charging 7.2v 800mah battery pack (6aaa) using dc power supply help

Discussion in 'General Electronics Chat' started by JOhnnyHandford, Mar 23, 2016.

  1. JOhnnyHandford

    Thread Starter New Member

    Mar 8, 2016
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    how can i charge it using a low cost regulated power supply.
    there is limited resources about this topic in net.
    there is no charger provided.

    1. should i connect resistor in series with battery to limit the current supplied to battery? what is the value of resistor
    2. what must be the polarity of the battery with reference to dc supply
    3. what must be the voltage value setting of the dc supply
    4. how long will i charge it
    5. should the temperature of battery rise as i charge it ?

    any other insights given the limited resources.thanks in advance
     
  2. InspectorGadget

    Active Member

    Nov 5, 2010
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    What is the type of battery? NiCd? NiMH? Those are the two likely chemistries for 7.2V. Supply a picture of the pack with any markings.

    There are LOTS AND LOTS of resources about charging a battery pack on the Internet.

    You need to know the chemistry and the capacity (mAh). If it's a NiCd or NiMH, you can use a current-limited charger. Set no-load voltage to the rated terminal voltage of the pack (in this case 7.2V) and limit charge current to 1/2 C and you should be fine. Just monitor end-of-charge conditions (when the terminal voltage reaches its proscribed level) and don't over-charge.

    1/2 C means if the pack has a 1000mAh capacity, charge it at 500mA max to be safe.

    It may get warm but it shouldn't get hot. If it gets hot to the touch then stop charging. Heat will destroy a rechargeable battery. If you charge a NiCd or NiMH at a limited current like 1/2C and it heats up, it means something is very wrong.

    Always connect the + terminal of the charger to the + terminal of the battery, and likewise with the - terminals. Pretty simple.

    If it's another chemistry, like Li-Ion or Li-Poly, don't mess around with a power supply. Get a proper charger. Lithium rechargeables can overheat and burst into flame or explode if mismanaged.

    If it's not NiCd, NiMH, Li-Ion or Li-Poly, don't try to recharge it.
     
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  3. #12

    Expert

    Nov 30, 2010
    16,337
    6,821
    1) yes, and that's negotiable.
    2) + to+ and - to-
    3) that's negotiable with the resistance of the resistor.
    4) That's negotiable with the voltage and the resistance.
    5) yes

    You have to tell which kind of battery you have.

    Poo! About 5 seconds behind Inspector Gadget and he did a better answer.:(
     
  4. JOhnnyHandford

    Thread Starter New Member

    Mar 8, 2016
    3
    0
    the packaging of battery
    in addition sir:

    1. what is the resistor to be used and how many watts.
    thank you so much #12 and gadget inspector
    NI-MH battery
    2. how long will it charge ?
    3. how about lead acid battery?
    pls help me with the math... so i can troubleshoot in case i will charge other types of battery
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    Charge it at 100mA for 8 hours, or 800mA for 1 hour, or any combination, use a constant current circuit like a Lm317t.
     
  6. hp1729

    Well-Known Member

    Nov 23, 2015
    1,961
    219
    Others gave good advice. You don't have to charge it at 1/2 C. If you charge it at, say, 100mA then charge a 800 mAH pack for 8 hours. Put out whatever you draw from it. You may want to monitor voltage and calculate current over time until you get to that 800 mA hour you want.
    Resistor size depends on output voltage of the power supply and the current you charge at.
    If you apply 7.2 V and the initial charge on the battery is 4 Volts, charging at 100 mA means a 33 ohm resistor, right. 7.2 - 4 = 3.2 V. At 100 mA that's a 33 ohm resistor, 1/2 watt. Or charging at 50 mA means 68 ohm 1/4 watt for twice as long.
    Take a reading every hour or so and monitor battery voltage, calculate current and count hours to get mA-hours of charge.
    If you want to get more specific you can build a current limited power supply with a 7.2 Volt output.

    If the batteries are individual and not in a pack it is a good idea to charge them individually. 6 batteries, 6 resistors, charging at 1.2 V and whatever current you choose.

    Charging non-rechargeable batteries is not suggested but if you charge them at a really low current over a long period you might have some success, but don't expect too much. 800 mA-hr? maybe 5 to 10 mA over a week or two. But they will seldom come back up to a full charge.
     
