Years ago I bought an 8-pack of solar lawn lights. They were fun for lighting up the back yard for a few hours after sunset, and I was curious how long they'd last. The answer was about five years, plus or minus a few years, depending on your standards because they gradually fade out. Both in terms of shorter duration after sunset, and in terms of dimmer illumination.

Now only one of the original eight offer any visible illumination, for less than half an hour after sunset. This behavior implies whatever internal energy storage device has degraded over years, and I wanted to take a look to see if I can confirm my hypothesis.

Solar lawn lights are very inexpensive now, and it's much easier to just buy a new set. The reason I wanted to try reviving these lights is because their center clear section is glass, not clear plastic as many current products use.

Glass won't yellow and fail after years of southern California sunshine, which is not something I can say for whatever non-glass material was used for the solar cell's top layer. The paint is also very visibly damaged by sunlight. I can repaint if I want to, but condition of that solar cell looks bad.

Flipping the lid over, I was surprised to see a battery compartment door. I had expected a "no user serviceable parts inside" arrangement. I opened the door and the molded plastic latch broke, brittle from years of baking under the sun.

Inside the door is a AAA NiMH battery cell, literally roasted by the sun while it lived inside a black enclosed compartment. After removing the battery, I took a multimeter and measured the open-circuit battery compartment terminals under sunlight. My meter read 2.095 volts. Yikes! That's a lot higher than the 1.25V nominal level for a NiMH battery cell. Are NiMH batteries expected to take this kind of abuse? I know NiMH batteries can handle over-voltage better than lithium chemistry batteries, but I didn't know to what degree. While excess energy can be dissipated as heat, it's hard to shed energy as heat when it is already very hot from sun. Between the high voltage and high temperature, these cells lived a harsh life.

Looking in my pile of NiMH batteries, I found two AAA-sized units and installed one of them. The LED lit up when I covered the solar panel to simulate night, so I decided the old battery must be completely dead. To my surprise, it wasn't! When I connected it to my "Joule Thief", the LED lit up and stayed illuminated for days. The battery isn't completely dead, but not well enough to run this solar lawn light. Connecting it to my bench power supply, I find the LED turns off when battery voltage drops below about 0.9V. In comparison, my Joule Thief will run all the way down to about 0.4V, which is much more demanding on the battery and a bad idea for longevity.

I don't have many NiMH AAA batteries on hand. I could buy some more, but I don't particularly relish the thought of buying new batteries just to sentence them to a quick and hot death. I decided to open up the light to see inside. There wasn't much: the solar panel is well-sealed by a blob of dark gray epoxy, and there's only a tiny circuit board inside.

The back of the circuit board shows all the signs of something left out in the elements.

As does the front, which showed a YX805 chip in charge of the operation. An online search found a Simplified Chinese datasheet which explained it was specifically designed to run solar lawn lights. Machine translation claimed battery over-discharge protection as a feature, explaining the deactivation at 0.9V. There's also mention of adjustable battery charge rate, but nothing about over-voltage protection. Either the machine translation missed it, or the designers decided it was OK for a 1.25V nominal voltage NiMH battery cell to face 2.095V charging input. Or perhaps the 2.095V I measured was just an artifact of an open circuit that fails to trigger the battery charge rate limiter?

Given its corroded condition implying it may fail elsewhere in short order, I'm not eager to spend money on new batteries. I have plenty of NiMH cells on hand, though, just not in AAA size. The biggest of which are from Neato battery packs. They're too old and tired to run a robot vacuum, but perhaps they can run a solar lawn light. I cut the spot-welded tabs in half and soldered to the half-tabs, sparing battery cells from heat of my soldering iron.

Sadly many of those old cells were too weak to get up over that 0.9V minimum bar, but a few of them were good enough to run a solar lawn light (though dimly) for a bit after sunset. Perhaps my solar lawn lights will be my way of giving some old NiMH battery cells yet another life, even if a hot and short one. Or I can use it as motivation to explore another technology: supercapacitors.