The battery and the digital load

Isidor Buchmann
Cadex Electronics Inc.
isidor.buchmann@cadex.com
www.buchmann.ca

Revised October 2002

 With the move from analog to digital communications devices, new demands are placed on the battery. Unlike analog transceivers that draw a steady current, the digital radio loads the battery with short, heavy current spikes. The new Tetra system (Terrestrial Trunked Radio), which is being implemented in Europe, draws current pulses of up to 3 Amperes when transmitting. Other systems, such as Project 25 used in North America, have similar requirements.

One of the urgent requirements of a battery for two-way radios is low internal resistance. Measured in milliohms (mΩ), the resistance is the gatekeeper of the battery that, to a large extent, determines the talk time. The lower the resistance, the less restriction the battery encounters in delivering the needed power spikes. A high mΩ reading often triggers an early ‘low battery’ indication on a seemingly good battery because the available energy cannot be delivered fully and remains in the battery.

Cold and hot temperatures also play a critical roll in battery performance. Similar to us humans, the battery performs best at room temperature. Depending on battery chemistry, the performance at freezing temperatures is reduced by 20-50%.

In Figure 1 we examine analog and digital radio transceivers and compare peak power and peak current requirements, which the battery must be able to supply during transmission.

Figure 1: Peak power requirements for PMR and cell phone systems.
Moving from analog to digital communications devices reduces the overall energy need but increases the peak current during load pulses. The wattage varies with signal strength.

¹ Wattage varies with signal strength
² Based on 7.2V battery

What’s the best battery for a digital two-way radio?

Today’s battery research is heavily focused on lithium systems, so much so that one could assume that all future applications will be lithium based. How well do these new battery systems perform in the rather harsh environment of digital transceivers? In Figure 2 we examine the relationship between energy density (capacity) and internal resistance of Nickel-cadmium, Nickel-metal-hydride and Lithium-ion batteries. To address longevity, we also include ‘best cycle life’. It should be noted that periodic discharge cycles are required to achieve the indicated cycle life of nickel-based batteries.

Figure 2: Characteristics of NiCd, NiMH and Li-ion in terms of energy density, internal resistance, self-discharge and cycle life. Although most durable, NiCd requires a largest amount of maintenance by applying monthly discharge cycles.  

¹ Internal wiring, contacts and protection circuits are taken into account. Readings vary with cell rating, charge state and   
   number of cells connected in series.
²  The discharge is highest in the first 24h, then tapers off. Self-discharge increases with battery age and high temperature.
³  Cycle life is based on depth of discharge. Shallow discharges provide more cycles than deep discharges.
⁴  Cycle life is based on maintenance procedure. Failing to apply periodic full discharge cycles may reduce the cycle life
   by a factor of three.

Although Nicked-metal-hydride and Lithium-ion batteries have proven to perform well for cell phones and laptop computers, the track record for Nicked-metal-hydride on two-way radios is less encouraging. Shorter than expected service life prompt some users to switch back to Nickel-cadmium or experiment with the more expensive Lithium-ion batteries.

Nickel-cadmium, and to some extend Nickel-metal-hydride, are high maintenance batteries that must be fully discharged once per month to prevent ‘memory’. The word ‘memory’ is a misnomer because the modern Nickel-cadmium battery is mostly void of this phenomenon. Memory is better explained in terms of crystalline formation that occurs on the cell plates. If no maintenance is applied for a period of four months or more, the capacity drops by as much as one third. Discharging the battery to one volt per cell at this stage may restore lost performance. However, full restoration becomes more difficult the longer service is withheld.

It is not recommended to discharge a battery before each charge because such activity wears down the battery and shortens life. Neither is it advisable to leave a nickel-based battery in the charger for more than two days. When not in use, the battery should be put on a shelf and charged before use.

Aging is another concern. Lithium-ion frequently fails after two or three years, whether used or not. Keeping the battery at moderate temperatures extends the life. Manufacturers are constantly improving Lithium-ion and the age limitation may one day be solved.

Although the overall energy requirement of a digital transceiver is less than that of the analog equivalent, batteries for digital transceivers must be capable of delivering high current pulses, which are often several times higher than their own rating. Let’s look at battery rating as expressed in C-rates.

A 1C discharge of a battery that is rated 1000 mili-ampere-hours (mAh) equals 1000mA. In comparison, a 2C discharge of the same battery is 2000mA. A Tetra transceiver powered by a 1000mA battery, which draws 3-ampere pulses, loads the battery with a whopping 3C discharge pulse.

