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I have a RAID card in my home server and the battery went dead. It's a lithium ion chemistry with the circuits to tell the RAID card miscellaneous information. I could buy another L-Ion battery to solder in there, but I'd rather upgrade to a supercap for about the same price.

My question is. What kind of affects would I get from just soldering a cap in place of the L-Ion pack? Will the charging circuit work? At the end of the day, the cap should be better than the l-ion pack in all regards, right?

Note: The charging circuit limits current to 500mA

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    \$\begingroup\$ 1) "At the end of the day, the cap should be better than the l-ion pack in all regards, right?" Why ? I think the battery will be better as a battery can store much more energy than a supercap. 2) The card is designed to be used with a Lithium based cell, replacing that with something else is not a good idea. \$\endgroup\$ – Bimpelrekkie May 3 '16 at 17:06
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    \$\begingroup\$ Reading on wikipedia gets you this chart showing that a Li-ion stores about 50-100 times more energy than a supercap. I'm not sure where you got the misconception that a supercap would be better but you'd better not trust that source again. :) \$\endgroup\$ – pipe May 3 '16 at 17:42
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    \$\begingroup\$ @pipe: The name has "super" in it. It must be better unless you get a superbattery. ;^) \$\endgroup\$ – Transistor May 3 '16 at 22:39
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Just substituting a supercap for a LIon is not going to be good.

A LIon dies if the voltage exceeds 4.2v, the charger will limit to handle this. A supercap dies if the voltage exceeds 2.7v. Oops!

Two series supercaps, with DC balancing resistors to protect each against overvoltage, might work OK, but with the charger stopping at a max voltage of 4.2v versus their capability of 5.4v, you won't use their full potential.

Cost for cost, a LIon is likely to have a much higher usable capacity than a supercap. As long as it stores enough energy for an orderly sync and shutdown, that difference may be OK for you. If the controller thinks it has a LIon connected, then it may over-estimate the energy it has remaining at its disposal, and fail to start the orderly shutdown soon enough.

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  • \$\begingroup\$ Those are all very good points. I figured the caps would never use their full potential, but I thought the percent capacity would be similar. As for the balancing, would this cap negate the need for such things? digikey.com/product-detail/en/vishay-bc-components/… \$\endgroup\$ – Eric Fossum May 3 '16 at 17:26
  • \$\begingroup\$ I've never seen a 4.2v supercap before! 7.5ohm ESR, that's going to support very little output current. How much does your RAID need? \$\endgroup\$ – Neil_UK May 3 '16 at 19:32
  • \$\begingroup\$ Well, the original battery is 3.7V1200mA and designed to last 12-24 hours. PS - I found a somewhat direct replacement on Amazon, so this is really a learning QA as of now. \$\endgroup\$ – Eric Fossum May 4 '16 at 3:46
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Other answers and comments are definitely correct, you'll have vastly reduced capacity. Another concern that I've seen is that the capacitor can discharge to zero, while the battery can't (without being destroyed). Charging circuits designed for batteries sometimes see a zero-volt capacitor, decide there is no battery there, and refuse to charge.

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