Batteries in a Portable World 2^nd Ed.
       A Handbook on Rechargeable Batteries for Non-Engineers

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16.2 The Correct Battery for the Job

What is the best battery choice? The requirements differ between personal users and fleet operators. The personal user can choose batteries in various sizes and chemistries. Cost is a factor for many. If a smaller and less energy-dense battery is chosen, a spare battery may be carried to assure continued service.

The energy requirements are quite different with fleet operators. The equipment is matched with a battery designed to run for a specified number of hours per shift. A degradation factor to compensate for battery aging is taken into account. A reserve capacity is added to allow for unforeseen activities. Allowing an aging degradation factor of 20 percent and providing a reserve capacity of 20 percent will reduce the usable battery capacity from 100 percent to 60 percent in a worst-case scenario. Such a large percentage of reserve capacity may not always be practical but the equipment manufacturers should consider these safety factors when fitting the portable devices with a battery.

The best choice is not necessarily an oversized battery, but one that has sufficient safety margin and is well maintained. This is especially true of NiCd batteries. When adding large safety margins, the reserve capacity should be depleted once per month, if this is not done already through normal use.

The NiMH also needs exercising but less often. Cycling lithium-based batteries is only recommended for the purpose of measuring the performance.

Many battery users have a choice of switching from NiCd to NiMH to obtain longer runtimes and/or reduce weight. Regulatory bodies advise using less toxic alternatives because of the environment. But will the NiMH battery perform as well as the NiCd in industries that require repetitive deep discharges?

The NiMH will not match the cycle count of the NiCd chemistry. This lower life expectancy has serious consequences on applications that need one or several recharges per day. However, in a recent study on battery choice for heart defibrillators for emergency applications, it was observed that a battery may cycle far less than anticipated. Instead of the expected 200-cycle count after two years of use, less than 60 cycles had been delivered. Such service information is now available with the use of ‘smart’ batteries. With fewer cycles needed, the switch to lighter and higher energy-dense batteries becomes practical for these applications.

In most cases, NiMH can be used as a direct replacement for NiCd. When doing so, the charger must be checked. A NiMH charger can charge NiCd batteries, but a charger designed only for the NiCd battery should not be used to charge NiMH. Battery damage may result due to inaccurate full-charge detection and excessive trickle charge while in ready mode. If no alternative exists, the battery should be removed as soon as the green ready light appears. Battery temperature during charge should also be observed.

Remote control racecar enthusiasts rely heavily on high current capabilities and quick charging. NiMH batteries are now available that can handle very high discharge currents. This makes the battery ideally suited for competitions, because the weight and size of the battery can be reduced.

For most hobbyists, the NiCd remains the preferred choice. The reasons are: more consistent performance, longer cycle life and lower cost. NiCd needs replacement less often than NiMH. RC racing experts claim that NiMH is fragile, temperamental, and can be hurt easily. The storage of the NiMH battery is also erratic. Some cells are flat after a few weeks of storage; others still retain a charge.

High load currents have been problematic for NiMH. Discharge currents of 0.5C and higher rob the battery of cycle life. In comparison, NiCd delivers repetitive high load currents with minimal side effects.

The ultra-high capacity NiCd does not perform as well compared to the standard version in terms of load characteristics and endurance. Packing more active material makes the NiCd behave more like a NiMH battery.

The Li-ion battery has limited current handling capabilities. In many cases, it cannot be used as a replacement for such applications as defibrillators and power tools, not to mention RC racing. In addition, Li-ion requires a different charging system than the nickel-based battery chemistries.

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