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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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