The same issue of New Scientist this question was published, 18 July 2009 included an article on rechargable battery technology. Refer to page 45 the diagrams showing the charge and discharge cycle for a rechargable battery. A rechargable battery not under load will slowly dissipate charge across the separator. I personally use Sanyo's eneloop batteries which  are stated to maintain their charge for up to a year after having the same grievance of rechargable batteries going flat when not being used.

Nickel-metal hydride batteries have a high rate of self-discharge - about 30 per cent per month. This means that every two months their charge diminishes by a factor of 2, and in a year they will discharge to about 1.4 per cent of their full charge - effectively dead.

Nickel-cadmium batteries are somewhat better, and lithium batteries are much better at only about 2 or 3 per cent discharge per month. In 2005, low self-discharge NiMH batteries were introduced. These are sold as pre-charged or ready-to-use. They are more expensive than old-fashioned NiMH batteries, but can be used in many applications, such as a clock, where normal NiMH batteries would be unsuitable - though in some of these applications it might be more economical just to use non-rechargeable batteries.

Eric Kvaalen, La Courneuve, France