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PWM Online Battery Charger

Discussion in 'Power Electronics' started by Xenobius, Aug 26, 2012.

  1. Xenobius

    Xenobius

    125
    0
    May 15, 2012
    Hi all,

    I have a project which needs to take 12VDC from a Battery however I cannot take it offline to charge it. I also have a 13.8VDC PWM Power Supply @ 15A.

    I need

    1. A Circuit that can take 13.8VDC from the power supply available to trickle charge the battery properly as it should be without monitoring or timers. (For example I do not wish to base the charging on a timer such that I assume that the battery needs 4 hours to charge... instead I would like the charger to be intelligent)

    2. I need to know how much charge the battery has on it.. maybe if its too hard to know during charging, at least I need to know for sure when there is no power connected.

    3. I need to use the battery while it is being charged.


    I also don't mind if I have to buy some specialised IC to make this easier as long as you can help me build a circuit with it :)

    Thanks so much for your help.
    PS: Even if you don't know how to answer one of these questions, still reply because anything helps at the moment :)

    I know I could use a computer UPS but these are more complex since they actually convert AC to DC and back to AC and from 230VAC to 13.8VDC and back to 230VAC and in my case this is totally unnecessary... not to mention that I would need to convert it back to 12VDC or do major modifications for the UPS. Also a UPS with all its problems won't give me feedback on how much the battery has left.
     
  2. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    Is your battery a lead-acid battery?

    You write "I also have a 13.8VDC PWM Power Supply @ 15A".
    Does this mean that the power supply gives the PWM power signal on the output (you have pulses on the output), or is the PWM just internal (you have a smoothed 13.8V DC output)?
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Are you sure you're not describing a 'Switch Mode' power supply? I've never seen a PWM power supply. Unless this is a high amperage lead acid battery, as ElectroBrains mentioned, I don't think you need a 15A charger, though I realize that you didn't say you did. Always post as much pertinent information that you can. ;)
     
  4. Xenobius

    Xenobius

    125
    0
    May 15, 2012
    Oh dear me,

    I meant to say Switch Mode :) apologies for the inconvenience.
    Regarding the battery, I haven't bought it so any thing you recommend will do. I don't want to limit my solution to the battery but the power supply yes it needs to be used.

    Thanks
     
  5. Xenobius

    Xenobius

    125
    0
    May 15, 2012
    Yes its a switch mode not pwm... i always confuse those :)
    Regarding the 15A .. well its there now so might as well use it :)

    Thanks
     
  6. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    As long as the power don't output more than 13.8V, AND depending on the ambient temperature, you could use the power directly as a trickle charger for a lead acid battery. Personally I would be more happy with 13.6V, but it all depends on the ambient temperature. The higher the temperature, the lower the voltage.

    TOK ;)
     
  7. Xenobius

    Xenobius

    125
    0
    May 15, 2012
    Oh I can adjust its output by +/- 1V so No worries there... what I need help with is understanding WHAT and HOW to build the schematic which will:

    1. Use my power supply to trickle charge a battery properly
    2. Give me a way of knowing how much juice is left
    3. Works like an online UPS where with or without the battery, power comes from the battery


    BTW.... what battery would you recommend? I am going to be taking 1.2A pulses from it very frequently.

    Thanks
     
  8. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    Lead-acid batteries are very robust and easy to work with. The charging level can also be measured easily (corresponds directly to the voltage).

    The most simple solution would be to just add a "window amplifier" to the battery and present the output on some type of analog or digital display. When the battery charger is disconnected, the display would indicate the state of the battery (after a couple of minutes without the charger).

    On the diagram I added a diode, that maybe is not necessary (as gorgon points out). I did this to:
    1) lower the voltage to a level that would allow for continuous battery charger connection. This would probably not be necessary if you can lower the voltage, or if you don't have the charger connected for very long time.
    2) to protect the charger from backwards going current. I suppose this would not be needed either, if the charger is well designed.

    The amplifier circuit is not so difficult to build.
     

    Attached Files:

    Last edited: Aug 27, 2012
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Judging by this statement your battery is being used for some sort of off grid alternate power. like wind or solar cell; though we shouldn't have to be guessing.

    Lead acid batteries are probably the most forgiving of batteries. They handle trickle charge and brute force (high current) nearly equally as well. If a battery charger supplies 13.8V to 14.5V and you monitor both charging current and battery voltage you will have a constant monitoring system of both charging status and battery health.
     
  10. Xenobius

    Xenobius

    125
    0
    May 15, 2012
    Regarding the 13.8VDC, I can lower the voltage +/- 1volt so the diode is not needed in Electrobrains diagram.

    Also the final application is a device which will be either used in an urban area where I can supply 230VAC or in a rural area where no power is available (hence the need for a battery.) The device will be connected wirelessly to a computer hence the reason why I said "I need to know for sure when there is no power connected." because if it is running on batteries, I would need to know however if it is running on 230VAC, I don't really care what battery level it is but would be nice to have.

