Discharging Battery

A fully-charged cell measures 1.4–1.45 V (unloaded), and supplies a nominal average 1.25 V/cell during discharge, down to about 1.0–1.1 V/cell (further discharge may cause permanent damage, and the risk is increased with multi-cell packs). This voltage varies depending on the discharge rate of the cell (lower discharge loads result in an increased voltage output for longer periods, approaching the 1.4 V unloaded cell voltage).

Under a light load (0.5 ampere), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts; some measure almost 1.5 volts. This voltage falls rapidly to about 1.25 volts at 10% depth of discharge (DOD) and then remains almost constant until the cell is over 80% discharged. The voltage then falls rapidly from about 1.2 volts down to 0.8–1.0 volts at which the cell is considered "flat" in most devices. Mid-discharge at a load of 1 ampere, the output is about 1.2 volts; at 2 amperes, about 1.15 volts; the total effective differential internal resistance is about 0.05 ohms. Nickel metal hydride batteries provide a relatively constant voltage for most of the discharge cycle, unlike a standard alkaline where the voltage falls steadily during discharge.

[edit] Over-discharging

A complete discharge of a cell until it goes into polarity reversal can cause permanent damage to the cell. This situation can occur in the common arrangement of four AA cells in series in a digital camera, where one will be completely discharged before the others due to small differences in capacity among the cells. When this happens, the good cells will start to drive the discharged cell in reverse, which can cause permanent damage to that cell. Some cameras, GPS receivers and PDAs detect the safe end-of-discharge voltage of the series cells and auto-shutdown, but devices like flashlights and some toys do not. A single cell driving a load can't suffer from polarity reversal, because there are no other cells to reverse-charge it when it becomes discharged.

Irreversible damage from polarity reversal is a particular danger in systems, even when a low voltage threshold cutout is employed, where cells in the battery are of different temperatures. This is because the capacity of NiMH cells significantly declines as the cells are cooled. This results in a lower voltage under load of the colder cells.

[edit] Self-discharge

NiMH cells historically had a somewhat higher self-discharge rate (equivalent to internal leakage) than NiCd cells. The self-discharge is 5–10% on the first day and stabilizes around 0.5–1% per day at room temperature. This is not a problem in the short term but makes them unsuitable for many light-duty uses, such as clocks, remote controls, or safety devices, where the battery would normally be expected to last many months or years. The rate is strongly affected by the temperature at which the batteries are stored with cooler storage temperatures leading to slower discharge rate and longer battery life. The highest capacity cells on the market (>8000 mA·h) are reported to have the highest self-discharge rates.

[edit] Low self-discharge cells

Main article: Low self-discharge NiMH battery such as Compaq Presario NX9010 battery, Compaq Presario NX9000 battery, Hp F2299A battery, Hp F3172A battery, Hp Pavilion ZT1000 battery, Hp Pavilion XZ200 battery, Hp Pavilion ZT1100 battery, Hp Omnibook XT1000 battery, Hp Omnibook XT1500 battery, Hp F2019 battery, Hp F2019A battery, Hp F2019B battery and Hp OmniBook 6000 battery

A new type of nickel-metal hydride cell was introduced in 2005 that reduces self-discharge and therefore lengthens shelf life. By using a new separator, manufacturers claim the cells retain 70% to 85% of their capacity after one year when stored at 20 °C (68 °F). These cells are marketed as "hybrid", "ready-to-use" or "pre-charged" rechargeables. Besides the longer shelf life, they are otherwise similar to normal NiMH batteries of equivalent capacity and can be charged in typical NiMH chargers.

Low self-discharge cells have lower capacity than some standard NiMH cells due to the larger area of the separator. The highest capacity low-self-discharge cells have 2000–2450 mA·h and 850 mA·h capacities for AA and AAA cells respectively, compared to 2800 mA·h and 1000 mA·h for standard AA and AAA cells. C types are typically higher than their usual NiMH cousins, with 4000 mA·h and the D type being 8000 mA·h.

However, after only a few weeks of storage, the retained capacity of low-self-discharge batteries often exceeds that of traditional NiMH batteries of higher capacity.