DC Mini Ups [Concepts !]

Discussion in 'The Projects Forum' started by ShayanFiroozi, Apr 26, 2016.

  1. ShayanFiroozi

    Thread Starter Member

    Nov 18, 2015
    Hi Experts !

    i'm gonna make a simple DC UPS 24 volts output
    this is what i done
    i need you suggestions for make it better

    and 2 questions :
    1-i could NOT found a good SLA battery discharge protection circuit , may you could help me
    2-it's better to use diodes for switching between adapter and battery or use a simple MOSFET which detects 220V mains failure and flow the battery current to the output

    thanks in advanced and sorry for bad English
  2. Dyslexicbloke

    Well-Known Member

    Sep 4, 2010
    Don,t worry about the bad English, I am English and cant spell anyway!

    Your concept is wrong ... well too complex anyway

    You need to think in two parts ...
    Charging the batteries and regulating your output, these are and must stay separate.

    The easiest way to go is to use a simple battery charging system, essentially just keeping them on a float voltage. If they get discharged, more on that later, they would take a long time to recharge. The best way is to charge in 3 stages but that is complicated by having to account for loads whilst charging. have a look at http://batteryuniversity.com/.

    When you are done reading you will find that you need to monitor battery current and voltage and then apply some logic to decide what voltage, or limited current needs to be applied. Whatever you decide, simple float or clever system, your charger manages the battery irrispective of the load you apply to the output.

    Next pick your battery voltage and output system... If you are not using any form of voltage boost the battery voltage must be higher than your desired output voltage even when the battery is under load and not fully charged. This means that 24V batteries cant supply a 24V load.

    Assuming you are going to buy or construct modules, charger and output, there are choices choices.

    Use an output regulator that requires its input to be above its output, a buck circuit, and choose batteries with an appropriate voltage.
    Use an output regulator that requires its input to be less than its output, a boost circuit, and choose batteries with an appropriate voltage.
    Use an output regulator that will buck or boost as needed and choose batteries with an appropriate voltage.

    The easiest way to go is to use 36V batteries, which could easily be at 44V when charging and will certainly float at around 40V.
    Use a buck circuit to bring this down to you 24V output.
    However cheap buck modules will oftern have a 36V maximum and you would want to stay below the absolute maximum anyway.

    Cheap buck/boost modules may be a better choice. You could use 24V or even 12V batteries and get regulated 24V out without having to worry about the battery voltage much. A heavy load on a partially charged battery would drop the terminal voltage below 24/12 but the output circuit would compensate. Charging, fast or float, would take the terminal voltage way above 24/12 but the output circuit would compensate.

    This is a commercial unit... Clever charging but essentially what I am talking about.

    If you don't expect to discharge the battery often then a simple float voltage will work. it how fire alarms and security systems usually do it.

    Mains supply > DC @ float voltage > DC @ output voltage
    Ac 120/230 > 13.8DC > 24DC

    One PSU, one battery, one boost converter, Switch it on and forget about it ....
    everything is always in circuit so no switchover is necessarry
    You would probably want to add a cutoff circuit to prevent the battery over discharging.

    Have fun...
  3. ShayanFiroozi

    Thread Starter Member

    Nov 18, 2015
    Thank you sir for such a complete answer
    Yeah actually I,m using float charger,and boost converter,I testes this circuit without low discharge protection and working well,my battery is standby use not cycle use,if main is present 24 volts power supply would take a lead and supply the load,if not battery would,working well without any interruption.
    My problem is when main is not present aftery 1..2 hours my battery voltage goes below 11.8 and its critical,I need to detect it and cutoff the load
    Also lets assume I need to make a 12 vots ups,so my power supply would be 12,on that situation I can not use diode switching because voltages are the same!!
    I need a simple electronic switch which able to detect 220v main is present or not and turn on or off a mosfet for switching
    Any suggestion or help.please

    Thank you again
  4. tcmtech

    Distinguished Member

    Nov 4, 2013
    I think that is an overly and unnecessarily complicated circuit for what should be a simple power system.
    If you need 24 volts at the end then just use 24 volt based components through the whole system all in series.

    220 VAC to ~28 volt DC 10 amp power supply to 24 volt battery set to 24 volt DC regulator stabilizer with a low voltage shutdown built in and out to the load.

