Charging Circuit for AAA in series?

Discussion in 'General Electronics Chat' started by wes, Sep 5, 2010.

  1. wes

    Thread Starter Active Member

    Aug 24, 2007
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    Hi, I need to create a charging circuit for 8 AAA in series? So how could I go about this? Can I just appy the charging voltage across all 8 aaa or do I need to apply the charging voltage across each battery so each battery recieves the same charging voltage?

    Again the batterys are in series
    I need to create a charging circuit that charge all 8 without taking them out
    So how could I go About this.
    I have some idea's but I just want to see if there might be a better way
    I am not sure about charging in series because wouldn't there be a voltage drop after each battery and so the next would recieve less voltage and thus take that much more time to charge?
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Most chargers I've seen in the stores seem to do exacly that - they can supply up to 10V or so and just limit the charging current.

    If you want to do it right pull out a specs page on the recommended charging rate for a typical AAA but remember there are a few different kinds. I don't know if they made a fast charge version but I think I've seen NiMH AAAs in addition to the common NiCds.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Since your cells are in series then they all share the same charging current. They are charged with current that must be limited.

    Modern AAA rechargeable cells are Ni-MH. Old fashioned Ni-Cads are used in cheap Chinese products like wireless telephones.

    AAA Ni-MH cells are 850mAh and Ni-Cads were 220mAh. Each cell will have a voltage of 1.4V to 1.5V when fully charged so the supply voltage for 8 cells in series must be at least 12V and have a current limiter.

    My Energizer Ni-MH charger charges AAA cells at 120mA and AA cells at 360mA. It has a timer to shut off after about 8 hours. The timer is stupid because it overcharges cells that are not completely dead and it severely overcharges charged cells after a power failure (because the timer starts again when the power returns).
     
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  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You might get away with charging them in series for awhile, but as the batteries age, they will wind up with different mAH capacities. This will lead to overcharging the batteries with the lower mAH capacities, and undercharging the higher capacity batteries, particularly if you are trying to charge them quickly.

    Anyway, see the attached schematics for some ideas.
    "Juan Battery Charger 4" was designed by another member here; can't remember who it was offhand. Basically, R2 sets the charge current, and VR1 sets the charge termination voltage. D5 is an LED that stays lit while the battery is charging; as it becomes charged the LED dims, then finally goes out. D6, another LED, turns on as D5 turns off.

    D1 and D2 are Schottky diodes. If you are powering the circuit from a DC source, they could be replaced with standard rectifier diodes, like the 1N400x series.

    The 2nd schematic has somewhat similar adjustments.
    The formula for R4 should actually read:
    Iout ~= 0.6v/R4

    [eta]
    Your charging current will need to be lower than these circuits are configured for.

    You should locate the manufacturers' datasheet for your particular batteries. If you don't know how to do this, then post your battery manufacturer's name and part number or type.
     
    Last edited: Sep 5, 2010
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    If you limit the current to a trickle ( a current that the cells can tollerate for an infinite time ) all cells will stabalize at full charge ( for their current capacity ). Or trickle charge at the end of fast charge cycle.
     
  6. Ghar

    Active Member

    Mar 8, 2010
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    I was worried that was the case... my Energizer NiMH batteries have been having awful performance but being naive I assumed the provided charger would be better than utter trash so I've been blaming my camera. The batteries were usually pretty hot after charging too...
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Heat is never a good sign. You are trading battery lifespan for speed of charge, at a very unfair rate. As has been mentioned, you are much better individually charging batteries to match their internal characteristics, which is also a moving target.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    In time the availability of LiION batteries and devices that use them will increase and eventually a price decrease will follow.

    The replacement 18V cells for my Makita portable tools aren't much more than their NiMH counterparts and the advantages already outweigh the differences in performance, charging time, ability to charge a half full cell, power:weight ratio etc.

    18V, 3.0 AH for around $62 including shipping at present. I have learned the hard way to only buy the genuine Makita ones though, bought one generic and that was a big mistake.
     
  9. wes

    Thread Starter Active Member

    Aug 24, 2007
    242
    2
    Thanks for the help everyone. I Guess I will go with a series charger just because of the simplicity.
     
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