# Is it possible to charge a capacitors, and in return charge a battery?

If suppose I have a Li-ion battery of 10000mAh Input/Output: 18W Max (5.1V/2.4A ,9V/2A , 12V/1.5A) How many 1000F capacitors would I require to charge the battery?.

Ok, simply put, I am trying to charge a Mi power bank(10000mah) using capacitors. How many capacitors of 1000farads would I require?.

I found there are ultracaps, for ex:- Maxwell Ultracaps 2.7v BCAP0350 , that can store 1300Joules. Now how should I make the capacitor charge the battery?

Won't the current flowing from the capacitor damage the battery?. Will using a resistor prevent the damage?

• how long is a piece of string? Commented Mar 3, 2020 at 4:30
• How many apples can my basket have? No details? Then no answers. What type of battery is it? How much current would you like to charge your battery with? Which capacitors would you like to use? Basically, the battery charge is $Q=i\ t \mathrm{[mA\ h]}$. Capacitor charge is $Q=C\ V$. With these formulas, you can calculate by yourself, at least theoretically. Commented Mar 3, 2020 at 4:32
• well, 0.0001F enough if you charge them with (infinity voltage) :) Commented Mar 3, 2020 at 5:12
• Have added the specifics, Didn't know these many specs are there for a battery. Commented Mar 3, 2020 at 5:13
• what you have added is (their maximum output related to their internal resistance .. i suspect these are power supply specs ! Commented Mar 3, 2020 at 5:14

A battery stores energy. If you want to charge from another source, then that has to supply that amount of energy, plus a bit more for losses.

How much energy does your power bank store? You say it's 10Ah. Unfortunately, you don't say at what voltage. Let's assume the manufacturer is trying to 'big up' their device, so we'll use the lowest plausible voltage of 3.7v. That's an energy of 10*3600*3.7 = 133kJ.

You've found an ultracap that can store 1300J. You would need around 100 of those if you could (a) extract all their energy and (b) move it to the power bank at 100% efficiency. With capacitors, you get 75% of their energy out with a 2:1 voltage swing, which keeps the converter design simple. Let's assume 80% converter efficiency. You would therefore need a stored energy of 133k / (0.75*0.8) = 222kJ, or 170 capacitors.

Batteries have a maximum charge rate specification. You can avoid damage to the battery by keeping your charge rate within specification.

Won't the current flowing from the capacitor damage the battery?

Yes. To prevent damaging the battery you should use a proper li-ion charging circuit.

Will using a resistor prevent the damage?

Yes, if it is large enough to limit current to 5A (0.5C charge rate) or less. However using a resistor for this is quite inefficient, since it wastes 50% of the stored energy as heat. You should use a switching regulator or charger which 'transforms' the voltage without wasting much energy (a good switching regulator could have over 90% efficiency).