Replace CMOS battery with super-capacitor?

[ben7]

Do you think it is an OK idea to replace an RTC battery with a super capacitor?
I have this motherboard (actually from a laptop) that uses, or, used, a two-cell NiMH battery for the RTC (or cmos, bios, whatever..). It has gone dead (the battery).

It has a part number (GDM710000041), but no other markings. I have a feeling, since it is two cells, that it is charged up to around 2.8V (1.4V per cell when fully charged, drops to about 1.2V when discharging). This is based on my somewhat limited knowledge of NiMH batteries.

So, would it be OK to replace it with say, a 1F 5.5V super capacitor?
Or, should I go with a higher capacitance.

I looked up the part number online, and I do see that there are many replacements being sold, but, I'm not sure how reliable it would be since they are from 3rd party vendors...

[mariushm]

Re: Replace CMOS battery with super-capacitor?

It would be better if you could use a larger one, and ideally one rated for 2.7v. The 5.5v and the coin cell type ones usually have a larger internal resistance and discharge faster.

However, the charging circuit probably pushes more than 2.7v so.. it's tricky... i guess it depends on how the charging circuit is built.

The circuit will probably auto-start charging the "battery" when the voltage goes below a certain amount, so while your laptop runs the supercap will be constantly charged (as soon as it goes down to ~ 2.4v it will try to get it up to about 2.8v.. so a lot of the energy will be wasted.

What you could do would be to use a boost regulator, for example TLV61224 to boost 0.7-3v to 3v and then put the diode in series with the output so that the charging circuitry will always see 2.9-3v and never try to charge the supercapacitor (the diode would block charging). This way you'd use much more than just energy between 2.4-2.9v and the settings will be kept for much longer time even if you remove the battery completely.

You'd also have to use a buck regulator (or find some 1.8v-2.5v voltage line in the laptop) to charge the supercapacitor, because the charging circuit will no longer charge it directly.

Some suggestions..

http://www.digikey.com/product-detai...204-ND/3878073
http://www.digikey.com/product-detai...1000-ND/946800
http://www.digikey.com/product-detai...198-ND/3878078

[stj]

Re: Replace CMOS battery with super-capacitor?

i'd use a lithium coincell + diode,
or a Lithium Thionyl Chloride rechargable.

the problem with those caps is last time i looked the maximum number of full charge-discharge cycles was 1000
that's why they destroy xboxes.

[ben7]

Re: Replace CMOS battery with super-capacitor?

i'd use a lithium coincell + diode

That's one solution I was thinking of, that will work too.

the problem with those caps is last time i looked the maximum number of full charge-discharge cycles was 1000
that's why they destroy xboxes.

Hm, I've never heard of other super-caps leaking like the Aerogel based ones, which are in the first gen xboxes.

I've also never heard of them having limited charge-discharge cycles.
At least, the "Gold caps" from panasonic (google it) supposedly have unlimited cycles. Of course, there are other factors that limit the life of the capacitor, but the unlimited is theoretical (since they don't last forever, IE, get thrown out, rusted out, etc...).

[stj]

Re: Replace CMOS battery with super-capacitor?

gold-caps dont have an unlimited life,
ask any of the old guys here.
when they where used in vcr's they lasted about a year!

there was a type used by SEGA that had a light-green sleeve on them that lasted a very long time,
but they where large and i cant remember the make.

[ben7]

Re: Replace CMOS battery with super-capacitor?

gold-caps dont have an unlimited life,
ask any of the old guys here.
when they where used in vcr's they lasted about a year!

there was a type used by SEGA that had a light-green sleeve on them that lasted a very long time,
but they where large and i cant remember the make.

Were they made by NEC? I have a 0.047F super-cap made by NEC, it has a light green sleeve.

[stj]

Re: Replace CMOS battery with super-capacitor?

could have been - it was in a steel can with a white plastic sealed base.