I've got an Black & Decker electric trimmer that gets used about 2 months out of the year. It uses a NiCd battery pack.
For optimum battery life, should I leave it plugged in all year when not in use? I hate to waste that power (minimal though it is).
I'm Ok with needing to charge the battery overnight before first use.
The "memory effect" is not typically a problem with modern batteries. The effect is generally "broken through" by large discharges, too, so if it's a battery for a power-tool, then it's a self-correcting problem.
Modern chargers won't over charge so you shouldn't have a problem. Power consumption for continuous "trickle charge" is tiny. Opening your refrigerator an extra time will probably amount to a year's worth of such charging.
Battery longevity may be reduced by it remaining warm for an extended period but it also degrades just sitting around.
Bottom line: If it'll be a problem for you when you need to use your electric whatever and there is no charged battery, then leave a battery in the charger.
http://batteryuniversity.com/ has some of the best information about batteries. I'm especially fond of this article on the maintenance of Li-Ion batteries: http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
I would say no for two reasons. Power consumption, the charger itself is using electricity even when not charging the battery, so it is costing you money. The other reason is the heat generated by the battery in the charger, if you have ever felt the battery after taking it out of the charger it is warm. The heat is detrimental to the battery in the long term.
NiCd batteries don't do as well when they're "topped off". It's better to let them run out and then recharge them.
