Will secondary batteries replace primaries?

Consumer market put aside, the largest users of primary (non-rechargeable) batteries are the military, specialty emergency services and forest fire fighters. High energy density, long storage and operational readiness are among their strong attributes. No charging and priming is required before use. Logistic is simple and battery power can be made available at remote locations that are unmanned and have no electrical power. Disposal is easy because most primary cells contain little toxic materials.

Primary batteries have the highest energy density. Although the secondary (rechargeable) batteries have improved, a regular household alkaline provides 50% more power than lithium-ion, one of the highest energy-dense secondary batteries. The primary lithium battery used in cameras holds more than three times the energy of a lithium-ion battery of same size.

The negative on the primary batteries is its relative high internal resistance, which inhibits current flow. High internal resistance has little affect when powering low-current devices such as a TV remote control or a kitchen clock. The problem arises with digital cameras and other power-hungry devices. A power drill on an alkaline would be unthinkable. The voltage would imply collapse.

The comparison of the energy densities was done in an unfair way. Whereas most secondary batteries are rated at a 1C discharge, the alkaline was discharged at only 25mA to 0.8 volts. We now take the same batteries and run them under a load. The yellow bars in Figure 2 represent the usable energy if the batteries were used in such a device as a digital camera.

Capacity rating of alkaline cells
With moderate load, the energy density of alkaline batteries is very high. Figure 3 illustrates the rated capacities of the AAA, AA, C, D cells and the 9V pack. Note that these batteries only deliver the specified mAh if discharged through a high resistive load typical of portable entertainment devices.

Run-time estimation
The most distinct limitation of the primary battery is its one-time use. Because of this, the cost of power is about 30 times higher than that of rechargeables. The pricing becomes even more excessive if the packs are replaced after each mission, regardless of length. A general of the US Army said that half of the batteries discarded still have 50% energy left. Throwing away partially used batteries is widespread. Keeping track of these packs in the military and public sector is time-consuming and awkward. It is much simpler to issue fresh packs before each activity.

It is possible to read the state-of-charge of primary batteries. The most basic method is measuring the open terminal voltage but the result is inaccurate. The increase in internal resistance with lower state-of-charge adds to run-time estimations. A better method is counting the out-flowing energy units, also known as coulombs. This requires a circuit and a display unit on the battery. Due to high cost and inherent inaccuracies, especially during pulsed loading, this method is seldom used on primary batteries. A more accurate state-of-charge measurement is possible with a rapid-test instrument that examines the chemical integrity of the battery. The test is non-invasive and takes only a few seconds to complete

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