Simple circuit to protect 2.7V supercapacitor from exploding

I need advice and help. I am newbie and kind of interested in building things. Please bear with me.

Situation:

I want to power my wireless modem/router using a solar panel so when there is no electricity I can still watch videos and surf internet as there are frequent brownouts in my area.

Buying a lead battery is expensive and not nature friendly, secondly I want to explore things or building things. So I started reading about solar panels, capacitors, diodes and transistors, etc.

I bought soldering (irons,leads) and digital multi-tester. I bought a super-capacitor rated as 100F 2.7V

I read that when the capacitor is charged or exposed to more than the values it is design it will explode.

I tested the solar panel using multi-tester and it shows 7.2V.

QUESTION:

If I connect my solar panel (7V) supply to my super-capacitor which is only 2.7V then my super-capacitor will explode, right? Is there a simple way (circuitry) you can suggest to avoid my capacitor from exploding?

The input is 7V and I read that capacitor explode when they are exposed to a higher voltage source.

I cannot buy resistors in my area so I am salvaging some resistors from older radios and TVs that are still working. I can order online but it will take weeks to come.

How to make sure that the input will never be above 2.7 volts since super-capacitor is rated only 2.7V 100F.

[EDITS]

Sorry I like to correct the capacitor value: 2.7V 10F

My location is always sunny and in-case there is a power cut and it's daytime I want to power my router/modem using the solar panel that we have.

The solar panel supply is not fixed depends on the sunlight. It sometimes produces 5V up to 9V.

I read that capacitors function like a temporary battery. The goal is to provide a steady flow of 9V for the router.

In case the solar power source drops then the super-capacitor will still have the power to supply the 9V.

Please note I have 6 super-capacitors (2.7V, 10F.) I bought them online. See the image below.

Bear with me. Thank you.

• I like the idea but it is always helpful to check if the it is reasonable or not: 1) can you report what the current intake of your router is? Normally it is written next to the power inlet. 2) Do you check if the brownout affects only your power grid or is the internet affect as well? Because chances are that it goes down as well, although those infrastructures tend to have backup systems/separate power grids. Commented May 5, 2020 at 9:24

Few underlying issues with your idea,

1. Lead acid is actually one of the few battery technologies that can be almost completely recycled. (That's why most scrap metal places separate them - just melt down the lead and purify the acid and you have a new battery.)

2. Super capacitors hold usually less than 1% of the same charge of an equivalent size battery, assuming you could use a boost converter to drain every last bit of charge in your capacitor. It only holds 0.1 Watt hours of energy, so say your router draws 5W, it would only store about 1 minute worth of power.

On to the actual questions you would be after a regulator, e.g. an LM317 with the correct resistors on the output to keep the output set to something like 2.65V. I am not clear what modem/routers have an input voltage as low as around 2.7V, and suspect there is another issue.

With older TVs, you may be able to find a zener diode for something easier to find, a 2.7V one "may" be suitable, but it will need a resistor between it and the capacitor to limit the current, and will get quite hot.

Edit: Based on your update of having 6 of the capacitors with a lower capacity, The bast configuration you could have would be 4 of them in series, with some balancing resistors in parallel with each capacitor, this would make your capacitor breakdown voltage 10.8V, you would still need a regulator or similar to make sure this is never exceeded, but would work better to your end goal,

So we have a 10.8V capacitor arrangement, assuming your solar panel + regulator can keep it held at say 10.6V, and we take a guess that the modem / router will not cut out until about 6.5V (assuming internal 5V rail) your left with about 21 Joules of energy, so about 35 seconds of power. It will get you something usable, but not much power to hold you over.

For a lead acid based approach you would use a panel that could charge the battery to say 13.2V, and use a regulator to drop that to the routers 9V, assuming a scooter battery which is 7Ah, and about 80 Watt hours, a 5 Watt load could be powered for at least half a day.

• Can I recharge the lead battery? Do I need to buy a charger for that? Thank you for this suggestion "LM317" I will find this tomorrow. I'm in lock down and I can only go to market once a week due to corona virus outbreak.
– Jeo
Commented May 5, 2020 at 9:00
• BTW, I made some edits on my situation above. Sorry I did not write properly.
– Jeo
Commented May 5, 2020 at 9:02
• The input for the modem/router is 9v so I already have a buck step-up to increase to voltage to 9v. I have few edits on my situation. I'm sorry I wrote incompletely.
– Jeo
Commented May 5, 2020 at 9:11
• Lead acid batteries can be recharged a large number of times, main point is how deeply your discharging the battery, the more pricey "Deep Cycle" batteries are built to survive being drained flat, compared to the cheaper "Starter" Batteries. Commented May 5, 2020 at 9:11

I am sorry, but a super capacitor will not be able to power your wireless modem for very long. A few minutes perhaps.

To answer your question, you could use a buck regulator to step down from 7V to 2.5V. But then you need a boot regulator to step up that voltage again to power your modem.

If you really want to use a capacitor for this purpose, then you should buy a capacitor with higher voltage rating.

• I bought only one buck regulator and I am planning to add this to the router to increase the voltage to 9v. I need to buy one so I can reduced the power source from solar panel going to capacitor.
– Jeo
Commented May 5, 2020 at 9:01
1. Event your voltage is 7 voltage is 7 voltage but it have limited current flow there for v(t) = (1/C) i(t) dt so your voltage to increase from zero overtime.
2. To limit voltage to below 2.7 voltage you need voltage regulator (recommended switching type) and set it below 2.7V or if your capacitor bug enough you can do voltage cut off with voltage comparator and mosfet.
• I made changes on my situation please re-read. I apologize. BTW. Do you mean that I can solder directly the solar panel to my capacitor directly since you said that voltage increases from zero overtime and since the capacity is high my capacitor is safe from exploding?
– Jeo
Commented May 5, 2020 at 9:09
• I mean it increasing over time while it not exceeds maximum limit it will fine but you need to cut if of before that happen. Commented May 5, 2020 at 9:18
• Oh you mean that. I need to do some testing. I also need to buy LM317 as @Reroute suggested. Thanks.
– Jeo
Commented May 5, 2020 at 9:22
• I don’t recommend linear type in prone to heat and less efficient. Commented May 5, 2020 at 9:37