    The circuitry works with 12VDC so what I started with is buying a 200W switch mode power supply. Electrobrains diagram makes it look very simple however will I have 12V output or will I have 13.8V on the output when it is connected?

    Ok so originally I needed:
    1. Use my power supply to trickle charge a battery properly - I will connect the PSU to a charging circuit which I have yet to find, and connect it to a Lead Acid Battery
    2. Give me a way of knowing how much juice is left - As assumed this will only work when the power supply is not switched on and I should be able to find this circuit somewhere as well.
    3. Works like an online UPS where with or without the battery, power comes from the battery - This will be done if I connect things as Electrobrain suggested.


    A. What should the voltage be to safely charge a battery? Is it 13.6V as gorgon said?
    B. Can I mount a lead acid battery horizontally ? I am talking about those sealed type just like inside a UPS.

    Thanks everyone for your help :)
    X
     
  11. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    1. The simple diagram that I showed, would give higher output voltage when the charger is attached.
    2. If you need exactly 12V, a Low-Drop Regulator (with a power MOSFET) could be put on the output. Then you would not be able to differ between charger or no charger.
    3. It would be easy to add a sensor, showing if the charger is used or not.
    Also, an alarm sensor could be added to the window amplifier, for empty battery. If you use a wireless system, you could OR whatever alarms you want and send them to some host or whatever.
    4. With a bit of extra effort, you could also test the battery with the charger connected (you would then probably need a little control circuit, disconnecting the battery, while testing it)
    5. A: The charging voltage depends a bit on the lead-battery type (see good description in Wikipedia) or if a continuous charger is supposed to be present.
    6: B: The battery position is defined by the manufacturer. Sure you have types that can be mounted horizontally.
     
  12. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    You don't need anything extra between the power and the battery, IF the charger don't load the battery when turned off. If so use the diode in Electrobrains schematic.

    Another thing is that you should disconnect the load from the battery if it goes below 10.5V loaded. If not you may ruin the battery by overdischarge it.

    Regarding the charge voltage. If you add a diode you need to compensate for this so the battery voltage is 13.6V. Remember always to measure the voltage without the battery connected.

    Regarding the 13.6V, this is a general trickle charge voltage at a given temperature. You should check the datasheet for the actual brand of battery. We are talking about geltype batteries here.
    You don't want to use a too high voltage, this will heat up the battery and destroy it eventually, due to the negative voltage / temperature relation.

    TOK ;)
     
  13. Xenobius

    Xenobius

    125
    0
    May 15, 2012
    Ok so let me get this straight, I need a power diode connected to the power supply. The diode should drop around .7V and I will sweak the power supply to get it exactly 13.6V WITHOUT anything connected.

    I will then connect the PSU with diode directly to the battery.... And this will trickle charge it. But wouldn't that eventually over charge it? I mean if I forget it connected for over 2 days, won't that over charge it? This is the part I don't understand.

    Also this is for geltype batteries... since I dont have it I can settle for geltype :p
     
  14. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    I think we all agree that you most probably don't need the diode (it will introduce unnecessary power loss).
    According to Wikipedia, you should have 13.4V from the charger, if you plan to leave it there and will be using gelled electrolyte.
     
  15. Xenobius

    Xenobius

    125
    0
    May 15, 2012
    Hmmph I see...

    Can you confirm to me that my switch mode power supply IS NOT a charger?
    Because everyone keeps on talking about the charger :) while I have a power supply :)

    Am I confused ? is a charger simply a power supply? I tought it had circuitry which will determine when the battery should stop being charged... a power supply doesn't

    Thanks
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Basically the main difference between a power supply and a battery charger is the absence of filter capacitors, which aren't normally needed or found in chargers. A DC power supply can do dual duty as either but an unfiltered charger can't serve as a DC power supply because they output pulsating DC. This is because lead acid and gel cell batteries don't require pure DC to charge them. On the other hand, it won't hurt them either. ;)
     
  17. Xenobius

    Xenobius

    125
    0
    May 15, 2012
    This is great! Am really happy to learn such a thing however the charger must have some kind of limit to the amount of power it feeds the battery... what happens if I connect the PSU to the battery for ... 10 days ? (am exaggerating on purpose here)

    Thanks
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I've attached a schematic and a DC Transfer Curve Plot that addresses your question. The current curve displayed are Dependant on the internal resistances of the Battery and the PSU. I chose a ballpark value for these as shown on the schematic. They can be a good deal lower than this depending on the battery and psu. None the less this plot gives you a good picture of how the charge current behaves from low to full charge. As you can see when the battery is fully charged the current just trickles.

    I'm going to post a another schematic and plot with a small lamp in series. It's a very old and simple method of limiting max charge current and provide a low cost indication of charging current.

    Chris
     

    Attached Files:

  19. Xenobius

    Xenobius

    125
    0
    May 15, 2012
  20. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Neither. Lead Acid and Gel Gells have little in common with Ni-CAD's Ni-MH or Lithiums.
    Out of the three Lithiums are the least finicky though.

    Are you desirous of complicating this? :rolleyes:
     
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