Understanding Battery Charging

Discussion in 'General Electronics Chat' started by Joster, Sep 13, 2014.

  1. Joster

    Thread Starter Member

    Jun 12, 2013
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    Hi All,
    Lets say I have a series-pass darlington pair with a 15V zener on the base. Therefore I will have 14.3V on the emitter wrt ground. Now, if I have a battery that is at 11V and connect it between the emitter and ground the voltage on the emitter will be pulled down by the battery right? I tried this on the bench and this was the case. Will the battery still charge up to 14.3V? I have heard of putting an emitter follower powered by the series pass darlington pair but not entirely sure how that works. Will this emitter follower act as a buffer to prevent the battery from pulling down the charging voltage? If anyone can help clarify this for me that would be great.

    Thanks!!
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You will have 13.6v on the emitter, because there are two transistors in a darlington, and the battery will be pulled up to that voltage as long as the psu and darlington will handle the current.
     
  3. #12

    Expert

    Nov 30, 2010
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    A good battery is a low impedance load. If you try to force a 12 volt battery that is discharged to 11 volts to suddenly be 13.6 volts, it could require quite a bit of current. Now...I designed a battery charger that can do that. 50 amps continuous, 200 amps surge. You probably don't have one like that. It doesn't matter. Your charger will do as much as it can and the battery will gradually charge up to 13.6 volts.

    If you keep adding transistor amplifiers, all you will do is arrive at the limit of your transformer power, then surpass it, and start burning up the transformer.
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    You have a choice.
    If you build a simplistic battery charger, it will only bring your battery up to about 70% of charge and will result in the battery becoming sulfated and having a very short lifetime.
    Or you could choose to build a semi-smart charger, which will charge your battery to max capacity and it will have a long and productive life.

    I like simple simple circuits, but, lead acid batteries are not simple devices. Their capacity and lifetime are directly affected by how they are charged.

    May I suggest the lead acid battery charger in the finished project section based of the BQ2031. This is a very very cool chip. Read the app note, it will blow you away.
     
    Last edited: Sep 14, 2014
  5. Joster

    Thread Starter Member

    Jun 12, 2013
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    Thanks for all the great replies.

    The bq2031 is indeed an incredible chip however i am after something more simple. What I am really looking to build is a charger that can deliver 2 Amps to a 12v 20 amphr batt untill it reaches 15v then float it at 100-200mA. I could do that with a regulator but not sure how to enter into float mode. I can build a comparator circuit to sense when the 15v is hit but how would I enter float mode. Maybe I could use an adjustable regulator?

    I would set the regulator to be 15V then once 15v is reached the output of the comparator would somehow alter the divider network on the regulator to pull down the output voltage of the regulator to just above the batt and then trickle charge it....any thoughts?

    I suppose this would be a voltage regulated method of charging....

    Perhaps though I would want to bulk charge at 2 amps until maybe about 14v then 1a to get to 15v then trickle after that...
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I don't know what a amphr battery is or why the 12 volt battery can be charged to 15 volts.

    In most battery technologies (lead acid/lithium polymer/...) configured to supply 11.1 or 12 V nominal voltage, the charge is only a 1.5 or 2 volts above the nominal voltage. In any case, your problem with trickle charging is kind of self-solving (if you plan to "keep it simple"). Assuming your battery has 1.0 ohm of internal resistance, as your battery approaches the charge of your charger, say, 14.9 volts on a 15.0 volt charger, the current will be - according to ohms law - is 0.1 volt/1 ohm = 0.1 amp or 100mA.
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    A lead acid battery has to be "re-conditioned" to some extent each time it is charged. If you view a LA battery like a big capacitor and charge it at 13.5 volts, thinking that you are just replacing the energy you used, you will soon have a badly sulfated battery. A LA battery has to be over charged slightly each time it is charged in order to suppress sulfation and equalize the cells. This occurs when the battery is held at 14.7 Volts (for a 12V battery).

    A full "Equalization charge" is where you extremely over charge the battery to further desulfate and equalize the cells. This is where a 12 volt battery is held at 15.5 Volts until the specific gravity of each cell stops changing. An equalization charge can be rough on a battery so it is normally done every few months based on battery usage.

    Note: The above only applies to a flooded lead acid battery.
     
  9. #12

    Expert

    Nov 30, 2010
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    Les, you seem to be well versed in the details...better than Battery University.com
    I encourage you to make a blog available with your experience in it.

    ps, I could also use details about the variations of lead-acid, like AGM.
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
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    #12
    Thank you for your kind words. Yes, I have thought about starting a blog on lead acid battery charging, and desulfating. I am relatively new to AAC and wanted to pay some dues before I did that.

    The Battery University.com is an interesting site. Nothing wrong with what he is doing, just a whole lot of words for very little information. Go to trojanbattery.com and download a few of their support pdf files. Tons more usable information and far less fluff than the Battery University.com.
     
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