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A recent development in battery technology is Nickel-Metal-Hydroxide (NiMH) batteries. NiMH batteries have a considerably greater capacity than the same sized NiCad batteries. They are also much better for the environment since they don't contain cadmium, a heavy metal. They are sold as an improvement for such energy intensive uses from camcorders or hybrid automobiles. I wanted to test them in solar landscape lighting to see if the theoretical improvement would translate to real improved performance. Until recently, most inexpensive solar landscape lighting came with Nickel-Cadmium (NiCad) batteries installed. NiCad batteries are relatively inexpensive and easy to recharge. NiCad batteries perform very well in solar landscape lighting since they can be recharged about 1000 times and charge very efficiently with a low current over a long period. In normal operation, they need to be replaced every 2-3 years in solar landscape lighting. 
Now that the price has come down on NiMH batteries, many higher quality solar landscape lights come equipped with them. Their theoretical performance is over twice that of NiCads. I happened to have a set of four bargain basement solar landscape lights that were about 3 years old and in need of replacement batteries so I decided to replace the NiCad batteries with NiMH and see how they worked. The lights I have were made by Intermatic and only cost about $12 each. They have a small, relatively low efficiency solar panel. They are essentially a solar battery charger with an LED attached. When the solar panel is charging the battery the circuit switches off the LED. When the panel is not producing power (or very little power) the LED is switched on and stays on until the battery voltage drops to a low level, indicating a discharged state. In a pinch they can be used as a battery charger by leaving them out during the day and removing the battery at night. In a couple of days of sunshine they will put a near full charge on the battery. Rechargeable batteries generally last longest if they are charged to about 90% and discharged to about 50%. The 3-year-old batteries in the lights I have were about shot. They would only provide 1-2 hours of light at best.
When the Intermatic lights were brand new 5 or 6 hours of sun would make them work for 5 or 6 hours at night. More sun didn't help much, they would still be very dim or out after 6 hours.
Switching to NiMH batteries resulted in a dramatic improvement. With the same 5-6 hours of sun the lights now stay on for 10-12 hours! The bottom line is the NiMH batteries performance was even better than I expected. The questions to be answered are: - Why do NiMH batteries perform so much better in solar landscape lights?
- Are there any safety issues with using NiMH batteries this way?
- Will NiMH batteries last as long as NiCad batteries?
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