A battery is an electrochemical device that produces a voltage potential when placing different metals in acid solutions. The open circuit voltage (OCV) attained varies according to the metals and acid solutions (electrolyte) used. Applying a charge or discharge places the battery in the closed circuit voltage (CCV) condition; charging raises the voltage and discharging lowers it. This voltage behavior is governed by the internal battery resistance; a low resistance produces less fluctuation under load or charge than a high one. Charging and discharging agitates the battery and a full stabilization takes up to 24 hours. Temperature also has a role; cold rises the voltage and heat lowers it.
Manufacturers rate a battery by assigning a nominal voltage and with a few exceptions, these voltages follow an agreed convention. Rating some Li-ion higher than the standard 3.6V/cell may help in product marketing but for the user, a chemistry-specific voltage counts. Here are the nominal voltages of the most common batteries in brief.
The nominal voltage of lead acid is 2.00 volts per cell, however when measuring the open circuit voltage (OCV), the voltage of a fully charged battery should be 2.10V/cell. Keeping lead acid below 2.10V/cell will cause the buildup of sulfation.
In consumer applications, NiCd and NiMH are rated at 1.2V/cell, industrial, aviation and military batteries adhere to the original 1.25V. There is no difference between the 1.2V and 1.25V cell; the marking is simply preference.
The nominal voltage of lithium-ion had been 3.60V/cell. This is a practical figure because it represents three nickel-based batteries connected in series (3 x 1.2V = 3.6V). Some cell manufacturers mark their Li-ion products as 3.70V/cell or higher. This poses a marketing advantage because of higher watt-hours on paper (multiplying voltage times current equals W). It also creates unfamiliar references of 11.1V and 14.8V when connecting three and four cells in series. Let this higher voltage not cause confusion; equipment manufacturers will always adhere to the nominal cell voltage of 3.60V for most Li-ion systems, and the standard designation of 10.8V and 14.4V will always work.
How did this higher voltage creep in? To calculate the nominal voltage, we take a fully charged battery that measures 4.20V and then fully discharge it to 3.00V at a rate of 0.5C while plotting the average voltage. For Li-cobalt, the average voltage comes to 3.6V/cell. Performing the same discharge on a fully charged Li-manganese with a lower internal resistance will result in a higher average voltage. Pure spinel has one of the lowest internal resistances, and the plotted voltage on a load moves up to between 3.70 and 3.80V/cell. This higher midpoint voltage does not change the full-charge and end-of-discharge voltage threshold.
The phosphate-based lithium-ion deviates from others in the Li-ion family and the nominal cell voltages are specified at between 3.20 and 3.30V. Because of the voltage difference, the two lithium-ion families are not interchangeable. New lithium-based batteries will have other voltages and specialty chargers may be needed.
The alkaline delivers 1.5V, silver-oxide 1.6V and primary lithium 3V. The 9-volt battery has six cells in series. Do not charge primary batteries because overcharge can produce explosive gases. See Reusable Alkaline.