I wanted to provide battery power for a system that cycles back and forth between drawing a few watts for a couple minutes and then draws 160 Watts for 5-6 seconds. I wanted to power it with (ideally) two 18650 or two 20700 lithium ion cells. Most cells are rated for 10A max discharge (give or take). The cells are great at delivering a little power over a long time, but lose capacity delivering lots of power over a short time. I wanted to add a capacitor to ease the load of the batteries during the bursts of power


How do I size a capacitor to dissipate ~150W for 6 seconds if it’s powered by two 18650 batteries in series (3.7v) and how to I add it into the circuit so that the batteries keep the capacitor charged and it helps to deliver current during those burst periods? I could ad a buck boost converter between the batteries and the capacitor, or just connect a large capacitor in parallel with the battery leads? Any advice on picking the correct sized capacitor and how to add it to the circuit would be greatly appreciated.

 

Estimate around 2 farads.
SG

 

I don't think you are going to get anywhere with this idea without it becoming a massive, expensive, inneficient monster.

Placing capacitors in parallel with the battery:

A vanishingly small portion of the energy stored in the capacitors can be utilized, as soon as the caps discharge to the batteries normal output voltage under load, they will cease to contribute current. Since the terminal voltage of a capacitor is directly proportional to the stored energy, (unlike a battery) you would need to discharge the capacitor down to some much lower voltage to extract the energy - this would require some fancy, inefficient and expensive boost converter circuitry.

Charge the caps to some higher voltage, then regulate it down:
Could be made to work- but all the conversion inefficiencies would probably eat the battery faster than using the battery by itself.

 

If you want to calculate it

Assume for now that the capacitor will supply all of the power and somehow you can get all of the energy stored in the capacitor out.

150 W for 6 s is 900 joules (1 J = 1 W·s)
Q = 0.5 * C * V^2 the charge on a capacitor in joules equals one half times the capacitance in farads times the potential in volts, squared
900 = 0.5 * C * 8^2 (assuming fairly fully charged cells)
C = 900/(0.5 * 64) = 28.125 farads

You can't recover all the charge in any practical way, but the battery will supply some of the load current. If the battery supplied 10 A at 8 volts for 6 seconds, that would be 480 joules. Now you're down to 14 farads or so, which is still totally unrealistic since you would still have to get all of the charge out and you can't.

If the battery voltage dropped by say 10% under 10 A load, you could calculate the amount of capacitance it would to supply the balance of the load requirement by considering the charge at full voltage and the charge that would remain at 90% of that voltage. Big huge giant enormous capacitor! You would be much better off with twice as much battery. It would be smaller lighter and cheaper.

 

Forgot to mention:

In most capacitor types the C*V product (capacitance times rated voltage) is constant for a given size. If you double the voltage rating you get half the capacitance in the same physical size. Because charge storage is proportional to the square of the voltage that doubling/halving would yield twice the charge storage. Ten times the voltage rating would yield ten times the charge storage in the same physical size. A 1000 µF 250 V good quality aluminum electrolytic capacitor will be about 60 mm high and 35 mm OD. Fully charged it stores 62.5 joules. A 10 000 µF 25 V cap would be the same size and store only 6.25 joules.

 

Thanks for all the advice, this has been extremely helpful! Even if the cap can assist the battery a little during the portions that draw a lot of power that might increase the pack capacity enough to make it worthwhile. Probably will buy some caps and test them to see if it's worth it. Any additional advice or experience welcome!

 

Thanks for all the advice, this has been extremely helpful! Even if the cap can assist the battery a little during the portions that draw a lot of power that might increase the pack capacity enough to make it worthwhile. Probably will buy some caps and test them to see if it's worth it. Any additional advice or experience welcome!

Re-read the answers given, don't bother buying anything, it's a waste of time.