  7. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    Without a "smart" charger to detect the end-of-charge properly (see the wikipedia article), I don't think you should charge it at 1C (800mA) or even C/2 (400mA). For "dumb" chargers, e.g. a current-limiting resistor, you should trickle-charge at C/10, or 80mA. This will allow you to monitor terminal voltage to detect end of charge. Sure, you could limit the charge time, but you don't know exactly how much charge you're starting out with, and NiMH don't like to be overcharged. (They don't like to be completely discharged, either, but more on that later.)

    If you really want to build a trickle charger, you can use a simple resistor, but the calculations are not quite what hp1729 says above. He implies that NiMH batteries can start charging at a lower voltage and that's not true, at least to the extent he shows. NiMH batteries have fairly flat discharge curves. They change in voltage only about 0.2V over their discharge. If the cell voltage is lower than about 0.9V, making it 5.4V for your pack, it's dead. It won't recharge properly and it won't have capacity because one of the cells in series won't hold a charge.

    Anyway, you calculate the current-limiting resistor for the battery pack's total rated voltage and your charge voltage source. If you're using a 12V regulated power supply, then use ohm's law where Vc is the charging supply, Vb is the battery pack rated voltage, and Ic is the charge current, in this case, C/10 or 80mA.

    Rc = (Vc - Vb) / Ic = (12 - 7.2) / 0.08 = 60 ohms

    Calculate minimum safe power using Watt's law, in this case P = I**2 x R

    P = 0.08 x 0.08 x 60 = 384mW, so I use at least a 1/2 W resistor. I would use a 1W resistor so it doesn't get as hot.

    So use a 12V regulated power supply and a 60 ohm 1W resistor. Check it every 1/2 hour or so after 8 hours. When the voltage has reached a constant level over three readings then stop the charge. Don't let it charge over 10 hours. If the pack is below 5.4V to begin with, throw it out. It won't be any good.

    NiMH cells really don't like to be over-discharged. And standard NiMH like the pack you have will self-discharge in about a month or two. After over-discharging, they will have a fraction of their capacity; they will be effectively dead.

    You should never discharge an NiMH pack to less than 1V per cell (6V for your pack). One cell will always be the weakest, and it will be driven down to way below the 0.8V safe limit. In fact, people who COMPLETELY discharge their NiMH packs thinking it will reduce "memory" effect (which it doesn't), will drive their weakest cell negative which will completely kill it.

    Wikipedia has a good article:
    https://en.wikipedia.org/wiki/Nickel–metal_hydride_battery

    Battery University has a good article, too:
    http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride
     
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  8. bertus

    Administrator

    Apr 5, 2008
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  9. JOhnnyHandford

    Thread Starter New Member

    Mar 8, 2016
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    0
    Thanks for the insights, i learned a lot from this forum. Last one question, this time about hte power supply. So during our 2nd year college, we were asked to assemble a low cost regulated power supply.

    1. I noticed that a transistor becomes very hot after few minutes of used, almost untouchable due to heat.
    2. i am concerned that charging the battery over long period of time will damage transistor?
    3. can i install a heatsink on the transistor without thermal paste and instead solder the heatsink to the transistor?
    4. can i use instead ac-dc adaptor, i think it is not regulated. thanks to all who helped me
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
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    Yes you can use a simple ac /dc adapter circuit
     
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  11. InspectorGadget

    Active Member

    Nov 5, 2010
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    That's pretty cool, Dave. A low-side constant-current charger. A lot more flexible than a resistor and regulated voltage supply.

    So JohnnyHandford, you could use this but you'd need to understand the LM317 constant-current mode and select resistor values that limit it to 80mA and you still have to monitor the battery voltage for end-of-charge.

    Referring back to your questions, a regulator [transistor] should not get hot if the amount of current drawn in the load is within the capability of the regulator/transistor, and the voltage drop is not too big to overcome the power dissipation. Remember, power dissipated = voltage drop in the regulator X current drawn. So if you use a current regulator like DodgyDave's reference above, you need to select a mains adapter that's close to your regulated voltage. Like, use a 9V supply instead of a 12V supply.
     
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  12. ian field

    Distinguished Member

    Oct 27, 2012
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    7.2V could also be 2 series lithium - terminal voltage has to be *ACURATELY* controlled to avoid fire/explosion risk. Series cells also require charge balancing.

    Nickel chemistry undergoes a voltage dip at full charge, and most "intelligent" chargers detect that to terminate charging. But the voltage change is more subtle on Ni-Mh which makes a DIY controller more tricky.

