Instructables

Replacing a UPS battery with Super-Capacitors

Picture of Replacing a UPS battery with Super-Capacitors
Got tired of replacing the lead-acid battery in a UPS (Uninterruptible Power Supply), so put together a super-capacitor array to go in its place.

Such units are now commercially available
http://www.marathon-power.com/EN/UPSProducts/SupercapacitorUPS/SupercapacitorUPS.html
 
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Step 1: Super-Capacitor array

Picture of Super-Capacitor array
Started with six used 2600F 2.5V Maxwell super-capacitors that I found for $70 with postage and cross-bars included (from www.goldmine-elec.com, but they no longer stock them). The six are wired in series to allow for the voltages seen by a 12V battery; fortunately the array was found to be so well balanced by itself, that no extra circuitry was needed to ensure that no capacitor experienced an over-voltage. A UPS that trickle charges its battery will overcharge such an array, so a protective Zener diode would need to be added; the UPS I used it with did not have this problem, so there was no need for any extra components.

Step 2: Carrier design

Picture of Carrier design
Designed a wood carrier that would take the place of the lead-acid battery. Tried to keep the design simple, and got it down to two dowels and two end-plates that needed holes drilled. The carrier is glued together permanently for strength, but still allows the capacitors to be removed and replaced. The two central dowels hold things together and stop the capacitors moving around; added a final cross-bar of wood across the output for physical strength.

Step 3: Possible alternative configuration

Picture of Possible alternative configuration
Considered the possibility of reconfiguring the array if the space available was narrower but higher; in my case the original design fitted well and was preferred for the support the inner dowels gave. In both designs the bolt heads reside part way into the end-plates, ensuring that the array cannot unfold.
dbonggon2 months ago

sir i have a question where can i find that kind of capacitor i am here in philippines

ashiekh (author)  dbonggon2 months ago
ebay; search for boostcap
sam5228 months ago

hello guys;

i have sun tracker
project, i wanted anyone can help me to know the specifications of my
supercapacitors ( my dc source voltage is 20 volt ) i need to have 12 volt and 1.2 amp from my supercapacitor

thanks

ashiekh (author) 1 year ago
Easily many KW if one wants; each capacitor is 2600F so the bank of 6 in series is 433 F
ndege ashiekh12 months ago
if they are hooked in series, you still get 2600F.
gene99 ndege10 months ago

No, you don't get 2600F. Check your physics textbook, or just listen to ashiekh, he really does know what he is talking about.

bigjeff5 gene9910 months ago

Give him a break, it's really counter intuitive.

The capacitance for series-connected capacitors is 1/Ctotal = 1/C1 + 1/C2 + 1/C3..., while the voltage goes Vtotal = V1 + V2 + V3...

The confusing part is that 1/C, to get the final capacitance you still have to invert it, so the final capacitance of the array is 2600 divided by 6.

Capacitance is not a direct measure of energy storage, it's a measure of the ability to store a charge. To get the total energy stored, you need to do a bit more math. C = joules / voltage squared, so a 2v capacitor at 2600 farads holds 5200 joules of energy, while a 12v capacitor at 433.3~ farads holds 31200 joules of energy, or 6 times the amount.

ashiekh (author)  bigjeff510 months ago
It gets even tougher when one connects a source of voltage V to a discharged capacitor and only half the energy gets to the capacitor no matter how low is the resistance connecting the two.
bigjeff5 bigjeff510 months ago

Er, C = 2 * joules / voltage squared

I got the calculation right though! :P

ashiekh (author)  ndege12 months ago
Actually not so; but they do carry 6 time the energy

One way to see this is the energy is 0.5 C V^2, and 6 in series carry 6 time the voltage, that is one way to see that the overall capacitance is now 1/6 th
wbond12 years ago
Please forgive my stupid comment, but when you say UPS, I am thinking United Postal Service. What does UPS mean hear?
wbond1 wbond12 years ago
I figured it out, Uninterupted Power supply, my bad.
ashiekh (author)  wbond111 months ago
I have modified the document to clarify this; I hate acronyms.
apirzada11 year ago
how much power do they provide? and capacitance?.:D
ashiekh (author)  apirzada111 months ago
In principle many kW is possible, but the UPS I use is only 500W; each capacitor is 2600F, so the series array is 433F
mashayekhi2 years ago
This is very nice idea. I am interest to negotiate with you about this idea, if it is possible please send me an email (mahyar.mashayekhi@gmail.com).
Thanks
ashiekh (author)  mashayekhi2 years ago
Such UPS units are now commercially available
http://www.marathon-power.com/EN/UPSProducts/SupercapacitorUPS/SupercapacitorUPS.html
carlos66ba2 years ago
This is a wonderful idea, but have you tested how long it lasts in operation (how many minutes)? The reason is that the discharge curve of a battery and a capacitor are very different, so the UPS may shut down the discharge assuming that the "battery" is nearly dead when in fact the capacitors still hold plenty of juice. (Batteries have a long "flat" region in the voltage vs. discharge charge, whereas capacitors V declines linearly with the discharge Q.)
ashiekh (author)  carlos66ba2 years ago
About 2 mins (as mentioned at the end); strictly the capacitor bank has 2Ah, but because the UPS shuts off early, it is only equivalent to 1Ah. I only intended it to carry across glitches, so 2 mins is more than I need; it would actually be closer to 3 mins but the UPS itself consumes about 75W

The real issue is when the UPS tests the 'battery' and in my case the capacitor array is seen to test as good, unless the UPS tests when I have a high load connected and then the UPS feels the voltage is falling faster than expected.
Can you elaborate how you charge this device? What kind of circuit you use? Im interested in something like this for home use.
ashiekh (author)  sunwukong19472 years ago
The super capacitor array goes inside a UPS, so the UPS itself does the charging; I was fortunate enough to have a UPS with a lot of space inside.
ashiekh (author) 2 years ago
It has been in service for over a year now holding up one computer and all the networking gear; as a result I have been able to retire all the other UPS units and don't so purchase anymore batteries.
Still, there are now lead acid batteries of claimed 12 year life http://www.csb-battery.com/english/01_product/02_detail.php?fid=19&pid=120 but hopefully the capacitors will live even longer than this.
Then again, one anticipates a new generation of UPSs based on lithium technology.
rcisneros2 years ago
great idea. Most power outages I've had last just a few seconds or cycle 2 or 3 times in just a few seconds. This would be perfect.
ooda552 years ago
Nice concept :)