Li-ion batteries act like capacitors, says David Heacock, Texas Instruments, so it doesn't really matter what state they're in when a user decides to charge them. "The only thing that will vary is the amount of time it takes for the battery to get charged. The last 30 to 40 percent takes a while, but there's no compelling reason to discharge them before juicing them back up again."
Vin D'Agostino, principal, BNS Solutions, compares this charging action to having two jars, one empty and one full, with a tube connecting the two. The empty jar will start filling quickly because of the amount of pressure. Then the rate slows considerably until the flow is next to nothing.
It's a different story for Ni-MH and Ni-Cd chemistries, however. Heacock says that users of these batteries should use or discharge all the power from a battery before recharging it. "Discharging all the cells keeps them in better working condition." Battery packs are made up of a number of cells, and over time, the cells that haven't been fully used develop a film on their internal plates, cauing a loss in discharge voltage. This is often referred to as the "voltage depression effect."
When charge flow falls below a certain threshold for Li-ion, the charger decides that the battery is full and reaches charge termination. For Ni-MH chemistries, temperature is measured; a drastic increase in temperature signals that the battery is charged.
For Ni-Cd batteries, the finesse of measuring temperature is more expensive than is logical to pay for this older chemistry. Rather, when the terminal voltage of this type of cell starts to droop, then falls, it's fully charged. A sudden increase or decrease in ambient temperature will affect the charger's ability to read the level of charge. More sophisticated systems check for the ambient temperatures surrounding the batteries, but these systems are reserved for batteries in devices such as LVA (left ventricular assist) pumps and are too expensive for the tools of mobile workers.