# Connecting a super capacitor to the solar battery in parallel

I find some people connect a super capacitor like (16v 88F capacitor bank) in parallel with the 12v 100Ah solar battery to optimize the surge current draws from the battery due to running heavy inductive load by the inverter(to increasing the battery lifespan).

But i'm wondering since charging/discharging curve of battery are different from the super capacitor, can this mismatching in charging/discharging behavior harm the battery or super cap. or both of them?

• Well, consider the use of bypass capacitors in smaller-scale electronics. It's the same idea, just on a bigger scale. – Hearth Feb 11 at 19:06

….can this mismatching in charging/discharging behavior harm the battery or super cap. or both of them?

NO, there will be no side effects.
The current will be shared between the capacitor and battery for both charge and discharge. In a solar panel usage configuration as you suggest, the current from the panel will be limited and the voltage will track the battery charge/discharge characteristics.

It is however very inefficient use of the Supercaps when you simply parallel them with a battery.

Consider that a 12V battery may have a fully charged state of 13.4V and a fully discharged state 10.5V. Putting a large supercap in parallel with the battery does not change the terminal characteristics. You still would have low voltage trips at 10.5V, and still classify as fully charged at 13.4V.

The charge stored in a capacitor is: W = 1/2 * C * V^2

For a capacitor in parallel with a 12V battery the total charge in the capacitor would be:

W = 1/2 * 88 * 13.4^2 ---> 7900 Joules

But since the lowest voltage is the fully discharged level of the battery you can only access a portion of the stored energy:

W =!/2 *88 * (13.4 - 10.5)^2 ---> 370 Joules

What a waste of an energy storage capability!

Does the supercap improve the battery system, absolutely it does. Large pulse current can be provided by the supercap so it can help where the output loads are DC-DC convertors (which tend to have high current spikes) especially where the battery is close to discharged (and has a higher impedance).

• Potential side effect is the current rush when connecting the capacitors the first time, using a current limited power supply or a resistor to pre charge the before connecting to the batteries might be a good idea. – RobinSt Feb 12 at 1:31
• @RobinSt Of course, and there has to a strategy for initialization of the Supercap before closing the connection. But the OP did not ask about that, so I assume there is some understanding there. – Jack Creasey Feb 13 at 2:42