overcharge protection for 12v Battery

Discussion in 'General Electronics Chat' started by tonir, Aug 22, 2012.

  1. tonir

    Thread Starter New Member

    Aug 9, 2012
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    Hi,

    I want to build a overcharge protection circuit for my 12v battery charger. I am using LM317 voltage regulator and the battery being used is a 12v 7ah sla battery.

    When the threshold voltage is reached, charging should be stopped.
    Any help appreciated.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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  3. #12

    Expert

    Nov 30, 2010
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    I float my lawn mower battery at 14.2 volts. Read the label. It shoud tell you what voltages to use.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Here is a simple 12V battery charger circuit:

    [​IMG]
     
  5. #12

    Expert

    Nov 30, 2010
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    Don't forget backflow prevention in case he connects the battery before he connects the power to the charger.
     
  6. tonir

    Thread Starter New Member

    Aug 9, 2012
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    Hi MrChips,

    I already built one using a LM317 as posted by you. I have set the output voltage in the range for cyclic use of the battery, as per the specifications for the battery. Now once completely charged i can't have the battery being charged at that voltage as it will damage the battery in the long run. So i wanted to cut off the charging process.

    But #12 says that he float charges the battery. So is there any way to float charge the battery with little modifications to the circuit posted by you, once the battery has attained full charge?

    Thank you.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    You could add a comparator and a flip-flop. The comparator is connected to the battery and to a reference voltage at which you want the charging to stop. When the comparator voltage is reached it trips the flip-flop when connects an addition resistor in parallel across R2 to lower its voltage to the float voltage.

    You add a power-on reset for the FF so removing and reapplying the power resets the FF for the next charge cycle.
     
  8. tonir

    Thread Starter New Member

    Aug 9, 2012
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    Hi crutschow,

    Thanks for your feedback. I have built the comparator part. I have set the comparator output to go high when the target voltage reaches. I couldn't understand the flip flop and resistor part of your idea.

    Could you post some sample circuit using the same.

    Thank you.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Coincidentally, I am right in the middle of putting together the very same circuit as shown. If you set the charge voltage to 14.2V the current will start to drop as the battery voltage to the charge voltage. It will take another 6 hours or more to complete the topping charge stage before moving to the float charge stage.

    I have 12V 12Ah SLAB on charge from a bench PSU set at 14.5V. At full charge they are drawing 15mA, not likely to damage the battery.
     
  10. tonir

    Thread Starter New Member

    Aug 9, 2012
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    Hi crutschow,

    Can i use a relay instead of a flip flop and achieve the same result?
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Yes, you can use a relay, but you need to configure it as a latch so that once it is triggered by the comparator, it will stay latched until reset or the power removed.

    What comparator are you using?
     
  12. tonir

    Thread Starter New Member

    Aug 9, 2012
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    hi crutschow,

    i'm using lm339.
     
  13. absf

    Senior Member

    Dec 29, 2010
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    OK, I am posting a schematic using 339 and 4013 flip flop as a latch and using a NPN transistor to shut down the LM317 when Batt is fully charged as you requested. Assuming the Vin of LM317 is 18V and the CMOS can only withstand 15V, I used a 78L12 to get the 12V for the 4013. The 339 is getting 18V directly from the Vin.

    Now on the working of the circuit. During normal operation, the output of 339 should be low. When the Batt voltage reaches the preset voltage (RV1), output of 339 will turn High and set the FF which in turn triggers on the transistor to switch off the LM317. To reset the 4013, either press the reset switch or power off/on the supply. There is a POR on the 4013 FF.

    I am not able to test the circuit so not sure if there is any mistake in it. SO use it at your own risk. You can replace the 4013 with a RS flip flop using a Quad NAND or NOR gates chip. The 78L12 may be substituted with a 12V zener and a resistor.

    Allen
     
  14. tonir

    Thread Starter New Member

    Aug 9, 2012
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    Thank you very much allen
     
  15. absf

    Senior Member

    Dec 29, 2010
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    I think the whole idea of detecting the battery voltage when the it is fully charged and then switching off the charging voltage is foul.

    When the output voltage of LM317 is lower than that of the battery, the current from the battery would tend to flow back to the regulator which is not desired. This might also spoil the LM317 in question. I simulated the circuit using a 24Ω resistor to represent the battery. When the LM317 cuts off, the output voltage drops to 1.37V which is too low to be safe.

    But if you insist, you can try it out at your own risk.

    Allen
     
  16. tonir

    Thread Starter New Member

    Aug 9, 2012
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    Hi Allen,

    But by including a diode after resistor(RS) in your circuit, the reverse flow of current from battery can be avoided right?
     
  17. absf

    Senior Member

    Dec 29, 2010
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    That you have to find out yourself.:D

    I found some mistakes in my schematics. Mainly on the 4013, as it wouldnt reset on power up as there was initially no output from the LM317 for the 339 to compare. The power_up_reset makes the "R" of 4013 High, but at the same time the "S" of 4013 is also High due to the 10K connected to Vcc. So Q1 would conduct due to the Q output of 4013 being High. I've made some modifications to the inputs of 339, adding Q2 & R8 as an inverter, and also R3 which I forgot to put in in my previous diagram.

    I suggest that you make the LM317+Heat Sink part permanent and do the 339, 4013, Q1 & Q2 part on the breadboard to see if it works or not before you commit to the design.

    The new circuit is attached for your reference.
    The left figure is the normal working outputting 14V,
    The right figure is the battery reaching the threshold, goes into protect mode.

    Allen
     
    Last edited: Aug 27, 2012
    tonir likes this.
  18. t06afre

    AAC Fanatic!

    May 11, 2009
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    I would have used a battery charger/management IC for lead acid batteries. They are quite common. And they do things correct. At least study the data sheets for such ICs. They may/will give you a lot of design hints
     
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  19. crutschow

    Expert

    Mar 14, 2008
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    If you want to drop the voltage to a trickle-charge value rather than shut it off, you can connect an appropriate value resistor in series with the transistor collector so that the parallel combination of that resistor and R2 gives you the desired voltage.
     
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