Choosing a battery that lasts

A battery is a perishable product that begins to deteriorate from the moment it leaves the factory. Similar to a spring under voltage, a battery aims to reverse that low denominator. The speed of wear is subject to the level of discharge, environmental conditions, loading and maintenance procedures or the lack of this. Each chemical battery is affected differently by wear and conditions of use.

In Figure 1 we compare three battery systems for broadcasting applications and examine each chemical composition in terms of energy density, internal resistance, self-discharge and life cycle. To simplify the comparison of the internal resistance we use a 3.6 volt battery.

As part of a research program to find the optimal battery system for broadcasting applications, Cadex has run a life cycle test on Nickel-Cadmium (NiCd), Nickel Hydrate (NiMH) and Lithium-Ion (Li-ion) batteries. All tests were performed by the Cadex C700 battery analyzer at Cadex Electronics Inc.'s test laboratories in Vancouver, Canada.

The batteries received an initial full charge and then endured a continuous regime of discharge and charge cycles. Internal resistance was measured by Cadex's proprietary method, OhmTest, and self-discharge was obtained from a reading performed from time to time, reading the capacity lost over a 48-hour period. The test housed 53 batteries of different models and chemical composition.

When a laboratory test is performed, it can be noticed that the performance in a protected environment is greater than that which can be presented in the field. Elements of pressure and inconsistency that may occur every day may not be properly simulated in the laboratory.

It has been observed that striving a battery to achieve maximum capacity, charging it quickly, generally brings more difficulties, reduces the ability to achieve high loads and shortens the life cycle. This is especially true in Nickel-based batteries.

In terms of lifecycle, the standard NiCd battery is the most durable. In Figure 2 we examine the capacity, internal resistance and self-discharge of a 7.2 V, 900 mA NiCd battery with standard cells. Due to the time of compulsion, the test was completed after 2200 cycles. During this period, the capacity remained firm, the internal resistance was at 75 milliohmnias and the self-discharge was stable. This battery received an "A" grade for a performance close to perfection.

Readings of an ultra-high-capacity NiCd are less favorable, but hold up better than others of another chemical class, speaking in terms of duration. Although above 60% in energy density than the standard NiCd version, Figure 3 shows a firm detachment of capacity during the given 2000 cycles. At the same time, the internal resistance rises slightly. The most serious degradation is the increase in self-discharge after 1000 cycles. This deficiency manifests itself in short periods because some energy is consumed by the battery even if it is not in use.

Figure 4 examines the NiMH battery; a battery that offers high energy density at a low cost. We observed good performance at the beginning but after the 300 cycle mark, the current performance begins to decline rapidly. The rate of increase in internal resistance and self-discharge can be observed after counting the 700 cycle. This shows that NiHM batteries may not be used for applications that require long life cycles. However, many users accept a short-lived service in exchange for a long period of charging.

The lithium (Li-ion) battery offers advantages that neither the NiCd nor the NiMH battery can offer. In Figure 4 we examine the capacity and internal resistance of a typical Li-ion battery. A moderate and predictable drop in capacity is observed over 1000 cycles and the internal resistance is slightly increased. Due to the low readings, self-download has been omitted in this test.

On the downside, the Li-ion is subject to wear even if it is not in use. The deterioration capacity is considerable after one year. After two years, the battery frequency runs out. This is why it is not recommended to keep a Li-ion battery in storage for a long time. Instead, the battery could be rotated as a perishable food. The buyer should look at the date of manufacture when he is making the change of a Lithium (Li-ion) battery. Unfortunately, this information is often encoded in an encrypted serial number only available to the manufacturer.