I 'm working on a device which according to the below diagram, has a solar panel witch gives at most 10v voltage and 30mA current (average 9v and 20mA) and also it has a 6v lead acid dry battery with 3.2Ah capacity. The system (main circuit) works with 3.3v and draws in standby 1mA and depending on the task, 6 mA or 20mA or 100mA or 300mA, say 8 times a day.

I wanna add some super capacitors to the device to store energy when the battery is full or to reduce the number of charging cycles (for lengthening the life of battery) and probably to remove the battery and also I wanna use a switching converter for reducing the power loss in the device instead of linear voltage regulator. Now my questions are: 1. Is the future circuit diagram is correct? Can 3 super cap.s be matched with battery in aspect of difference peak voltage that they will have? 2. Is the capacitance of the super-caps is enough to supply enough power to the system or we need the battery as well? Can we use a different, smaller, batteries from different type? 3. Do you have any recommendation for a suitable IC as DC/DC buck converter? If you have any recommendation or comment regarding the this problem, please share it. Thanks.

diagram of the system with power source and storage device


Your main problem is that the panel is a poor match for the rest of the system. It's killing the battery by overcharging it.

The lead-acid battery functions as a voltage regulator; you can't force the voltage drop across it to more than about 2.4 V per cell. This means that your solar panel will be forcing about 30 mA into the battery, even when it is already fully charged. You really need to add a circuit that limits the float-charge voltage going to the battery.

This is just as important if you add or switch to supercapacitors as your energy storage device. Supercapacitors have very strict voltage limits, and if you exceed them, you will severely shorten their lifetime. Also, they have less ability to equalize the voltage among the individual series-connected devices, so you'd need to add circuitry to handle that, too.

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  • \$\begingroup\$ Dear David, thanks. Actually that diagram was simple but there is a mechanism using microcontroller to cut off and avoid overcharging the battery but not the super-capacitor. BTW I didn't the last point you mentioned. I appreciate if other people answer other questions or give a feedback about that idea. Thanks again David. \$\endgroup\$ – PersianEngineer Jun 26 '14 at 10:56

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