Simple AA Solar Charger Help

Discussion in 'The Projects Forum' started by brandonros, Apr 7, 2011.

  1. brandonros

    Thread Starter New Member

    Apr 7, 2011
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    Hi,
    I've been searching forums for the past hour but can't find the answer to this...
    I have a small solar panel that in bright noon sun provides 8.5 volts and about 250 mA's. I'm trying to make it charge AA batteries. I originally tried 6 Energizer 2400 mah batteries in series from the panel, but with the diode from the panel, it dropped my voltage down to a little over 7 volts and I guess this wasn't enough for the batteries so they never really charged.

    Now I'm trying to use a 5v 1amp voltage regulator to charge 4 AA batteries in series. I was thinking this would work since each battery is 1.25 volts, but it's acting the same way and they never quite reach 5 volts. I know that AC chargers charge each battery a lot higher than 1.25 volts, but I'm not sure if that's necessary.

    My question now is, should I just use the high voltage straight from my solar panel (~7.5 volts with the diode), to charge 4 AA batteries? Will this work? If so, I guess I'd need to watch my time closely to prevent overcharging? And at would voltage on these AA's would be dangerous to charge to?

    Thanks so much for any advice and help,
    Brandon
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    What type of batterys? From mAh, guess Ni-mh, which is difficult for a simple charger. NI-Cd might be best as more tolerant of over charge on trickle, & over discharge.
     
  3. brandonros

    Thread Starter New Member

    Apr 7, 2011
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    Thanks for the reply Bernard. Yeah, sorry I forgot to mention, but they're NiMH batteries. I don't think Energizer makes NICd AA's. But if I can, I'd rather stick with the batteries I have. I didn't want to spend much more money than I have already on this project. I got the solar panel from a broken spotlight.

    Do you think around 7 volts is too high to charge 4 AA NiMH? I know that AC chargers can charge these as high as 1.5 volts immediately after coming off of the charger, but even with that, it would make a total voltage of 6 volts not 7.
    Thanks again
     
  4. russpatterson

    Member

    Feb 1, 2010
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    I'm not familiar with NiMH batteries or if there are safety concerns you should be aware of. I would find that out before you do too much experimenting though. I've done a lot of solar battery chargers with 12V lead acid batteries and the panels to charge those are usually 18 - 22V open circuit. You also need to be aware of how much current you put in to the batteries (again I don't know for NiMH but I'm sure that info is available). The rule of thumb for lead acid is not to exceed about 25% of the rated amp hours (so a 10aH battery could handle a charge current of 2.5 amps).

    So given that this is probably all different with your batteries, it sounds like maybe your 8V panel is cutting it close for 6V worth of batteries. I suspect given enough time you're batteries would get charged. Yes definitely don't watch the voltage level and don't overcharge them. Get a second multimeter that measures current and see how much current is going from the panel to the batteries too. Then you can really see what's going on. You'll see the panel voltage collapse to that of the batteries once the load of the batteries is applied. Then voltage will slowly climb and current will decline as the batteries charge.

    Get an Arduino. Make a voltage divider out of two resistors (so the battery voltage is cut down and can't get higher than the 5V your Arduino board is running) put the output of the voltage divider into an analog input pin on the Arduino. Then you can write some code to turn off a relay (or transistor) and cut power from your panel to your batteries once they're fully charged. That's probably a bigger project than you wanted but you'd have an automated solar charger once you were done.
     
  5. brandonros

    Thread Starter New Member

    Apr 7, 2011
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    @russpatterson, Thanks for the great advice! Yeah, I haven't been able to find out too much about about charging voltages. I've read someone's advice in one of the forums that you need a range of 6-9 volts to charge 4 NiMH AA's. So my solar panel may be alright if this is correct. Keeping the panel on the dash or seat of my vehicle may help keep the current down a little too. But this panel is about the size of a greeting card and the most amperage I've ever seen out of it is 250 milliamps, so I'm not worried about coming within the 25% of the 2400 mAh of the batteries.

    Also, I know this is not a practical application of diodes, but I've connected 2 diodes (instead of the 1 to prevent discharging through the panel) in order to drop the voltage of the panel down below 7 volts.

    I also read up on Wikipedia about trickle charging NiMH batteries and they say, "Some equipment manufacturers consider that NiMH cells can be safely charged in simple fixed, low-current chargers with or without timers, and that permanent overcharging is permissible with currents up to 0.1 C (where C is the current equivalent to the capacity of the battery divided by one hour)." So is this saying that some manufacturers say I couldn't overcharge these 2400 mAh batteries as long as I stay below 240 milliamps?

    And thanks for suggesting an Arduino. I've never really looked into one before, but that would be pretty fun project. I'll have to check it out when I can come up with the little extra cash.

    Thank you
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Why don't you look at the datasheet of the AA Ni-MH cell on Energizer's website? They even have an Applications Manual that tells you everything.
    The 2400mAh cells can be charged at 240mA for about 14 hours if they are dead but a solar panel has varying amounts of current with none at night.

    Here is the charging voltage from Energizer's website. It is much higher than you thought. The voltage from your solar panel must be high enough when it is fully loaded, not just with no load.
     
  7. iONic

    AAC Fanatic!

