Solar charging 1.2V NiMh battery

Discussion in 'The Projects Forum' started by Coyote Mike, Sep 22, 2014.

  1. Coyote Mike

    Thread Starter New Member

    Sep 22, 2014
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    I'd like to charge a 600mAh NiMh rechargeable battery using a small photo voltaic panel. The smallest panel I found is 2V, 200mA. In ideal conditions, that's enough panel to over charge the battery. Does this matter? Will the battery be damaged on a sunny summer day?

    I'd consider making a tiny charge controller if anyone could point me at a circuit.

    The panel and battery will be built into an art project. The battery will power a Joule Thief which drives a colour changing LED. I have the circuit working on breadboard and will transfer it to strip board when I'm confident I've done all I need to do to preserve the battery.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Do you have a blocking diode to prevent reverse current in the dark? Once you add that, I'm not sure that panel will even charge your battery, let alone overcharge it. It really depends on the details - the I-V curve of the PV panel. Against a charged 1.2V battery (and with the blocking diode), that panel might not drive more than a few mA, which the battery might tolerate as a trickle. The devil is in the details.
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    a schottky diode might just get you to the sweet spot for a slow charge if your 2.0 volt cell is really 2.0 volts.
     
  4. Coyote Mike

    Thread Starter New Member

    Sep 22, 2014
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    Yes, I have a diode to prevent the battery discharging through the solar panel at night. I haven't actually measured the charge current. I'll give that a try tomorrow morning (the sun is setting here).
     
  5. Coyote Mike

    Thread Starter New Member

    Sep 22, 2014
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    I'm using a 1N4001 I think -- some random diode found in a box of components. I have some Schottky diodes, so I'll try one of those when the sun comes out.

    The panel definitely charges the battery -- I'm getting close to 1.3V after several hours.

    Mike
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Your "worst case" is ironically if the PV panel forces too much current through a fully charged cell. The low rent solution to that problem might be to cover a fraction of the panel. A second diode in series is another way to cripple the output. The most sophisticated solution is an IC charge controller suited to the chemistry of your battery. In between is a current shunt, that can "short" a portion of the PV panel's current once the battery reaches a set voltage.

    I wouldn't do anything else until collecting the data to see what your actual charge current is in full sun. You may not have any problem to solve.
     
  7. fooeywuffle

    New Member

    Feb 17, 2014
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    The Chinese have been doing this for many years, both NiCad and NiMH, in those solar powered LED things. I have had a lot of them, and the only problem I found is the heat here finally dries the electrolyte out. The little circuits are cheap on eBay, and it takes care of charging and powering a 20 ma LED.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Is the 2 V nominal or open ckt.?
     
  9. Coyote Mike

    Thread Starter New Member

    Sep 22, 2014
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    I did some tests this morning. Open circuit, the cell is 2.2V. Connected to a low charge battery, through a 1N4001, in full autumn sun, mid morning, the cell is putting 22mA into a single cell NiMh battery. If the sun stays out, I'll see if that reduces as the battery approaches full charge.

    I planned to try it through a Schottky diode, but the sun went in.

    My guess is that in ideal conditions in the UK, this solar panel won't put more than 50mA into the battery which is below the recommended C/10 I've read about (the battery is 600mAh). The battery is advertised as being good for 1,000 charges. On a daily cycle, that's about three years. Ask me in 2017 how the battery is doing :)

    The solar panel is a 65mm square thing going cheap on eBay. It may expire before the battery.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    In my experience with cheap solar lights, the mechanical connection to the battery is usually the weak link, followed by the battery, followed by the PCB. These all corrode and degrade performance or fail. Ants love the protected environment inside, and an ant colony is not the best thing for a PCB.

    If they'd just pay attention to metallurgy and design to prevent corrosion, these things would last a LOT longer. I don't think I've had more than 1 PV panel go bad. If the light is out, "it's never the panel".
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    2.2 V is just a llitle short; 2.2 X 75 % = 1.65 V - .33V [ 1N92, Ge ] = 1.32 V & need about 1.45 V.
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think I would just wire it up using the diode. If you have the room an AA battery would for sure be safe.
     
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