    My favourite DIY method is temperature sensing. Nickel types are endothermic during charge, but the energy going in starts turning into heat once full charge is reached.

    The drop in terminal voltage on nickel cells means that a constant voltage charger would force *MORE* energy into the fully charged cell and cause destructive overheating.
     
  13. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    Look at post #4 -- the photo of the battery pack. It has three cylindrical subunits. It says Ni-MH. It's not lithium. This is the first thing I asked about. I also cautioned against amateur charging of Li-Ion/Li-Poly.

    Besides, nobody rates Li-Ion/Li-Poly at 7.2V. They're always 7.4V. At 3.6V, a lithium cell is dead. At 3.8V, it's in the middle of its discharge cycle.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    Actually, it'd be 8.4V for the fully charged terminal voltage, but it never hurts to get the reminder out there about the safety aspects.
     
  15. InspectorGadget

    Active Member

    Nov 5, 2010
    211
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    Of course end-of-charge terminal voltage is 4.2V per cell, but I was talking about "rated" voltages. Lithium rechargeables are never rated at 4.2V/8.4V; they're always rated at 3.7V-3.8V per cell. Mostly 3.7V per cell, very popular in the hobby R/C device world.

    BTW, if you want a longer cycle lifetime on the cell, you only charge it to 4.0V or 4.1V terminal end-of-charge.
     
  16. RichardO

    Well-Known Member

    May 4, 2013
    1,238
    384
    Too hot to hold your finger on is too hot for the transistor --
    Bob Pease "5 second rule": If you can hold your finger on a hot device for 5 seconds, the heat sink is about right, and the case temperature is about 85 deg C.

    The thin layer of thermal paste is used to reduce the thermal resistance between the transistor and the heat sink. It must be used.

    No, you cannot solder the tranistor to the heat sink for several reasons. First, the soldering process will get the transistor too hot; damaging it. A good heat sink will require a huge amount of heat from a soldering iron to solder to it. Most heat sink are made of aluminum which can not be soldered to. A stee heat sink (wich can be soldered to) is a very poor heat sink. Note that copper heat sinks will take solder, unlike aluminum, but they will require an even more powerful soldering iron.
     
  17. MrAl

    Well-Known Member

    Jun 17, 2014
    2,439
    492
    Hi,

    I am assuming NiMH cells.

    Without detecting the delta V at the end of charge it is best to go with a lower current charge so that if one of the cells gets to full charge it wont over charge too much. The lower the current the less it gets damaged.

    This makes a taper charge method attractive for very low cost chargers.
    Using a resistor in series, as the cells charge the voltage rises, and if the power source is constant the current will decrease. This provides a tapered charge current profile.
    This means you can start off at more current like 200ma, then taper off to 100ma.

    Otherwise 100ma isnt too bad. However, the charge time is not exactly 800ma for 1 hour due to the charge acceptance at lower currents. At lower currents the charge acceptance for the cells is lower, so at least a factor of 1.1 is recommended. That means treat the cells like they are 800*1.1 or 880mAH cells. That means 100ma for 8.8 hours, or 88ma for 10 hours. Test the cells to make sure they dont get hot, but at that low current they probably will not get hot at all.

    The resistor value and power rating depend on the power source being used. Higher value source voltages require more resistance and more power, while lower source voltages require lower values and power rating.

    Do some tests on the first couple of runs, to make sure the current is not too high before and at the end of the charge. Make sure the cells dont get hot.
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    Apparently; there is a minimum safe charge rate - under that will also do the cells no good.

    The old Ni-Cd cells thrived on "robust" charge and discharge rates, but the Ni-Mh cells seem rather fragile in comparison.
     
  19. MrAl

    Well-Known Member

    Jun 17, 2014
    2,439
    492
    Hi,

    Yes there would be some minimum, but i have no data for NiMH cells. C/10 should be good enough though, and using a charge acceptance factor of 1.1 is probable good enough too. I can charge 2000mAH cells at 120ma for example, and that's even less than C/10. C/20 seems very safe for the cells using a simple timer algorithm.

    Here is a quick snapshot of such a charger, almost completed just have to solder the battery holder wires to the board...
    Using a Nano as the controller, the LCD shows remaining time to complete, current level, and ampere hours so far.
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
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    Probably anything less than C/20 is bad for the cells. The information isn't easy to find.
     
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