Just read an article from a highly reliable newsletter (Windows Secrets) which generally discusses issues related to PC's, but some articles are relevant to other technologies ... in this case, the article was about lithium batteries, and how to prevent early failure of these. This tip might be relevant to your problem:
"Overcharging — leaving a battery connected to a too-high voltage source for too long — can reduce a Li-ion battery's ability to hold a charge, shorten its life, or kill it outright. Most consumer-grade Li-ion batteries are designed to operate at around 3.6 volts per cell but will accept a temporary overvoltage of around 4.2 volts while charging. If a charger outputs the higher voltage for too long, internal battery damage can occur. In severe cases, overcharging can lead to what battery engineers delicately refer to as "catastrophic failure." Even in moderate instances, the excess heat produced by overcharging will negatively affect battery life. High-quality chargers can work in concert with circuitry inside well-designed Li-ion-powered devices and their batteries, reducing the danger of overcharging by properly tapering off the charging current. But the simplest, can't-fail method is not to leave your Li-ion devices connected to any charger longer than is needed."
I know that the Nikon charger is supposed to shut off when the battery reaches full charge, but you might want to check to see if this is actually happening when you leave your battery in the charger overnight.