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What voltage to use to charge a 7.2 V Battery?

Discussion in 'General Electronics Chat' started by Lac, Apr 21, 2004.

  1. Lac

    Lac New Member

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    I'm about making a batterycharger, charging a Ni-Cd Battery rated at 7.2V 1500mAh, but what should the charge voltage be? 7.2 V? at the orginal charger I measured around 11.8V when the charger was ON but the battery disconnected.

    Anyone know the right voltage?

    Cheers!
    Lac.
     
  2. stevez

    stevez Active Member

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    A very simple answer is enough voltage to yeild the desired amount of current to flow into the battery. The voltage applied will always have to be slighly above the battery voltage at any given moment in order for the battery to charge. The open circuit voltage of a simple charger may be very high relative to the desired battery voltage but because of it's design only a limited current will flow. The resulting voltage settles where it settles based on internal resistance and other things.

    There are limits to the rate at which you can charge a battery. Use of a charger based on voltage alone might ruin the battery or worse, create a dangerous situation. Some batteries are made to be charged at high rates, others aren't. Sorry to be so unclear but it's not a simple question.

    If you aren't concerned about the time charging takes you might construct a simple current limited charger with a 10 or 12 vdc power supply capable of delivering 50 to 100 milliamps. See the datasheet for an LM317 - it shows a simple current regulator that you could set up for 50 or 75 ma. It might take 24 hrs to charge the battery but you stay way away from overheating and other problems. There are numerous ways to charge batteries at faster rates - some very complex but that preserve battery life - some simpler at the expense of time or some battery life. Many of these are described at websites- you might search prior posts under battery chargers. Some of these are really sophisticated but thanks to modest cost ICs are within the range of a person with modest electronics skills.

    As a general rule, from what I've seen - a simple, low-cost charger can be used if time is not an issue. As the desire to reduce the time becomes a bigger factor then more elaborate schemes are applied to increase the charge rates in a way that minimizes danger -possibly at some expense in battery life. My son used these batteries in so called fast chargers. They did work and the batteries got hot but didn't explode. I think that it was more "cut and try" rather than good design on the part of the charger manufacturer. I'm also convinced that battery life was much less than it could have been but then like everything else in life you trade on thing for another.

    You could "cut and try" using the scheme I described, increasing the current. That IC (LM317)is good for an amp or more. I can't tell you how hot is too hot for the battery or at what point it might let go. An amp gets the charge time down to a few hours or less. I've no way to determine if you apply 12 volts if the battery will charge at 1 amp though I'd think it would.
     
  3. ante

    ante New Member

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    Hi Lac

    Use the peak detect technique to determine the voltage needed.

    Ante :roll:
     
  4. dave

    Dave New Member

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  5. Lac

    Lac New Member

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    Geez! your good stevez! This battery is a part of a backup circuit where the battery charges, as long as the circuit gets power from a walloutlet, but once the power from the walloutlet is cut, the battery kicks in.

    So the charging time doesn't need to be fast, I think 15mA is enough, 1%. So 9V (or more?) would be enough to charge the bat?

    Thanks!
    Lac.
     
  6. stevez

    stevez Active Member

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    If you plan to leave this on line constantly there might be some issues with a constant trickle charge - though with very low current the likelihood of a problem ought to be low.

    It would be nice to put something in to turn the charger off, turn it on. The problem is that the difference between a fully charged battery and partially discharged one isn't great - and all of that changes with temperature and from battery to battery making it very difficult to know with any certainty what level of charge is contained in a battery. By the time a really profound difference exists you'll have a nearly dead battery.


    Really good chargers account for temperature and watch the incremental changes in voltage over time to indicate where things are at. Good project for a PIC probably but too many things on my to-do list already.
     
  7. k7elp60

    k7elp60 Active Member

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    Correct voltage for charging

    Nicad batterys have very close to 1.5V per cell across their terminals at room temperature. If you construct a constant current charger of the Capacity of the cells times 0.095 the charger can be left on all the time with out damage to the cells, this is at normal room temperature about 74 degrees F.

    In your case 1500Ma X .0.095 = 142.5Ma. But as others have suggested
    about 100Ma would be fine. As long as the charger will supply 9V at your charging current everything should be great.
     
  8. Lac

    Lac New Member

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    thanks! Just one more question: Can I use 12V instead of 9V to charge the bat?

    Thanks!
    Lac.
     
  9. ante

    ante New Member

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    Lac
    You don’t charge nicads with Volts you charge with Amps the voltage is irrelevant.

    Ante :roll:
     
  10. k7elp60

    k7elp60 Active Member

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    Nicad Charger

    I agree you charge nicads with current, but the charger must be capable of supplying about 1.5 volts per cell at the battery terminals to fully charge the battery.
     
  11. ante

    ante New Member

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    Absolutely, but if the current is correct why measure the voltage? There should of curse be some slack regarding the voltage, I would recommend 2Volts / cell to be on the safe side. The higher the charge current the higher the voltage get over the cells.

    Ante :roll:
     
  12. Juglenaut

    Juglenaut New Member

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    I have used anywhere between 12-14 volts.

    Back when I started in Radio control, I used a crude method to charge batteries, which included a timer switch and feeling it to see if the battery got to hot.

    Trickle charging gets the cells in a battery to equal levels, not all cells will be necessarily full. That is why alot of professional Radio control racers used Matched cells.
     
  13. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    NICAD's should be charged with a constant current, an extremely simple way to do this is a high value resistor from a high voltage. Assuming a high enough voltage isn't available, you can use an electronic constant current source to do the same thing - I would suggest 12V or so for charging 7.2V from a constant current source.
     

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