Charging 8x NiMH AA cells

Discussion in 'The Projects Forum' started by jj_alukkas, Aug 7, 2013.

  1. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Hello everyone,

    I've got into RC hobby recently and my transmitter requires 8x AA batteries in series. I've got 8x 1100mAh NiMH AA batteries but don't have a dedicated 8 battery charger. For several years I was charging pairs of NiMH at 80-100mA for abt 18hrs and they served fine for years. Now that I have 4 pairs, charging has become a problem. Dedicated 8 cell chargers are very costly here in India and dedicated MAX NiMH charge IC's are not available either. All the transformers available are for 12v 500mA at the least which will kill my batteries soon, so I tried building 2 circuits to control the current. The first was this with values set for 100mA and 10V. On charging the batteries, the voltage was observed to be arnd 10.4v but charge current was less than 10mA. The circuit is working fine and supplying max current if I try to light a bulb but cannot charge the batteries properly. Next I built a common current regulator with LM317 and a 12ohms resistor to provide arnd 100mA but the same thing happened with a very low charge current. Now Im charging as pairs as I used to as I donot want to overdose with the 500mA transformer setup. should I try charging as 4 each or 4+4 as parallel or something? I just need to get a simple charging circuit, no fancy timers or auto chargers as the batteries will be monitored while being charged. Please help.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    One thing first - the capacity of the charger is not a concern unless it's not big enough. If you need 100mA and the charger is rated for 500mA or 500A, it doesn't matter. The current is controlled by the battery or the circuitry, not the capacity of the charger.

    The circuit you linked worked as intended, I believe. It reached the voltage cutoff and reduced charge current. You cannot specify both current and voltage at the same time.

    The LM317 circuit - perhaps you could post your schematic?
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Just buy another 4 x charger (or 3 more 2 x chargers)
    4 in one charger and 4 in another
    (Or 2 in each of the 2 x chargers)
     
  4. Ramussons

    Active Member

    May 3, 2013
    557
    92
    mcgyvr, you beat me to it :D

    Ramesh
     
  5. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Thank you for the reply, but the reason I mentioned the chargers available are 500mA is that, I used to charge NiMH batteries without any circuitry for years but just a 100mA 3v transformer and a diode and even now I do it. So I wanted to build one which can give 12v 100mA and since the transformer is not available here for less than 500mA, I now use a wall charger with 12v 1.5A, but if I plug it, the battery will just die soon due to charging overcurrent. I've read that C/10 is the safe range and has lesser risks to damage due to overcharge. So inorder to tackle the problem, I built the above circuit but when I connect all the 8 batteries in series to it, the batteries dont seem to draw more than 10mA. The batteries are almost empty at 1.1v each. To cofirm the problem of high resistance in one cell resulting in this thing, I charged them as pairs individually till half capacity and then retried but still no draw.
     
  6. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    The thing is that, I could have gone for a Lipo pack which costs only as much as 8 batteries + 2x 4AA chargers, but i chose to go for the AA as I was more of the DIY type and chose to charge the 8 batteries with a simple circuit. If I had got the lipo, I could have used the 2s/3s Lipo balancer and charger which I use to charge my heli batteries. So could you help me on this? like what to diagonize or where to begin?
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Limiting current by using a low capacity charger is not a good idea, as this causes the charger to operate at or above its rating, which can be dangerous and will likely - at the least - shorten the life of your charger. The preferred method is to control current and/or voltage using a charging circuit.

    Your LM317 might allow you to charge to constant voltage or to charge at a constant current. Constant voltage works pretty well for lead-acid batteries but not so well for NiMH. Constant current is common for NiCd batteries. I think it's tough to do a good charge on NiMH without a smart controller, but maybe constant low current isn't too bad.

    Anyway, we can't tell what you should do without more detail about your wall wart. Is it really an old fashioned, unregulated transformer? Is the output already rectified to DC? Can you measure the unloaded DC voltage?
     
  8. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    The DC supply I'm presently using is a 12v 1.5A regulated smps from a wifi router. It gives around 12.4v unloaded and it is passed through the lm317 in constant current mode set for 100mA but the batteries are only drawing 10mA from the charger.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    You don't have enough voltage to drive 100mA through 8 AAs, after allowing for some dropout across the regulator. The regulator is maxing out and applying as much voltage as it can, but it's only enough to drive 10mA. You need a higher voltage source. It wouldn't take much. In fact, try removing one cell and see what it does with 7 in series.
     
  10. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    How dumb of me! I thought the LM317 only needed an extra 1.25 volts than the output! I hooked it up with a 18v supply and now it is working as intended! Thanks a lot for the heads up! a silly stupid mistake!
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Which I only knew to mention because I've done it myself.

    We all make mistakes, but there's a reasonable chance you won't make that one again. ;)
     
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