    Nov 16, 2007
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    ...again, you would have to watch the charge voltage, but using (2) diodes drawing ~1.4V would reduce your charge voltage to Max. 8V - 1.4V = 6.6V. Charge 4 NiMH 1.2V batteries to ~1.4 to 1.5V Max. Using this method would leave ~0.6V left over... perhaps (3) diodes.
    Thus 8V - 2.1V = 5.9V. 5.9V/4 = ~1.48V. ou will, of course loose some charge current, but at least you really can't overcharge the batteries.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    In my opinion, yes, this will work with little risk. At full charge, the AAs will drop 6v or so across themselves, leaving only 1.5v "extra" voltage. The current forced thru the batteries at that low ∆V will be far less than the 250mA spec of the panel, which is likely for a shorted condition. My guess would be the current will be under 20mA, and I think that's OK for prolonged trickle to NiMH AAs. You should verify these two assumptions, that the current in full sun, to charged batteries, is in fact OK for the batteries you're using.
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I always thought that the ideal solution would be to take a store-bought battery charger and check its Voltage and current requirements. The find a solar panel that can provide these requirements. Then remove the AC Adapter to power it with a regulated voltage.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Store-bought battery chargers vary in quality and price.
    The Energizer charger that came with my Ni-MH cells is stupid:
    1) It is simply just a timer.
    2) It doesn't measure that cells already are charged or slightly discharged so it severely overcharges them.
    3) it doesn't detect when a lower capacity cell is fully charged so it severely over charges it.
    4) It doesn't detect that a high capacity cell is not fully charged yet then its timer turns it off.
    5) If the power fails during charging then it starts the charging timer over again when the power resumes which severely over charges the battery.
     
  11. iONic

    AAC Fanatic!

    Nov 16, 2007
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    AG, then you bought a "Dumb" Charger as apposed to a "Smart" Charger!
    It's true, even some "Smart" chargers are also dumb too, I'm merely suggesting that the OP find one at least better than directly connecting batteries to a solar panel and babysitting it.
     
  12. brandonros

    Thread Starter New Member

    Apr 7, 2011
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    Well, I checked my Energizer charger's voltages and currents. It was actually pretty shocking... For each 1.2 volt battery that was discharged, it was putting out pulses of 1.29 - 1.43 volts under the load and pulses of 13 volts without a load. And for each battery, it would also pulse 1.45 Amps on a discharged battery, which got much lower as the battery charged. But unfortunately, it may be one of their "dumb" chargers. I see on the back of the charger, it has a DC output rating of 1.4 V at 360 mA's and trickle charge at 50 mA's for AA batteries. This charger info is also available here if you're curious which one gave me these numbers... http://data.energizer.com/PDFs/chcc.pdf

    But it looks like all of their chargers do have a "timer protection." Even their own labeled "smart" charger has timer protection... http://data.energizer.com/PDFs/chp4.pdf I guess it's a smart charger because it has "primary battery detection." But they all seem to also have reverse polarity protection to shut them off. Do you guys know if it would be do-able for me to create a similar reverse polarity protection?

    Energizer also has a new solar charger of their own. I wasn't able to find any specs on it yet. I couldn't even find anywhere online where it was available for sale yet, but here's their webpage for it... http://energizer.com/products/recharge/Pages/SolarCharger.aspx

    To update you on my own project, I've tested charging these 4 batteries under the solar panel. I started with all 4 batteries at a total of 4.5 volts. I checked the batteries a few hours later and they were at 5.31 volts total. I checked 30 minutes later and they were 5.43 volts and I checked the load of the panel and it was putting out 5.50 volts with batteries connected. I then checked another 30 minutes later and the batteries were at 5.50 volts with a panel load of 5.56 volts. I took the batteries off after this since they were about 1.39 volts each. If I get brave, sometime I may let the panel take them all the way up the 1.4-something that Energizer's chargers can take them.

    So far this seems like it might work if I "babysit" the batteries, but it's better than nothing and spending money I don't have right now. I do have an extra diode laying around and I'll try to connect that and see if it gets even better. I really wish I knew enough about circuitry to dismantle one of my AC chargers and connect it the solar panel and make it provide my trickle charging and reverse polarity protection.

    Thanks again
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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  14. wayneh

    Expert

    Sep 9, 2010
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    If you can provide any details about what you have, this should be trivial. BUT, I'd first be skeptical that the charger is doing anything fancy for you, anything better than just letting the current drop as the voltage climbs. Same thing that's happening "naturally".

    You really should measure your charging current, by putting a DMM in series with the panel and batteries. I suspect it's quite low near full charge and there may be nothing at all for you to worry about, especially once you add the blocking diode.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    The Energizer charger that you have is simply a timer. It is "dumb" so it does not detect that you inserted a fully charged battery so it will overcharge it for 8.5 hours.

    The better Energizer charger uses "delta-V" which detects that the battery is fully charged then it shuts off. A timer is used as a backup in case the delta-V messes up.
    I don't know which battery charger IC they used so I don't know if it is smart.

    A simple rectifier diode in series wirh the battery provides reverse polarity protection but you must inform the charger circuit about it.
     
  16. iONic

    AAC Fanatic!

    Nov 16, 2007
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    If your solar panel load was 5.56V you would not be able to overcharge the batteries unless the panel voltage increases. In this case adding another diode would reduce your max battery voltage to 1.215V each. I'd only add another diode if you know that the panel voltage gets higher than 6.0V.
     
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