AA Charging Cutoff Switch

Discussion in 'The Projects Forum' started by awesometastic1, Feb 27, 2010.

  1. awesometastic1

    Thread Starter New Member

    Feb 27, 2010

    I have a nice little solar usb charger I made a while back, which works great. But I'd like to modify it so the little solar panel I have will charge AA batteries and that then will be tied into the USB charger part; so that i'll be able to charge even when the sun's not out, assuming the AA batteries still have a good charge.

    In any event, I've looked around at various "simple" charge regulator circuits and they all have been a bit over my head (I'm most definitely a novice in all this; my philosophy with EE stuff has always been "I'll just use a micro-controller" to get around as many circuitry requirements as I can :)

    In any event, all the charge regulators I've found, while I'm sure simple to any experienced EE person, have been a bit over my head. I could just use a microcontroller, but this seems like a waste of valuable power on such a little solar panel (the microcontrollers I typically use run about 20ma/5 volts)

    So, I'm wondering if I could just use an appropriate zener diode coupled with a reed relay to work as an effective charge controller.

    For instance, I've got 4 AA rechargeable batteries each at about 1.2 volts fully charged. So couldn't I just use an appropriate zener diode so that when it gets to about 4.8 volts it switches a reed relay to shut off the charging? Something like that? All of the examples of charge controllers I've seen are quite a bit more complicated than that, so I figure I must not be understanding something about how this would work (or wouldn't, as the case may be)...

    Is there an easier way to do this? Is this fairly power efficient?
  2. russ_hensel

    Distinguished Member

    Jan 11, 2009
    I think you want the zeener in series with the relay? I think that it will not work because when the zeener starts conducting the current will begin to flow thru the relay coil, but not enough to turn the relay on. At some voltage of course the relay will close but in the meantime you are wasting power.

    If you are comfortable with a uc then use it. A PIC without external loads at 4 meg hz usese about 2 ma, not 20. Slowing the clock will greatly reduce this. Use a fet as a low side switch to control the charge. You can also use the pic's adc to control the voltage. Chips other than the pic may also work fine, check the current consumption at low clock rates.
  3. Audioguru


    Dec 20, 2007
    You did not say which type of AA battery cells you have. Ni-Cad or Ni-MH?

    They are 1.25V when fully charged and away from the charger for at least a few hours.
    They are 1.2V when discharging at a fairly high current.
    They are 1.4V to 1.6V (depending on how much charging current) when fully charged and are still in the charger.

    Why not use a smart battery charger IC? It detects the small drop in voltage when the battery is fully charged.
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Your small solar panel might charge at the blue line, killing ability to detect V roll-over & temperature rise. Stuck with just V detection, which does work. Side note on Ni-MH: when disconnecting a load via comparator detector, battery V rises when load is removed which can cause a slow oscillation.
  5. awesometastic1

    Thread Starter New Member

    Feb 27, 2010
    I am using the Energizer NiMH AA's.

    What are some simple alternatives to a microcontroller then considering it sounds like the zener diode thing wouldn't work?

    The microcontrollers I usually use are the Basic stamps, Basic Atom Pro, or my new favorite, the Basic Micro Nano line. I've never used a PIC (although technically the Basic Atom Pro has a PIC at its heart I believe; and I think the nano line does as well).

    The Nano line in sleep mode uses about 10 ma, so I think I could just program it to go in sleep mode and periodically wake up and check if it should switch off the charging. My panels give about 500-600-ish ma peak, so it's not too bad.

    Mainly I'm just actually trying to learn to get better at actually doing circuits like these without the microcontroller when it makes sense, instead of always using a microcontroller even when it's a huge overkill. So in this case, is their a simple circuit or cheap IC out there that would do this that would use less power than the nano and cost less ($7) or should I just go with the microcontroller here?
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Your SP is a little larger than i was thinking, so maybe you can charge at almost 1/3C. I'm going to try a comparator with -in , 100k to divided down batteryV, + in 100k & 100uF, to create a differential lag.