    When the AC power is present the DC power supply charges the battery plus supplies the power to the voltage regulator/stabilizer which in turn gives you your stable 24.2 VDC power to what ever it is you need it for.
  5. ShayanFiroozi

    Thread Starter Member

    Nov 18, 2015
    Thank you,but what about switching between battery ans power supply ?? What method should I use??
    Diode or mosfet?? I,m pretty new on electronic but I know for diode voltages can not be the same.
    Also what about battery deep discharge protection circuit ??
    Thank you for replying
  6. tcmtech

    Distinguished Member

    Nov 4, 2013
    You don't need to do any switching between them if they are both connected together. Which ever one that has the higher output voltage will supply the power. It's simple electrical physics and works on the same principle of how a vehicles alternator and battery are tied to together and work.

    If the AC power is on the main power supply will supply power to both the load and to recharging battery but if it's off then it has no output thusly the battery is what will supply the power to the load.

    More importantly, what is the load that needs an exact 24.2 VDC?
  7. Dyslexicbloke

    Well-Known Member

    Sep 4, 2010
    Look up maintained UPS and compare that to a UPS with a static switch....
    A maintained UPS is always on in that the output is always supplied from the DC bus, whatever it is.

    An older approach with older technology, which was less efficient and only capable of relatively short working life, would have used some form of solid state switch...
    When the mains was on it would be in bypass with the battery charger running but the load supplied from the main supply.
    When the main supply failed the the battery would be switched in to maintain the load.
    From your question I assume that this is what you think needs to happen.... It doesn't.

    Modern electronics, specifically the switches, ate far more robust than they used to be and can continually supply the load from the DC bus.
    The DC bus is directly connected to the battery, all the time, and is held at the batteries float voltage.
    If the main supply fails the DC bus voltage will fall but the output inverter/convertyer supports load using current from the battery which was already connected.
    When the supply comes back on the DC bus voltage go's up again, charging the battery AND supporting the load via the output inverter.
    There is nothing to switch...

    To ensure the battery inst damaged you would shut down the output when the terminal voltage reached some critical level.
    The output would then remain shut down until the battery voltage indicated it was on charge, all you need is a voltage sensing circuit on the DC bus with a little hysteresis. look at the spec of the device in the link I sent, it is exactly what you are looking for and if you are building one you should copy that methodology. There is usually a timer too that offers early shutdown.

    These things are very common, I seldom build a control panel without one. power go's off, control supply stays on holding up the PLC and providing power for actuators. Plant is shut down and made safe and then I allow the UPS to shut off so that the battery inst needlessly drained.

    When the power comes back on the battery starts to charge and the output switches back on.
    you could do this with two voltage sensing relays, it doesn't need to be complicated. Of course if you can also do it hundreds of other ways it rather depends on your application.

    Would it help if a made you a little block diagram?
    If so give me a few more details about your application, load, how long you need it maintained, if yiu need to do anything before the UPS actually shuts the load off...
  8. ShayanFiroozi

    Thread Starter Member

    Nov 18, 2015
    It,s no require to be exact 24.2,I set it higher to prevent diodes continuous switching,what I seen in many circuits is to connect 2 power supply with diodes between them,so diod will pass the higher voltages
    It a normal consumer like a network switch
  9. ShayanFiroozi

    Thread Starter Member

    Nov 18, 2015
    Thank you very much
    I need to design a 24 volts dc ups which supports up to 5 amp,but my battery and charger are 12 volts and I wanna use dc to dc boost converter which I found it,it,s rated 6amp so it,s not my problem
    Also I have a 24volts 6 amp switching power supply,
    My problem is how to switch between power supply and battery without interruption ,once I used diodes for switching and works well,but problem in this technique is power supply voltages should be a little higher than output of dc to dc boost converter,s voltages
    Also my.battery should be standby for better lifetime,I don,t want to use cycle use
    Also I have a good float charger,so that,s not the problem
    Problem are switching between power supply and battery And battert deep discharge in case of long time mains failure
    Thank you again for your help sir
  10. Dyslexicbloke

    Well-Known Member

    Sep 4, 2010
    No it isnt, you are missing the point, actually all the points from both of us.
    Look at the examples...
    Mains > 24DC > 12V Battery (Via a buck converter) > 24VDC (Via a boost converter)
    that's it unless you add low battery voltage detection, it will just work.
    There is no switching to do if the mains go's off the load stays on
    When the battery is charging the load stays on.

    To add low battery voltage detection read the voltage and switch off the output when some minimum is reached.
    only switch it on again when the battery starts charging.

    If you are determined to make it hard there is an endless list of possibilities but little point in asking for help...
  11. ShayanFiroozi

    Thread Starter Member

    Nov 18, 2015
    thank you so how can i detect when the minimum is reached ??