rechargable batteries for solar lights

I'm generally OK with electronical things, but hardly an expert on rechargables, from reading the posts here. So, thanks in advance; here's the question:

Like a lot of folks, i have a pile of those solar yard lights with a couple of rechargable AA nicads, which have lost their capacity to store a charge. (trying to recharge the cells with a nicad charger fails, substituting alkaline AA cells in the lights makes them work) It's easier to find NiMH these days so I tried a couple of lights with newly purchased NiMH cells; these babies were rated at 2200 mAH compared to 300 for the original NiCads. I wasn't expecting any better performance, mind you, assuming that the solar cell output is the limiting factor; but I'm not entirely sure that I'm not getting less output from them now. It's midsummer here, max daylight, and these NiMH are only charging up enough to run the lights for a couple of hours every night. I'm pretty sure I used to do better than that with the original Nicads. So here's the part where my knowledge of rechargables runs out; is it possible that the NIMH cells actually charge less than NiCads on the little cheapo solar charger in the light? Or is it just my own faulty memory?

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Hi Z

Interesting question, as I had the same problem with my rechargable garden lights. I concluded that, even in the hot sun, the solar cells were not putting out enough juice to fully charge the batteries. I tried improving the circuit (a cheap Chinese circuit made up of a whole ton of bipolars) and could not. I concluded things would not improve so I binned them.

In answer to your question, NiCads and NiMH batteries have roughly the same charging characteristic, so the charging results should be similar for a given input current. NiMH batteries hold about 40% more capacity than NiCads, but NiCads can kick out more current (should not be a problem with LED lighting). These are the 2 main differences.

I would be interested in any comments on this subject to fill a gap in my knowledge


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I don't know if this is a factor in your application, but ISTR people mentioning that higher capacity NiMH cells often have higher self-discharge current.

Wikipedia says that NiMH typically have higher self-discharge than NiCd, but there exist "low self-discharge" types (LSD NiMH), sometimes sold pre-charged.

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I'd try a set of Ni-Cads like the things were designed for; in my experience, NiMH's are crap, and especially with a charger that's optimized for Ni-Cads.

Good Luck! Rich

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Rich Grise

The output of amorphous solar cells reduces over time and it is a safe bet they weren't "A grade" material to start with. I liked the idea of the low self discharge NiMH.

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David Eather


** Chances are, you solar cells only deliver a small charge current ( like 25 to 30mA ) so must NOT be used with high capacity cells like 2200mAH NiMHs.

Charging the above cells requires a steady current input of 100mA for 24 hours or longer - at 25 mA they will barely charge at all or maybe even go backwards.

Try some AAA size NiCds, usually rated at 250mAH.

.... Phil

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Phil Allison

NiMH have higher self discharge than NiCd, so will waste more of what little charge you get from the solar cell. Also a larger capacity NiMH cell has larger self discharge, so you're getting bitten twice.

You can get 2x 300mAh cells for =A31 from poundland, hopefully they'll restore your lost energy. Or 800mAh =A31 each.

Another trick is to arrange an external reflector that gives more light to the PV cell at times when the sun isn't full strength. To be effective the reflected light needs to be diffused a bit rather than pin sharp.


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Well, on a long dark night the cells will completely discharge. A day on the solar cell, that would fully (or nearly) charge 600 mAh NiCd cells, will one-third charge the NiMH. So instead of deep-cycling NiCd cells (which is OK), you'd be shallow-cycling partly charged NiMH cells. I'd stick with NiCd, and if it doesn't work well, I'd assume it will NEVER work well. I have some lead-acid gizmos of this sort, they killed batteries quicker than any reasonable person would tolerate...

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