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3.3V SuperCap for Real-Time Clock?

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digi001

digi001

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Can anyone recommend a 3.3V SuperCap for a Real Time clock? It needs to last for around 4 months. The current draw of the Real-Time Clock is 500nA.
 

Did you calculate the required capacitance and refer to the specification of available supercaps?

As far as I'm aware of, supercaps aren't yet feasible for the intended backup time range.
 
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    digi001

    digi001

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Yes. I have used Maxim's Online Supercap Calculator:
**broken link removed**

Using this tool I have determined that 4 Farads will get me 4.1 Months @ 600nA from a charge of 3V down to 1.3V.

However I'm worried about the leakage current? Will this dominate the 600nA supply current and cause the part to discharge faster than this online calculator?
 

600 nA is 'nothing', so it's likely that leakage (internally in capacitor, or elsewhere in the circuit) will be a dominant factor.

Just get a sample, charge up to nominal voltage & leave it out for a few hours or days with nothing connected (both terminals in the air). Then check voltage with plain digital multimeter, and you'll probably see that voltage has dropped significantly. Read: only suitable for spanning hours, perhaps a number of days. Very probably not months... :|
 
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    digi001

    digi001

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Ok yes I'm starting to realize this.

I have one in the lab now. Charged to 3V with just 10Mega-Ohm probe hooked up. (3/10,000,000=300nA).

And already dropped 0.1V in an hour.

---------- Post added at 13:13 ---------- Previous post was at 12:22 ----------

If i cannot find a Supercap with low enough leakage current, What type of battery would you recommend? Rechargable or Non-Rechargable?
 

Depends on how often it's plugged in to allow recharge... If it's for a device that spends long periods of time sitting in a closet, a long-lasting battery like ordinary CR2032 lithium cell might be best choice.

If device will be regularly hooked up to mains, a small rechargeable battery might be better choice. In this case, choose a type that doesn't mind being fully charged all the time. Say NiMH for example (unlike common Li batteries where full charge (+heat!) is worst-case for longterm life).
 

I previously used NiCd and later NiMH batteries for similar purposes. The instruments are in service since more than 20 years now, and it turned out the limited battery life was one of the (minor) weaknesses of the design. At worst case they became leaky.

With todays low RTC currents, non-rechargeable Li-coin-cells are optimal in my view. Newer rechargeable batteries like Panasonic VL, ML or MT type may be interesting as well. I'm not aware however of a specification, that guarantees a similar or possibly longer lifetime than observed with nonrechargeable Lithium coin cells.

**broken link removed**
 

Rechargeable batteries sitting on the shelf show a self-discharge rate. NIMH discharge faster than nicads.

Don't know if a 3V button cell will put out high enough voltage.

Ordinary alkaline batteries will do the job. However two new batteries (at 1.55 or 1.6 V each) may not provide the needed supply V, so...

If it were me I'd test some half-used alkalines. Typical voltage 1.1 V. Put together three of them to add up to 3.3V (or a tiny bit more). Try to use cells that are not yet past their expiration date.

The larger the cells the longer they'll put out the current you stated.
 

Rechargeable batteries sitting on the shelf show a self-discharge rate. NIMH discharge faster than nicads.
Click to expand...
Self-discharge of dedicated backup cells is acceptable (in contrast to high current "sinter" type). But lifetime is limited to about 5 or 10 years, also by manufacturer specification.
 

0.47F capacitor is available from China.

---------- Post added at 15:35 ---------- Previous post was at 15:33 ----------

also china is manufacturing a 3.6V battery whose backup timing is 10 years
 

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