This topic is a bit trivial, but it always surfaces this time of year as the autumn days grow shorter and darker.
And oddly enough, it provides a mini lesson in the calculus of off-grid solar.
Solar yard lights are nice in theory, but as you may have experienced, they are not designed for our Canadian low winter light levels. Arizona & California, sure, but not here. Not only is the daylight weak, but it is also short, which accordingly means that with longer nights comes longer run-times for the solar not to keep-up with.
What’s Inside Solar Yard Lights?
These lights usually have 1 or 2 batteries in them (300mAh @ 1.2V x2 = 0.72Wh).
The lights are low-power LEDs of about 0.1W
The solar is usually thin-film in the 0.15W range
The solar cell acts as the daylight sensor, so no power is consumed during the day for that function.
The Performance Numbers…
In summer, the solar will charge the batteries by about 0.6Wh each day (given good sky exposure)
But, the LEDs will consume this energy in about 6hrs overnight (ie from 9pm through 3am)
In winter, you might get 0.1Wh of charging energy & 1-hour of run time (ie from 5pm to 6pm)
The LEDs will take what they can get, so tend to run at very low output as the battery voltage drops, so you might see a vey dim light for longer hours, but you get the idea.
This low-performance in winter is disappointing, but not surprising, and maybe we could simply deal with that. After all, we aren’t throwing garden parties or impressing the neighbours with our landscaping too much in winter.
But, the bigger problem is the deficit cycling of the batteries. Due to the nature of the design, the batteries used are very low capacity in the first place. Compare the 300mAh to the 2700mAh NiMh rechargeables you can buy for a flashlight or radio. No point in having all that capacity if you can’t use it. Plus, the deficit cycling will age the batteries faster, and ultimately they will stop accepting a charge after a year or two. NiMh rechargeables only have 500-1,000 cycles of life anyway, so 2 years is about it.
What to do?
Solar yard lights look great, and they are very handy for marking driveways and paths without having to run wire everywhere. So what can be done to improve their engineering?
1. Choose The Right Battery
Make sure you purchase lights that use NiMh batteries in them.
NiCd batteries are bad… they have a memory effect, so the partial charging will age them very quickly, plus Cd (Cadmium) is highly toxic, so this amounts to a product designed to grab your dollars & then head for the landfill. Please don’t support such crap, even if they are on sale!
Most importantly, if it doesn’t say what battery type it uses on the box, or in the user manual (open the box and read it before you buy it), then leave it on the shelf. It’s probably NiCd.
2. Removable Battery
Some lights are actually designed so that you can pop-off the top and replace the battery when needed. A much better idea! For two reasons…
a. You can readily purchase NiMh 300mAh replacement rechargeable batteries at the dollar store, so 2 years down the road, you can replace.
b. During the winter, you can take the batteries out of the lights once a week and give them a full charge in a battery charger. If you use full-capacity 2700mAh NiMh cells instead, they will give you several full nights without any daylight charging at all. But, always remember to give them a full charge at least once a month, even in the summer.
3. Increase The Solar (DIY hobby fun)
If you enjoy taking things apart, you might enjoy adding an extra solar panel to each light, or run wire from one larger solar module (at the appropriate voltage) to each of the lights as a daisy chain. Usually this only involves sneaking two wires into the battery compartment.
That’s about all I will say, and it’s probably enough, about solar yard lights.
If you have any specific questions, please feel free to reach out.