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How would you charge a dozen AA cells with a 12V 5W solar panel? — Parallax Forums

How would you charge a dozen AA cells with a 12V 5W solar panel?

MicrocontrolledMicrocontrolled Posts: 2,461
edited 2011-03-24 18:06 in General Discussion
Hi,

A friend of mine asked me how you might go about charging cells with solar power. He is planning on using this solar panel: http://www.northerntool.com/shop/tools/product_200422047_200422047
However, he wants to charge a dozen or so cells without converting from DC to AC then back. Is there a solution or chain of solutions that allows this to be done? It would need to be compatible with the connector(s) shown.

Thanks,
Microcontrolled

P.S. You'll notice I put "cells" instead of "batteries" so that those persnickety forum users out there don't give me several posts describing the difference between the two. :-)

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2011-03-22 18:22
    It depends a lot on what kind of cells you're talking about. Remember that a 12V panel will have a very wide range of output voltage, probably from 10V or less to 14-15V in full sunlight. Assuming you're talking about NiMH cells, best would be to use two strings of 6 cells and charge them at constant current using some kind of regulator (like a 7800 series). The cells all need to be the same current rating. Typical AA NiMH might be 2300mAh. You'd charge them at 1/10 of that by setting the regulator current to 230mA. You'd need two regulators, one for each string of cells. The regulator datasheet shows a constant current circuit and gives the formula for the resistance value needed.
  • icepuckicepuck Posts: 466
    edited 2011-03-22 18:23
    Assuming 1.2v cell voltage x12=14.4v you might be able to get away with using a simple charge controller
    http://www.northerntool.com/shop/tools/product_200422043_200422043
    I had a cheap ICP 7amp which couldn't do any better than 5amp
    Granted there's probably better ways to do this but it will get you started, no doubt the smarter people here will give you a better suggestion.
    -dan
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2011-03-23 00:32
    Mike is right about using a 7800 in constant current mode. It's very
    easy to do this...and it's cheap.

    I think if the voltage from the solar panel was higher it would be easier
    to use a single 7800 to charge all the cells together. The 7800 should be
    able to take at least 30v input. Perhaps your friend could find two smaller
    panels and hook them together for 24v or so?

    There are websites that have 7800 constant current calculators on them..
    Just google for them :-)
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2011-03-23 15:05
    [QUOTE= http://www.northerntool.com/shop/tools/product_200422043_200422043[/QUOTE]

    There solar pannels very good I have a 15 watt one
  • kwinnkwinn Posts: 8,697
    edited 2011-03-23 16:33
    With a 12V panel you can charge 8 or more 1.2V cells in series and the current will be limited by the amount of light falling on the solar panel so no additional circuitry is really required. If you wanted to get fancy you could monitor the voltage, current and time to allow the cells to be fully charged.
  • MicrocontrolledMicrocontrolled Posts: 2,461
    edited 2011-03-24 09:14
    Wow, guys, thanks for all the input. Does this mean that I'll need nothing but a 5V regulator to charge the batteries (I imagine you mean 1 per series of 4)? No limiting circuitry? Nothing to keep them from getting "overcharged"? If this is the case then great! Thanks for the help so far!
  • Mike GreenMike Green Posts: 23,101
    edited 2011-03-24 11:32
    The regulator is what keeps the cells from getting overcharged. Do be careful though. NiCd cells will handle an indefinite "trickle" charge of 1/10C (C=capacity). NiMH cells can get overcharged at 1/10C and usually (read the datasheet!) need a trickle charge of as low as 1/40C. If you figure out things properly, you might be able to connect everything up one evening, let things charge the next day, and disconnect things (all charged up!) the following evening.
  • Peter KG6LSEPeter KG6LSE Posts: 1,383
    edited 2011-03-24 17:08
    MIke I was just about to word up on the Rate issue

    NiCds are more forgiving then NiMh .


    Just last night i made a LM7805 based drop in charger with a lifting diode on the Common terminal on the reg to lif it just enough to give a 50mA charge( at like 5.6V) to the cells in the FRS radio I use a for NOAA ( as it has the ALERT mode on it ) .
    It uses 4 600mAH AAAs ( in side the radio I added a Shottkey Diode In series with the added pins on the bottom to keep them from shorting the pack from the outside .

    the radio is on standby most of the time so all the charger does is offset Cell losses and run the radio at the same time
    ( draw is 40mA or so )

    I preferr C/30 trickel for NiCds AA size and smaller .
    or C/40 NiMh .


    I plan on a 5 H charge cycle for Solar based chargers .

    this suits most cells with in reason


    Peter
  • Spiral_72Spiral_72 Posts: 791
    edited 2011-03-24 18:06
    The Triton charger will do this up to I think 36V from a 12V source. I would recommend using a 12V lead acid or other battery between to absorb surges.
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