Battery Reverse Charging Protection

Discussion in 'Power Electronics' started by Vinayakm, Sep 18, 2017.

  1. Vinayakm

    Thread Starter New Member

    Sep 18, 2017
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    Dear All,

    I have two batteries of 12-16.8V connected in parallel. I need to protect each other battery for reverse charging when load is not connected. For example if first battery is gooin low to 12. second battery connected in parallel should not charge first battery. How can we protect this? Can we diode with 5A capacity or P-Mosfet at positive terminals of the both the batteries?
     
  2. tcmtech

    Distinguished Member

    Nov 4, 2013
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    It's a non issue.
    If they are paralleled both batteries are also connected to the load and thusly both batteries are being drawn down equally so there will never be a voltage mismatch like you are thinking of.
     
  3. Vinayakm

    Thread Starter New Member

    Sep 18, 2017
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    Yes, I got it. During swapping the battery. I'm not sure whether both the batteries are fully charged. If load has been operated without connecting second battery for some time.. Discharging would have happened in first battery. When i connect new second battery with fully charged.It should not charge first battery instead of providing power to load. I will be monitoring both the battery by SM bus. If there is any mismatch in the charge with both the battery. I need to avoid second battery charging first battery. How to implement this?
     
  4. tcmtech

    Distinguished Member

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    A diode on each battery would solve the back feed issue.
     
  5. MrAl

    Distinguished Member

    Jun 17, 2014
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    Hi,

    If this is not a regular occurrence you can use a fuse between batteries with the right rating. This would allow the batteries to equalize as long as they are not too different, but if they are too different the fuse would blow.
    The fuse of course has to be able to handle the full load current too though.

    If you want active protection then you'd have to use mosfets.
    If you dont mind a voltage drop (something often critical in battery applications) then you can use diodes.
     
  6. Vinayakm

    Thread Starter New Member

    Sep 18, 2017
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    Thanks for your response.. I feel diodes might have more losses in terms of voltage drop and heat... Can we put ideal diodes or p MOSFET for the same. Where we may less losses and power consumption
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Yes you could possibly use a couple of ideal diode circuits using MOSFETs.
    What's the maximum load current?
     
  8. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Somewhere in the Everyday Practical Electronics archive is an explanation of using a MOSFET as an ultra low Vf diode.

    They use the trick against reversed battery very often in projects that can't tolerate a diode drop.
     
  9. tcmtech

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    If your battery voltage can fluctuate between ~ 16.8 at full charge and 12 at low the additional .3 - .7 volt loss any common germanium or silicon type of diode introduces is likely negligible.
    Same with heat production. Unless these are huge batteries supplying 10's of amps of current for extended periods the energy loses in the diode will be negligible compared to the overall energy transferred through them.

    You can make things as complicated as you wish but overall the invested time and materials to do so have to self justify at some point.
     
  10. Vinayakm

    Thread Starter New Member

    Sep 18, 2017
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    Max current will be 5A during discharge.if we apply ideal diode in series for battery reverse charging and p MOSFET in parallel to VCC and gnd to avoid reverse polarity protection. Is these can solve both the issue like reverse charging and reverse polarity issues?
     
  11. crutschow

    Expert

    Mar 14, 2008
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    The ideal diode in series should work to prevent reverse charging, but I don't see how the P-MOSFET in parallel with VCC and ground will protect against reverse polarity any better than a diode.
    For that you could just use a Schottky diode.
     
  12. Vinayakm

    Thread Starter New Member

    Sep 18, 2017
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    If I implement using ideal diode, my losses will be more. If there is any drop it will 0.3 or 0.7, my overall voltage will have 0.7 drop.16.8 will become 16V. need to avoid that, so I was thinking to use P mosfet to avoid reverse charging and also reverse polarity. There is any alternate for Ideal diode to reduce reverse charging and reverse polarity?
     
  13. crutschow

    Expert

    Mar 14, 2008
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    As I noted, you can use an ideal diode to prevent reverse charging with a low forward voltage drop, but you don't need an ideal diode for reverse polarity protection, as it won't readily work as an ideal diode for that purpose.
    So use an ideal diode for reverse charge prevention and a Schottky diode for reverse voltage protection.
     
  14. tcmtech

    Distinguished Member

    Nov 4, 2013
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    How so? You already admit/imply your battery drops down to 12 volts when run down.

    Also with a germanium diode at a ~.4 volt drop on a 5 amp load that only 2 watts peak loss in a circuit that carrying ~ 16.8 x 5 = 84 watts peak which is only a ~2.3% loss at most. Which if your battery run time is that critical that you can't lose 2.3% of the run time how do you compensate for battery aging and the resulting 50+% losses it inflicts as the battery ages? o_O
     
  15. crutschow

    Expert

    Mar 14, 2008
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    Here's a writeup on a simple ideal diode circuit, if interested.
     
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