
As a jump-off point for the rechargeable battery route, you could try something like this:

http://www.digikey.com/product-detail/en/MS412FE-FL26E/728-1053-ND/1889204

RTC.png
 
I think the biggest update of a superCap is not worrying about lifespan of the battery. That said, I don't know how much one needs to worry about lifespan of the battery.

I don't know if you can expect, say, a simple 150mAh LiPo to be happy at 4.2V for 15 years. If it is, then I'm pretty happy, because a 150mAh lipo is $1-3, and a basic linear charging chip is $0.40 (octoparts common parts library.) + a coupl components, and assuming you have 5V available.


I also know there is some sort of common coin cell rechargable battery, see https://www.adafruit.com/product/1572. In wins in compactness, but it's not drastically cheaper than a larger lipo from RC tech, simply because I think suppliers are less common for the LIR2450 (i'm sure it's much cheaper for manufactorers.)

*I think what it comes down to* for many many people, the best solution is to use a non-rechargable CR2032 kind of battery. Do you want 10 yrs lifespan? Do 1. If you insist on a 30 yr lifespan, maybe 3 will work for you (I dont actually think they're rated to last that long, but they are quite long-lasting. Eventually self-discharge will get you so increasing capacity doesn't help much).)


the supercap is really cool as a simple circuit design, but it would not be particularly long-lasting compared to any of the battery circuits.

Conclusion: I have argued for the use of the supercap, the large rechargeable battery, or the nonrechargable battery. heh, not very helpful.
 
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I was looking around at cheap DS3231 real-time clocks recently for a Raspberry Pi, that needs time keeping, but won't be connected to the network to set the time, and I noticed that most of the units now come with a rechargeable LIR2032 battery (for example, http://www.dx.com/p/ds3231-high-pre...gclid=CJXCypaDxsECFeRj7AoduzIA_g#.VEqolXX0LqQ). I can not tell from the ads, whether the DS3231 actually charges the battery when it is getting power from the VCC line in the i2c.

In the past, when I picked up a Chronodot (https://www.adafruit.com/products/255), it did not come with a battery, and I used a normal non-rechargeable coin cell battery good for 8 years or so. And I believe the Teensy 3.x does not charge the battery used for the RTC either.

What I would like would be a DS3231 variant that has an EEPROM setting of the last n times when power was supplied to VCC and then removed, so that you could hook up a battery powered device, and then let it run until the battery drains, and then do an i2c query to tell when the device was started and ended to get an idea of the runtime (yeah, there are various other ways to do it, but if it came for free with a DS3231, it would be nice).
 
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PaulStoffregen said:
Usually non-rechargeable lithium batteries are used with RTCs.
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That's what I thought, but then I noticed these new RTC's that all ship with rechargeable LIR2032 batteries. I didn't know if the unit actually could charge the battery, or if there was a special sale on LIR2032's that made it cheaper than the equivalent non-rechargeable battery. Or maybe, given some recent fires related to batteries, the asian companies can no longer ship the old batteries.
 
Can I interrupt this thread to point out MichaelMeissner's link at dx.com has a complete DS3231 board, including headers, battery, and AT24C32 (32K I2C EEPROM) for $2.54??? Digikey and Mouser don't even sell the bare DS3231 chip for less than $3.30 even when you buy a full reel of 2500.
 
Yup. Makes you wonder if it's been removed from an existing board and/or faked, eh?

For RTC purposes I don't quite understand the need for anything other than a CR2032 or like cell. I like the BK5058 battery holder because it's about as wide as a Teensy and relatively inexpensive.
 
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Constantin said:
For RTC purposes I don't quite understand the need for anything other than a CR2032 or like cell. I like the BK5058 battery holder because it's about as wide as a Teensy and relatively inexpensive.
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Funny -- I just got some of that exact same part last week to evaluate.

As to the why -- I'm working on a design where it's not going to be easy for the user to get inside the box to replace a battery so I am looking at options of having something to keep the clock running for a few hours when there's a power outage, the device is being moved, etc.
 
Today I installed the last missing thermostat in the basement - long story - and they used a pretty nifty system for the CR2032.

Imagine a small divot in the plastic clamshell enveloping the electronics. said divot has a rectangular hole with the right dimensions for the CR2032. It allows the installation of the CR2032 and removal by homeowner without opening the case. Works for Tstats that aren't moved much. Plus the mounting plate on the wall prevents the CR2032 from dropping out. All in all a pretty good package, even if I dislike the OEM (Honeywell) for their anti-DIY stance.

I hope you like the BK5058. I happen to think the thing is the bees knees because its secure, skinny, and easy to mount next to a Teensy without taking up much room.
 
potatotron said:
Can I interrupt this thread to point out MichaelMeissner's link at dx.com has a complete DS3231 board, including headers, battery, and AT24C32 (32K I2C EEPROM) for $2.54??? Digikey and Mouser don't even sell the bare DS3231 chip for less than $3.30 even when you buy a full reel of 2500.
Click to expand...

And yesterday, I thought I was getting a good deal at amazon.com, buying a Sunfounder DS3231 for $8.99 bringing my order up past $35 for free shipping. I didn't see the cheaper Chinese deals until I was posting my first response. However, I'm not inclined to wait for delivery. I checked on ebay, and they have even cheaper versions, a $1.55 board that ships with a non-rechargeable CR2032. And a $1.92 board that ships with the rechargeable LIR2032 battery. However, in looking at these various ads, I still can't determine if the unit actually can charge the LIR battery.
 
I've seen some circuit diagrams with the LIR2032 hooked up directly to 3.3v. That would end in tears rather quickly. But there are small charger chips that do a good job and whose circuitry comes out to less than a dollar, all in.

What I wonder is if the long term reliability of a LIR2032 is any better than a regular 2032. That is, come year 5 or 6, will the thing, even if properly charger and handled, still take a charge as expected and so on.

For this reason, I'd lean towards a supercapacitor and the surrounding chips to serve it. Sort of like a tantalum vs a electrolytic cap discussion at elevated temperatures. I'd expect the supercapacitor to be marginally more reliable in a long term application than a rechargeable chemistry-based cell. But that's just my intuition!
 
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