Will secondary batteries replace primaries?
Figure 1: Energy comparison of rechargeable and non-rechargeable batteries
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. The lithium primary used in cameras holds almost double the energy of a Lithium-ion battery of same size. Figure 1 illustrates the gravimetric energy density of Lead-acid, Nickel-metal-hydride, Lithium-ion, Alkaline and lithium batteries.
Based on the high energy density of the primary cells, efforts were made to make Alkaline and lithium rechargeable. The attempt has had limited success. The reusable Alkaline only holds half the energy of Lithium-ion and each recharge reduces the capacity further. (Refer to “Will the reusable Alkaline battery have a future?” See BD 78-14). The rechargeable lithium provides high capacity but is unsafe for commercial use.
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 terminal Voltage but the result is inaccurate. 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 provide 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.
Aging is subject to 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 only, whether used or not. 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.