Will secondary batteries replace primaries? (BU50)

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.

Figure 1: Energy comparison of rechargeable and non-rechargeable batteries.

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.

Figure 2: Energy comparison under load. The alkaline works well for a kitchen clock but fails on 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.

Battery type	Nominal Voltage	Rated Capacity* mAh	Voltage cut-off	Rated Load
9V	9 volts	570	4.8 volts	620 Ohm
AAA	1.5 volts	1,150	0.8 volts	75 Ohm
AA	1.5 volts	2,870	0.8 volts	75 Ohm
C	1.5 volts	7,800	0.8 volts	39 Ohm
D	1.5 volts	17,000	0.8 volts	39 Ohm

Figure 3: Industrial Alkaline Battery specifications (Courtesy of Panasonic).
* The mAh ratings are for reference only. The actual ratings may vary depending on discharge rate

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

The switch to secondary batteries

During the last ten years, armies and emergency response teams have gradually been switching to rechargeable batteries. Improvements in battery technology, better charge methods and more readily available power sources made this possible. The most important single reason, however, is cost.

In the US Army, rechargeable batteries are being used predominately for training. Officials are now exploring their suitability for combat missions. Rechargeables have advantages that go beyond cost issues. For one, the batteries can be re-used and do not burden the supply channels. In the absence of electric power, charging can be done through solar power, windmills and hand-crank generators. Even kinetic power is being explored in which an electric generator is built in the sole of the soldier's boot. Rechargeable batteries are able to providing power when no supply of fresh batteries is possible.

Another advantage of secondary batteries is low internal resistance. This allows high current on demand, an attribute that is essential for digital devices and instruments needing high inrush currents. Power tools, for example, could not be run effectively on alkaline batteries.
But rechargeable batteries have their limitations. Beside marginal energy density, secondary batteries have a defined shelf life and lose the ability to hold charge as they age. Similar to a spring under tension, a secondary battery seeks to revert back to its lowest denominator.

Battery aging is subject to cycling, storage temperature and state-of-charge. While a primary battery has a shelf life of 10 years, lithium-based batteries are good for 2-3 years in normal use. Cool storage at a 40% charge level prolongs longevity. Nickel-based batteries are good for 5 years and longer but require priming to regain performance after long storage.

Another negative of rechargeable batteries is the high self-discharge. nickel-based batteries exhibit a 10-20% self-discharge per month. This compares with 5-10% for lithium and lead-based batteries. The self-discharge increases at higher temperatures. For this reason, secondary batteries are not an effective media for long-term energy storage; primary batteries are better suited. A secondary battery can never be removed too far from the charger. It needs to be fed before each activity.

Secondary batteries have a limited cycle count. The number of cycles achieved is based on the depth of discharge, environmental conditions, charge methods and maintenance procedures. Each battery chemistry behaves differently in terms of aging and wear.

Secondary batteries need some level of battery maintenance. Only if periodically discharged do nickel-based batteries provide the cost-effective and reliable service expected in a fleet environment. The deep discharge reverses the crystalline formation (memory) that occurs if a nickel-based battery is repeatedly charged on top of a residual charge. lithium and lead-based batteries have no memory and an occasional discharge is applied to verify performance. Battery maintenance is best done with a battery analyzer.
_________________________
Created: March 2003, Last edited: November 2005