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[18650] rechargeable battery + super capacitor

Posted: Thu May 09, 2019 9:41 pm
by solar3000
I'm building a pocket project that I'm going to put in my pocket and walk around with.
Inside is a battery and a 3.6v to 5 v boost. Also inside is a raspberry pi zero, but it could be any small electronics as well.
When I need to change battery, I would have to shut down the load. I'm wondering if I put a super capacitor of some sort, I can then switch battery with-in n-seconds. I have tested the load and it drains from 110mA to 160mA.

Re: [18650] rechargeable battery + super capacitor

Posted: Fri May 10, 2019 7:12 am
by dyarker
How long is n seconds, and how much can the voltage drop without causing a problem?

Then use RC time constant formula for the capacitor size.

110mA to 160mA seems heavy. Does the raspberry have a low current mode (memory maintained, real time runs, no IO or LEDs etc)? Because 5mA to 10mA (or less) would be easier to design to.


Re: [18650] rechargeable battery + super capacitor

Posted: Fri May 10, 2019 10:40 am
by haklesup
I don't think you can fit a capacitor big enough in your pocket. Assuming you don't want to engineer a sleep mode or its not paratactical, then a secondary external battery hookup so you can temporarily power it externally while you swap the internal battery seems like the simplest approach.

If you are only worried about losing contents of memory, then there is probably a sleep mode or a way to save that to flash temporarily then reload on power up. In sleep mode, a coin battery may be enough to keep the ram powered.

The equivalent R of your device is 5V/160mA or ~31.25 ohms. so a 1F cap gets you half a minute to swap, those are big caps. A 33000uF will get you about 1 second. I think I did that right. Super Cap means either for high capacity or smaller than normal package for a value

Re: [18650] rechargeable battery + super capacitor

Posted: Fri May 10, 2019 4:15 pm
by solar3000
Well, I'm thinking a super cap in parallel with the li ion battery, before the USB booster.
So what's a li ion, 3 volts to 4 volts?
And how long does it take to switch a battery? Perhaps 5 to 10 seconds?
I'll stick to the high side of the drain, say 160 mA.

I saw a super cap rated at 5v at 4F. Some at 15 F too.

What do you think? can it be parallel with the battery? Then I would switch cells in 5 to 10 seconds? You think it will hold?
I'm asking because I don't have the materials. This is hypothetical, so before I buy a $450 super duper capacitor, I'd thought I ask first.

Re: [18650] rechargeable battery + super capacitor

Posted: Fri May 10, 2019 6:47 pm
by solar3000
what about this guy: a 45F 5.6 volt super duper capacitor. And how is it only $11USD? ... ND/7318587

Re: [18650] rechargeable battery + super capacitor

Posted: Sat May 11, 2019 4:23 am
by dyarker

Haklesup, that looks like one RC time. I don't believe the device can operate at 1/3 of normal supply voltage. Though super caps are small enough for pocket size project.

solar3000, At 160mA, I'll GUESS you'll need 3 or more (maybe 10) 45F caps to allow time to change internal battery. And remember the voltage is already down, or you wouldn't be changing batteries. Temporary external battery (or wall wart) idea of Haklesup looks better. though now you diodes to prevent external supply trying to charge old batteries.

So again, nominal and minimum operating voltage of device, and how long to change battery, and is there a sleep mode to reduce the required current? Specs first, math second, pick parts third.


Re: [18650] rechargeable battery + super capacitor

Posted: Sat May 11, 2019 8:44 am
by CeaSaR
Being as you are using a MC (the Raspberry Pi Zero), you can either include a battery voltage monitor routine that automatically puts the MC into low power / sleep mode, or a button push that would do the same. Once that is done, you can use a smaller capacitor since the drain will be greatly reduced. Then you can use the formulae in the above posts to calculate what you need.

Of course you could use something more ready made like these: ... nk---black
where you could have 2 bank, in the project and 1 in reserve, and have a Y cord that you could plug the second one in whIle you unplug the first. No need for a backup cap.


Re: [18650] rechargeable battery + super capacitor

Posted: Mon May 13, 2019 3:23 pm
by haklesup
Those Vishay super caps look like a good match but read the datasheet carefully, the only spec that might get in your way is the DC ESR which is as high as 70 ohms for some models. This will limit the discharge current you can effectively use by lowering the effective voltage (if I understand that correctly) . Also watch the max surge voltage, these things can be damaged by a moderate overvoltage

For the price, its worth a trial run. Maybe you can find one where they don't ding you so much for not meeting a min order. Maybe ebay or something

The spec for min storage energy in Ws should be an easier unit to calculate back to run time since P=IV. Calculate Wattage of the device and divide by capacity and you should be left with Seconds of run time