Batteries and Battery Chargers

Discussion in 'General Electronics Chat' started by prodigyaj, Jan 2, 2008.

  1. prodigyaj

    Thread Starter Active Member

    Dec 11, 2007
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    I have a 9.6V 2.2Ah Ni-Mh Batteries ! I plan to make the charging circuit on my own. I have known that a good charger must have the following properties:

    1 - An Ideal charger must have supply constant currrent and should have a current limiter

    2 - The constant current should be a minimum of 0.1 * battery Ah (Ideal)or maximum of 0.4* battery Ah(Maximum limit).

    3 - The charging potential should be equal to battery potential ( maximum of +10% of battery potential )

    any other parameters i need to consider ? and any links to good charging circuits ?
     
  2. John Luciani

    Active Member

    Apr 3, 2007
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    Linear Technology and Maxim make battery charger ICs. You should be able to
    find some application notes at both websites.

    (* jcl *)
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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  4. prodigyaj

    Thread Starter Active Member

    Dec 11, 2007
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    thank you for your replies !
    A query i would want to put forward ?
    I have a 9.6V 2.2Ah Ni-Mh batteries(brand new ones) !
    Currently if i check the voltage of the batteries voltage is 6.5V
    I have a 750mA 9V power source ! will my batteris charge ? if not till 9.6V but till 9V ?
    Can some one give me a detailed behaviour with current voltage parameters on my batteries ?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    If your power supply is rated for 750mA @ 9V, then it will attempt to charge your batteries at a 750mA rate until they explode from overheating due to the excessive current across the batteries' internal resistance, unless just by sheer luck they happen to charge to nearly 9V before they explode, spraying highly alkaline chemicals and metal fragments everywhere.

    I do not know what the optimum temperature is for charging NiMH batteries. However, if you get them too hot, bang!

    Check out the ICs that have already been recommended; they are highly efficient and will charge your batteries safely and quickly.

    Another approach would be to use an LM317 or similar in a constant current configuration. However, that IC is a linear regulator and will expend energy in waste heat, rather than using it to do useful work.

    You have not provided a part number for your batteries.

    However, if you go to the manufacturer's website, they could supply you with that information - and they're actually paid to do so.
     
  6. prodigyaj

    Thread Starter Active Member

    Dec 11, 2007
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    but the batteries rated at 2.2Amps cant it handle a current of 750mA ?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Why don't you try it and let us know how it worked out for you?

    ;)
     
  8. prodigyaj

    Thread Starter Active Member

    Dec 11, 2007
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    was that sarcastic or a serious suggestion ? anyways i promise i will let you know !!
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Wear full-face protection. You'll be glad you did. Just because the batteries can source a given current safely does not mean they won't be damaged by overcharging.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    896
    Nope.
    The 750mA power supply will overheat, not the battery.
    The battery overheats when it is overcharged that won't happen with the supply voltage that is too low.

    Ni-MH battery cells don't explode, they hiss and maybe leak a little fluid from their safety vents if they are drastically overcharged.
     
  11. Gadget

    Distinguished Member

    Jan 10, 2006
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    NiMH batteries CAN be fast charged at a high current, and in fact prefer it as opposed to NiCd batteries. The problem of coarse, is detecting when the batteries have reached a full charge state, otherwise YES, they will go bang... and leak lots corrosive gunk (voice of experiance). Most commercial chargers use either Delta T or Delta V (or both) sensing methods, using sensors to detect either a sharp rise in Temperature, or a slight "Knee" in the terminal voltage. I am guessing these batteries are either old Power tool batteries or similar, in which case they may already have a temperature sensor built inside the pack..(how many terminals are on the pack..?). The Original charger probably would have used this sensor to detect when the battery was close to full charge, and switched from Fast charge mode, to Trickle charge mode for final top up.
    Your 9 volt 750mA power supply wont charge these cells. If all cells are in good condition and can be charged to 9.6 v, the charge current will just drop off as the battery approaches 9 volts, and the battery will stop accepting charge. You need more voltage. A simple approach to current control would just be to put a resistor in series with this higher charge voltage, or you could make a simple current regulator from an LM317.
    Low charge currents are indeed safer for prolonged charging of these batteries, but can reduce the long term life due to Dendrite formation. Fast charging is my recomendation, but full charge sensing MUST be used to avoid a catastropy.
     
  12. prodigyaj

    Thread Starter Active Member

    Dec 11, 2007
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    thanks a lot for giving CONSIDERATE replies :) i just laid my hands on 130mA 11.3 volts charger ! i guess that should do the trick ?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    You need a plan to prevent overcharging; whether through charging over a period of time, or sensing the voltage on the battery.

    A constant 130mA rate sounds reasonable, but you need to taper off the charge rate when the battery nears being fully charged.

    The ICs previously recommended have circuitry in them to take care of such things. I suggest that you find the datasheets for them and read up, along with the applications notes. Even though you may not use the circuits, it will give you an insight as to how they work.

    And if you believe that my previous reply was inconsiderate - well, it made you start to consider things, didn't it?

    This world isn't safe, and in your prior thread regarding your stepper drivers, you seemed bound and determined to destroy your project. I did make a small mistake early on (by suggesting increasing Vsense to 0.8-something Ohms), but as the thread evolved that was corrected. Yet you kept insisting on taking a path that would've defeated the point of the entire circuit.

    The best suggestions have already been made, yet you continue to introduce obliques. It is no wonder that replies to your threads are becoming further and fewer between.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    Is the 11.3V "charger" current-limited at 130mA or is that its max allowed current?
    If it doesn't have a current limiting circuit then this charger will overheat when trying to charge a dead battery.

    Is the 11.3V regulated or will it rise as the charging current drops as the battery charges?
    When the 9.6V battery is fully charged then its voltage will be about 11.2V.
    If the charger's voltage rises then the battery will become overcharged and warm which reduces its life.
     
  15. prodigyaj

    Thread Starter Active Member

    Dec 11, 2007
    48
    0
    Thanks again for all your replies !
    well it is always nice to know how things are to be done perfectly but I guess I always use forums to know if there could be some other way of doing things not because of sheer stubbornness but the fact being limited resources and very limited time !!!
    in my thread of stepper motors although i was not inclined in cutting the tracks eventually i realized i couldn't push the fact that there was an obvious error and i had oblige to ground realities and i m sincerely thankful ! so it is not that i like to challenge the integrity of the answers but would definitely want the thinking caps of the experts on current situation of the trouble i face and if there is any scope of using them safely !!
    Sorry if the queries had meant to offend some one !!
     
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