Explore why some battery packs pose odd voltages and how this affects the user.
A battery is an electrochemical device that produces a voltage potential when placing metals of different affinities into an acid solutions (electrolyte). The open circuit voltage (OCV) that develops as part of an electrochemical reaction varies according to the metals and electrolyte used.
Applying a charge or discharge places the battery into the closed circuit voltage (CCV) condition. Charging raises the voltage and discharging lowers it. The voltage behavior under a load and charge is governed by the current flow and the internal battery resistance. A low resistance produces low fluctuation under load or charge; a high resistance causes the voltage to swing excessively. Charging and discharging agitates the battery and full voltage stabilization takes up to 24 hours. Temperature also plays a role; a cold temperature lowers the voltage and heat raises it.
Manufacturers rate a battery by assigning a nominal voltage, and with a few exceptions these voltages follow an agreed convention. 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 the voltage of a charged battery should be 2.10V/cell. Keeping lead acid much below 2.10V/cell will cause the buildup of sulfation.
In consumer applications, NiCd and NiMH are rated at 1.20V/cell, industrial, aviation and military batteries adhere to the original 1.25V. There is no difference between the 1.20V and 1.25V cell; the marking is simply preference.
The nominal voltage of lithium-ion is 3.60V/cell and represents three nickel-based batteries connected in series (3 x 1.20V = 3.60V). Some cell manufacturers mark their Li-ion as 3.70V/cell or higher. This offers a marketing advantage because the higher voltage boosts the watt-hours on paper (voltage times current equals watts). The 3.70V/cell rating also creates unfamiliar references of 11.1V and 14.8V when connecting three and four cells in series rather than the more familiar 10.80V and 14.40V respectively. Equipment manufacturers adhere to the nominal cell voltage of 3.60V for most Li-ion systems.
How did this higher voltage creep in? The cell manufacturer plots the voltage of a fully charged cell that measures 4.20V, discharges it at 0.5C to 3.00V and takes the mid-way point. For Li-cobalt the mid-way point is about 3.60V. The same scan done on Li-manganese with a lower internal resistance gives an average voltage of about 3.70V. It should be noted that the higher voltage is arbitrary and does not affect the operation of the portable devices or the setting of the charger.
The phosphate-based lithium-ion has a nominal cell voltage of 3.20 and 3.30V. This makes this battery incompatible with regular Li-ion and their chargers.
The alkaline delivers 1.5V, silver-oxide 1.60V and primary lithium 3.00V. The 9-volt battery has six cells in series. Do not charge primary batteries because overcharge can produce explosive gases. (See also BU-211: Alternate Battery Systems – Reusable Alkaline)
Last Updated 2/08/2015
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