A 3C rate discharge is acceptable for a battery with very low internal resistance. However, aging batteries pose a challenge because the mΩ readings increase with usage and time.

Improved performance can be achieved by using a larger battery, also known as extended pack. Bulkier and heavier, the extended pack offers a typical rating of about 2000mAh or roughly double that of the standard pack. In terms of C-rate, the 3C discharge is reduced to 1.5C when using a 2000mAh instead of a 1000mAh battery.

Why do seemingly good batteries fail in digital equipment?

As part of ongoing research to find the best battery system for wireless communications devices, Cadex Electronics has examined NiCd, NiMH and Li-ion at various discharge rates. These batteries had been in use for a while and generated good capacity readings when tested with a battery analyzer that draws a mild load. When discharged at a higher rate, which is the case in a digital transceiver, the performance dropped sharply. The reason is high internal battery resistance on some packs. Nickel-cadmium shows a low 155 mΩ, Nickel-metal-hydride a high 778 mΩ and Lithium-ion a moderate 320 mΩ resistance reading. At capacities of 113%, 107% and 94% respectively, the batteries appear normal.

From the charts below we observe that the talk time is in direct relationship with the battery’s internal resistance. Nickel-cadmium performs best under the circumstances and provides a talk time of 120 minutes at 3C discharge. Nickel-metal-hydride performs only at 1C and fails at 3C. The Lithium-ion allows 50 minutes talk time at 3C. Although the batteries tested are used for GSM phones (Global System for Mobile Communications), there is a parallel in performance with the Tetra system.

Figure 3:  Discharge and resulting talk-time of a NiCd battery at 1C, 2C and 3C under the GSM load schedule. The battery tested has a capacity of 113%, the internal resistance is a low 155mΩ.

Figure 4: Discharge and resulting talk-time of a NiMH battery at 1C, 2C and 3C under the GSM load schedule. The battery tested has a capacity of 107%, the internal resistance is a high 778mΩ.

Figure 5: Discharge and resulting talk-time of a Li-ion battery at 1C, 2C and 3C under the GSM load schedule. The battery tested has a capacity of 94%, the internal resistance is 320mΩ..

How can battery performance be measured?

Measuring the battery performance by applying a full discharge is reliable but the service is slow. In addition, a discharge with a steady current does not assure battery performance under a digital load. During the last few years, instruments have been introduced that measure internal battery resistance. Although fast, resistance diagnosis often provides conflicting capacity readings and the predicted battery performance in unreliable.

Cadex Electronics has developed a method to measure the state-of-health (SoH) of a battery through a method called QuickTest™. The service lasts 3 minutes and tests batteries for two-way radios, cell phones, laptops, scanners, medical equipment, video cameras and more.

QuickTest™ uses a unique inference algorithm to fuse data from a number of different variables. It evaluates capacity, internal resistance, self-discharge, charge acceptance, discharge capabilities and mobility of electrolyte. All of these variables are combined into one number called SoH.

The QuickTest™ program is built into the Cadex C7200 and C7400 battery analyzers and services nickel, lithium and lead-based batteries. The analyzers are user-programmable and also perform prime, recondition, fast-charge, life-test and boost functions. Figure 6 illustrates the four-station Cadex C7400 battery analyzer with QuickTest™.

QuickTest™ makes use of battery specific matrices that are obtained using the analyzer’s trend learning process. The ability to learn allows adapting to new batteries in the field.

The matrices are stored in custom-made battery adapters, which automatically configure the analyzer to the correct battery setting. The battery adapters commonly include the QuickTest™ matrix at time of purchase. If missing, the user can obtain the matrix by scanning a known good battery on the analyzer’s Learn program. The codes can be copied to other adapters, erased and re-entered. The required charge level to perform QuickTest™ is 20-90%. If outside this range, the analyzer automatically applies a brief charge or discharge.

Summary

Portable communications devices are only as reliable as the battery. To this day, the battery remains the renegade, especially after the pack has been in service for a while. No alternative technology is on the horizon to replace the somewhat temperamental electro-chemical battery in use today.

Although batteries have been improving, the immediate emphasis should be on battery maintenance. This is in form of exercising batteries to prolong life, reconditioning those that have become weak and retiring packs that are unserviceable. Only with a properly managed maintenance program can a battery fleet remain reliable and the operating costs reduced. While battery maintenance for nickel-based batteries consists of periodic discharges to eliminate ‘memory’, maintenance-free Lithium-ion packs benefit from quick-test methods.

About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC. Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.

Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.