The number of portable gadgets has grown substantially in the last few decades. Many of these, such as TV remote controls, some digital cameras, handheld GPS units, and flashlights, are designed to operate on either disposable alkaline batteries or rechargeable and reusable NI-MH batteries.

The earliest consumer rechargeable batteries were Ni-CD batteries. These batteries typically had only about 10% to 20% of the capacity of disposable alkalines of the same size. Devices using these batteries would run for only 1/10th to 1/5th as long as they would on alkalines. Fortunately, when the batteries did run down, they could be recharged. Properly cared for, even these early rechargeable batteries would last for several hundred recharges.

A serious concern with NI-CD rechargeable batteries is that cadmium is a very toxic metal, making battery disposal problematic. Many NI-CD batteries have made their way to landfills, contaminating our environment. Now, recycling of NI-CD batteries is mandated by law in most western countries.

NI-MH batteries had very similar properties to NI-CD ones, but with higher capacity, and more importantly, no super-toxic components. The only drawback of NI-MH compared to NI-CD was a lower maximum current. High-current applications like cordless power tools and electric powered model aircraft and cars continued to use NI-CD batteries for some time. As of 2008, some power tools still use NI-CD batteries, although model aircraft have switched almost entirely to NI-MH and Polymer Lithium ion Battery.

Compared to disposable alkaline batteries, traditional NI-MH batteries have one serious disadvantage. That is self-discharge. A qualified alkaline battery will have a shelf life of about three to five years. You can leave it in a drawer until you need it, open the package, and expect it to still have virtually all the capacity it ever had.

Until recently, most NI-MH batteries had a very high self-discharge rate. They would lose capacity even when not in use. A good NI-MH battery had a self discharge rate of about 1% per day. After each day of sitting idle, it would have only 99% of the capacity that it had the day before. After about a week, it would be at 93% and after a month, 73%.