Raspberry uninteruptable power suppy



  • So, I built myself a nice little UPS for my raspberry pi, because it is supposed to still work without power and send some SMS if a battery sensor sends a certain signal via an attached UMTS stick.

    Simplest design: A 5 V power bank with 2 A that can charge and discharge at the same time, and after that I have 3 supercaps summing up to 6.9 V and 3.33 F.

    However, I realize that the power arriving and the raspberry is only 4.77 V, which sounds too little to me.

    Questions:

    1. Why is that? Why is it not 5 V?
    2. How can I improve the UPS? I have a stepup module here, 1-5 V -> 5V. Maybe I should use that? Just behind the supercaps? Or would anything else be better?

    Any suggestions?

    Edit: I just checked: The step up module has only 500 mA. That is not enough. How do I get my 5 V back?


  • Hero Member

    @karl261 try this Most power banks contain a boost converter which in your case is probably set too low.
    So another option is to open up the power bank and adjust... (could be tough)



  • @AWI Thanks! I'll Check.

    What about this module:

    http://pages.ebay.com/link/?nav=item.view&alt=web&id=232075363633&globalID=EBAY-DE

    How did you get the Ali express image in there?


  • Mod

    @karl261 the forum auto-generates images for sites it recognizes. ali, ebay (but not your link for some reason), openhardware, codebender and maybe some more.



  • @mfalkvidd ah, cool, thanks. I see. Have to figure out the right link...


  • Mod



  • @AWI I just tested it: The power bank is fine. The supercaps cause the voltage decrease. But how? I don't understand enough of caps...


  • Hero Member

    @karl261 There could be some interference between the caps an the power bank. If the caps are of decent quality they act as storage only (not a load). Another possibility is that your voltage meter inacurately measures the "ripple voltage" of the power bank as 5v.

    I quess you don't have acces to an oscilloscope?



  • @AWI No, I don't have an oscilloscope. Would be nice though. Searching the net it seems that other people who tried also see low voltages. I think these caps are responsible. If each cap has a voltage drop of 0.8 V then I easily get down to 4.77 V...


  • Mod

    @karl261 what does your multimeter say when connecting 1 or 2 caps?



  • @mfalkvidd You mean take away one cap? That's not possible, because then the 5V is too much. Two caps would be only 4.6 V in total and giving them 5 V will kill them.


  • Mod

    @karl261 oh, you are connecting the caps in series. Sorry. I thought you had 3 caps at 1.11F each and you connected them in parallel to get 3.33F.



  • @mfalkvidd No no, those are 10 F caps 2.3 V each. If you connect them in series you get a 3.33 F and 6.9 V Supercap.



  • I found this on the net:

    http://www.hackerspace-ffm.de/wiki/index.php?title=Raspi_EDLC_UPS

    I added the 100R resistors. And I guess I will try with a step up module behind. Which one do you think is better? Of the two?



  • @AWI Ok, so the bosst converter has arrived. Now I have a stupid question: Should I add the boost converter before of after the caps? Somehow I cannot figure out the advantages / disadvantages of one or the other method.

    The problem with my power bank is, that the voltage drops to 4.5 V when the power bank is charging. When I disconnect the power bank from its supply, the voltage is 5 V as expected.



  • do i understand correctly your setup ?
    ac -> power supply for the power bank -> power bank -> super capacitors -> raspberry pi

    1. why do you need super capacitors ?
    2. the power bank is made like this: charger ic -> battery -> step up/down converter (up or down depends what the battery voltage is) so the current at the charger ic is equal to charging current + raspberry pi current. so if you want this to work then a) power bank supply should be able to deliver this amount of current, b ) charger ic should also be able to deliver the same amount of current, c) step up/down converter must deliver at least raspberry pi current. Because in this particular case the time when the charger will actualy charge the battery is much less than the time when battery is not charged the charging ic may be set up to charge at a low rate (so the charging current is low) e.g 200mA, then if power bank and charging ic can deliver 2A there is 1.8A for the pi. it is propably easier to build such power bank than to find one on the market, but if you already found one that can charge at 2A and deliver output current at 2A then you just need to set the charger ic to operate at lower rate (this is done usualy by choosing correct value for the charging current resistor and is described in the ic manual).


  • My UPS is working fine now for the past several months. My current setup is like this:

    AC -- Power suppply power bank -- power bank -- supercaps -- voltage regulator 5V -- raspi.

    The supercaps are needd because when the powerbank switches there is a short power interruption. The supercaps are buffering this.

    The 5V voltage regulator is needed, because when the power bank is charging and discharging at the same time its output voltage drops to 4.6 V or so. The voltage regulator keeps this at 5 V at all times.


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