Premature Voltage Cut-off
Not all stored battery energy can or should be used on discharge, and some reserve is almost always left behind on purpose after the equipment cuts off. There are several reasons for this.
Most cell phones, laptops and other portable devices turn off when the lithium-ion battery reaches 3V/cell on discharge, and at this point the battery has about five percent capacity left. Manufacturers choose this voltage threshold to enable some time before recharging. This grace period can be several months until self-discharge lowers the voltage to about 2.5V/cell, at which point the protection circuit opens. Most packs become unserviceable when this occurs. See How to Awaken Sleeping Li-ion.
A battery on a hybrid car is seldom fully charged or discharged; most operate between 20 to 80 percent state-of-charge. This is the most effective working bandwidth of a battery; it also delivers the longest service life. A deep discharge causes undue stress, and the charge acceptance above 80 percent diminishes. The emphasis on an electric powertrain and industrial applications is to maximize service life rather than optimize runtime, as it is the case with consumer products.
Power tools and medical devices with high current draw tend to push the battery voltage to an early cut-off. This is especially true if one of the cells has a high internal resistance, or if operating at cold temperature. These batteries may still have ample capacity left after the “cut-off;” discharging them with a battery analyzer at a moderate load will often give a residual capacity of 30 percent. Figure 1 illustrates the cut-off voltage graphically.
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Figure 1: Illustration of equipment with high cut-off voltage Portable devices do not utilize all available battery power and leave some energy behind. Courtesy of Cadex |
Alkaline batteries are not suitable for high load applications because of elevated internal resistance. Cold temperature and a partially depleted cell cause the internal resistance to rise further. This advances the cut-off and much of the energy is left behind. See Function of Primary Batteries.
Comments
You would probably see your charger put back in about 2.2 A-h if your pack were discharged down to the 3.2-3.3 V/cell range. Keep in mind that many ESCs have a soft cut-off (which can usually be changed or turned off) that will protect you from over- discharging the Li-Po battery.
On my 2.2 amp-hr r/c helicopter Li-Po battery, I land at about 3.8 v/cell and charge to about 4.2 v/cell. Over this delta V, my charger says it “put in” about 1.47 amp-hrs to get back to 4.2. If it could, at what voltage would the pack give up all 2.2 amp-hrs?