# Supercaps and discharge

I know that capacitors discharge quickly, but it seems like there has to be a way to replace deep cycle batteries. A capacitor with a constant current source that is rated to drain the cap slowly, should allow the cap to act like a battery, yes? Isn't a boost converter a constant current source (or variable but not so high as to drain the cap bank)?

To be clear, I am wanting to use caps for a datacenter 8 hour backup, but I imagine solar and other enthusiasts want to know.

• Fast discharge is synonymous with low energy density = bad for 8 hours with or without a boost converter. You're missing the forest for the trees by focusing on boosting the voltage. – DKNguyen Jul 29 '19 at 14:36
• Irrelevant because the low energy density problem is MUCH more severe than you are thinking. – DKNguyen Jul 29 '19 at 14:40
• 1) the energy stored in a capacitor is E = CV^2 (see: hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html). 2) Determine how much energy is needed for "datacenter 8 hour backup" 3) Play with the calculator from the hyperphysics link and find a solution to get the amount of energy you need. 4) compare to a battery based solution. 5) conclude that super capacitor technology simply isn't there yet, you'd need far more volume (for all the caps) and money compared to a traditional battery based solution. – Bimpelrekkie Jul 29 '19 at 14:40
• What about a typical home? Don't change the question! My procedure is universal as it simply compares energy stored (in a cap) to energy needed. So for a "typical home" or a supercapacitor powered gadget, the equations are the same. If you want to know: do the calculation. Also see where supercaps are actually used in products because that's where there is an advantage (if not the manufacturer would have used a battery). I'll give one example: a real-time clock inside a measurement product. Even for that solution: I simple coin cell is hard to beat. – Bimpelrekkie Jul 29 '19 at 14:47
• "... but it seems like there has to be a way to replace deep cycle batteries." - Why does it seem like there has to be? – marcelm Jul 29 '19 at 15:27

Yes, you can use switching power supply topologies to slowly and safely discharge a large capacitor bank. These can be controlled to create exactly the load current you need. Basically every switching power supply has some DC intermediate circuit with a capacitor bank, exactly as you described above.

Still, the remark from the comments seems to be limiting factor here. First you talked about datacenter usage, then you changed to a typical home.
Let's just assume an average consumption of 2kW during the 8h. This seems legit for home usage, but is way to low for a datacenter.

$$\ 2 kW * 8h = 16 kWh = 57.6 MJ.\$$
This is the energy we somehow need to store in the capacitors.

$$\ 57.6 MJ = E_{Cap} = \frac{1}{2} \cdot CU^2\$$

I just checked at a big electronics distributor and the "best" solution I could find would be an 8.4 V supercap with 15 F.

$$\ C = \frac{ 2\cdot E}{U^2} = \frac{2 \cdot 57.6 MJ}{(8.4 V)^2} = 1.6 MF \$$