Solar walkway light cells... NiCd, NiMH, or... rechargeable alkalines?

Turbo DV8

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First, I have internet access only one hour each week, so do not think lack of a quick reply to suggestions here demonstrates disinterest on my part. I will check back each week.

I picked up some solar path lights at Target. They have rechargeable alkaline cells. I also have seen some use NiCd, which I've heard handle deep discharge better than NiMH. However, I have also seen some solar path lights that use, in fact, NiMH! My core question hinges around the rechargeable alkalines which coem with the Target lights. One obvious plus is that they all measure over 1.5 volts out of the package, as opposed to NiCd's. But, how well do rechargeable alkaline's handle nightly deep (complete) discharges and then recharges? I thought recharging alkalines was only viable if they were not allowed to deep discharge between charges, and in any case, they would lose life quickly, leak, etc. Is a dedicted "rechargeable alkaline" a different animal than simply trying to recharge a normal alkaline with a "Buddy L" charger? (Egads, remember those? :sick2:) What about availability when it comes time to replace them? I can't recall ever seeing any for sale. Any advice is appreciated.
 

SilverFox

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Hello Turbo DV8,

Bummer, 1 hour a week is really cutting things down...

NiCd would be best, NiMh also works very good. If you live in an area where the cells never get run all the way down, the rechargeable alkaline cells should work, but if you over discharge them your cycle life will suffer. They will work for awhile, but both NiCd and NiMh cells will last longer.

In the winter in my area, I have to pull the cells about once a week to charge them. There isn't enough light to keep them going.

Tom
 

Lynx_Arc

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My experience with rechargeable alkalines (ram cells) is that they don't like deep discharges and tend to last longer if not discharged deeply. They also tend to lose capacity every time you recharge them IMO.... the amount may not be huge but I am guessing those lights won't be very bright a few years down the road. I would use them as is until they start getting too weak then buy whatever cells nimh or nicd that will fit them that are the cheapest although if you got the ones I am thinking of costing under $5 you may just be better off getting new lights for the cost of new batteries. I saw some there for $2 and $3 that were tempting with some sort of green 2/3 AAA cell in them.
 

BoarHunter

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Too late folks, you answered more than one hour after the OP !
Got to wait till next week ! :)
 

qwertyydude

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Nicd or nimh it almost doesn't matter. The biggest difference would be if the deep discharge reverse charges one of the cells. With that said the best thing you could do would be to match the cell's capacities as close as possible so they both die at the same time. This will extend the cycle life. The advantage of the nimh would be much longer battery life. But only if the solar cell can charge it to full capacity.
 

Lynx_Arc

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Nicd or nimh it almost doesn't matter. The biggest difference would be if the deep discharge reverse charges one of the cells. With that said the best thing you could do would be to match the cell's capacities as close as possible so they both die at the same time. This will extend the cycle life. The advantage of the nimh would be much longer battery life. But only if the solar cell can charge it to full capacity.

I believe this only have one battery in them. nimh doesn't like trickle charging so it is possible it would not last as long in use while nicads can be trickle charged indefinitely without problems. If the nicad has enough capacity for a full day of sunlight to capture all the energy without wasting any then there is not a lot of difference otherwise but if there is more power than the nicad can hold the nimh would run it longer when it gets dark out.
 

qwertyydude

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All the lights I've seen use two cells. If they use white led's they need to have at least two cells unless there's a boost circuit which I doubt since it's cheaper to just put a second cell than a boost circuit. And nimh can be trickle charged at the low currents like that found on these lights <100 ma. Trickle current is usually defined as less than c/10 but in reality it should be less than c/10 AND maximum 100 ma. They both have a recombination catalyst to recombine excess gas from trickle charging but with the higher capacity for hydrogen generation because of the larger capacity in nimh it's more sensitive to overpressurizing and venting hence why there's a more absolute maximum trickle charge current, but they can be trickle charged.
 

jayflash

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Lynx's point about cell capacity is noteworthy. I experimented, using Hybrid LSD cells of four times the capacity of the NiCd's which came in my solar lights. The solar PV cells provided much more power, during long summer days, than the original NiCd batteries could hold. The 2000mAh Hybrids would keep the LEDs lit until daybreak instead of midnight for the NiCd's.
 

Turbo DV8

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I'm baaaack! Thanks for the useful input. The lights do take only one cell, and they are white LED's. I got sixteen lights on clearance, at $7.50 per four (normally $30) at Target. Since I posted, I read some really discouraging stuff in the manual. My gut says they stuck alkaline in there to be "green," and the consumer gets stuck with the tradeoff in sub-par performance. The manual says to "check and replace the battery periodically throughout the year, especially after winter months," then gives a handy number to call to buy replacements. That sounds like I can expect pretty dismal performance and longevity!

I would expect these things to completely discharge in the winter. On a fresh cell, the LED draws 10 mA. In blazing overhead sun, it charges at 18 mA... filtered sun can charge as low as 3-5 mA. Doing the math with daylight/nightime hours during the winter, and it would seem that in the winter the cells will die every night. I could go the route of removing the cells every week or so to charge (NiMH) but with sixteen lights that feel flimsy enough to break every time I open them, well, I dunno!

Another thought... aren't alkaline cells prone to freezing and failing? I've had regular alkalines outdoors fail this way in the winter, and would rechargeable alkaline be any different?
 

Lynx_Arc

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Not sure if rechargeable alkalines leak or freeze less or not. If the lights only draw 10ma with a boost circuit they must be rather dim and be useless before midnight. You may consider wiring the lights in parallel and adding a nimh AA pack to the mix to boost the output
 
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srfreddy

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Not sure if rechargeable alkalines leak or freeze less or not. If the lights only draw 10ma with a boost circuit they must be rather dim and be useless before midnight. You may consider wiring the lights in parallel and adding a nimh AA pack to the mix to boost the output

5mm leds should really be driven only at 10ma or so, so the best output increase would be gotten from switching the LED's to Cree's or GS's.
 

Lynx_Arc

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5mm leds should really be driven only at 10ma or so, so the best output increase would be gotten from switching the LED's to Cree's or GS's.

I'm thinking he meant the draw from the battery was 10ma which means from a 1.5v battery you are getting less than half that to the LED or perhaps 4-5ma because of boost circuit and inefficiency also but I could be wrong as to what he meant.
 

Turbo DV8

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Yup, 10 mA through the cell. It is not a 5mm, it is a very tiny square chip. Although not "bright" by any definition, it is the brightest of three types I tried. A big limiting factor on some of these path lights is the absurd design of some of the lenses. The ones I tried from Costco have crenulations in the lens which focus all the light into tiny strips of light, instead of being an area light, rendering them absolutely worthless for anything other than decoration. The target lights are somewhat better. I don't understand why none I have seen have simple, clear, unadorned lenses.
 

Lynx_Arc

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I'm not sure why there is lousy lenses. Maybe they have people designing lights and not using them, I have noticed chinese products in the past made such that you wonder if they even tested them to see if they worked as most did not work well on the shelves of the store. I sort of concluded the $2 lights I saw would be similar in use to runway marker lights giving very little effective light to the area if there was any sort of light pollution present. I have seen larger solar lights in use and most of them after about 11pm at night are only bright enough to see the light itself.
 

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