Changing battery to super caps

Discussion in 'The Projects Forum' started by PersianEngineer, Jun 27, 2014.

1. PersianEngineer Thread Starter New Member

Aug 15, 2013
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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 and solar cells voltage 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.

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3. Potato Pudding Well-Known Member

Jun 11, 2010
684
92
You need load balancers in your supercap stack or one of them is likely to go overvoltage and POW!

With ten volts maximum output of the solar cells you need at least four super caps. You know what happens to the combined capacitance of series stacked capacitors right? Reduce your total as you add more in the series, because you are increasing the combined electrostatic gap.

What are the leakage current ratings including range from min to max of your supercaps? That alone might make this a problem. For the load balancer to work your stack will get the leakage current of the leakiest capacitor in the stack. It is possible with some that they might need more than the 30mA rating of your solar cell. More importantly is that even a few mA will cut the solar shoulders off from your charging and make you very vulnerable to other efficiency reductions like indirect sunlight due to changing angles with seasons and time of day, dust on the panel and clouds in the sky.

I could give more concerns but I need to go to work and that is enough for you to think about for now.

Last edited: Jun 27, 2014
4. wayneh Expert

Sep 9, 2010
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If your goal is only to reduce battery issues, I'd just use a bigger battery. Much cheaper, easier, and smaller. Big caps are handy in AC applications, but batteries are far superior for most DC applications.

atferrari likes this.
5. Potato Pudding Well-Known Member

Jun 11, 2010
684
92
Turning to a switching regulator is an excellent idea.
That alone will extend the life of your battery. From 6V to 3.3V with a linear regulator is going to be only 55% efficiency between the battery and load.

Your 3.2Ah battery is only good for 1.76Ah at the load with the linear regulator.
The switcher should allow 2.8Ah at the load.

An 88% efficient switching supply will mean that you can draw 60% longer, or draw 62.5% fewer amp hours from the battery for the same load profile.
Your battery will thank you very much.

The other concern that I get from your post is that you don't mention any type of charge controller. A battery that is not being charged properly will not last very long.

A 6 Volt battery hooked up to 10PeakVolts of low power solar cells will sort of work for a while. If that is what you have then that is a major source of battery problems that you need to fix in order to extend battery life.

I am not going to make a recommendation for the charge controller or switching regulator IC's, beyond telling you that your needs are basic enough that you should be able to get several options under a dollar a chip. Look for ones that need fewer extra components.

I am afraid my earlier reply was not very helpful. This post should actually help get you on the right track.

6. NorthGuy Active Member

Jun 28, 2014
611
122
Super-caps will not help much. They will always be at the same voltage as the battery, which changes very little. The energy stored in the capacitor is C*V^2/2. If your battery goes between 6V to 7V, energy in the capacitors will change between C*18 and C*25, that is C*7. This is only about 25% of capacity being used.

Buck converter is a very good idea.

7. Potato Pudding Well-Known Member

Jun 11, 2010
684
92
A Supercap datasheet.

Note they have 0.5(per Farad)mA leakage current. To get the 10F capacity wanted with a series stack means that you need 4 capacitors of 40 Farads each. That means they would have 20mA of leakage. That completely kills the supercap option. This is what I was trying to explain in my first post without getting into specifics.