Chapter 16: Practical Battery Tips
Batteries seem to have a mind of their own. Their stubborn and unpredictable behavior has left many battery users in awkward situations. In fact, the British Army could have lost the Falkland War in 1982 because of uncooperative batteries. The army assumed that a battery would always follow rigid military specifications. Not so. When the order was given to launch the portable missiles, nothing happened and the missiles did not fly that day. Such battery-induced letdowns happen on a daily basis. Some are simply a nuisance, others have serious consequences.
In this section we examine what the user can reasonably expect from a battery. We learn how to cope with the many moods of a battery and how to come to terms with its limitations.
Personal Field Observations
While working with General Electric, I had the opportunity to examine the behavior of many NiCd batteries for two-way radios. I noticed a trend with these batteries that was unique to NiCd. These particularities repeated themselves in various other applications.
A certain organization continually experienced NiCd battery failure after a relatively short service time. Although the batteries performed at 100 percent when new, their capacity dropped to 20 percent and below within one year. We discovered that their two-way radios were under-utilized; yet the batteries received a full recharge after each short field use.
After replacing the batteries, we advised the organization to exercise the new batteries once per month by discharging them to one-volt-per cell with a subsequent recharge. The first exercise took place after the batteries had been in service for four months. At that stage, we were anxious to find out how much the batteries had deteriorated. Here is what we found:
On half of the batteries tested, the capacity loss was between 25 to 30 percent; on the other half, the losses were around 10 to 20 percent. With exercise — and some needed recondition cycles — all batteries were fully restored. Had maintenance been omitted for much longer, the probability of a full recovery would have been jeopardized.
On another occasion, I noticed that two-way radios used by construction workers experienced fewer NiCd battery problems than those used by security guards. The construction workers often did not turn off the radios when they put down their hammers. As a result, the batteries got their exercise and kept performing well until they fell apart from old age. In many cases the batteries were held together with electrician’s tape.
In comparison, the security guards pampered their batteries to death by giving them light duty and plenty of recharge. These batteries still looked new when they had to be discarded after only 12 months of service. Because of the advanced state of memory, recondition was no longer effective to restore these batteries.
On a further application, I studied the performance of a two-way radio that was available with batteries of different capacities. It soon was apparent that the smaller battery lasted much longer, whereas the larger packs needed replacing more often. The small battery had to work harder and received more exercise during a daily routine.
Equipment manufacturers are aware of the weak link — the battery. For a more reliable energy source, higher capacity batteries are recommended. Not only are oversized batteries bulky, heavy and expensive, they hold more residual charge prior to recharge than smaller units. If the residual energy is never fully consumed before a recharge, and no exercise is applied, the nickel-based battery will eventually lose its ability to hold charge due to memory.
On the lithium and lead-base systems, a slightly oversized battery offers an advantage because the pack is less stressed on deep discharges. The battery does not need to be discharged as low for the given application. A high residual charge before recharge is a benefit rather than a disadvantage for these chemistries.
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.
Battery Analyzers for Critical Missions
Occasionally, a customer will call Cadex because their battery analyzer appears faulty. The complaint: the battery no longer indicates correct capacity readings. In most cases, the customer has just purchased new batteries. When testing these new packs, the capacities read 50 to 70 percent. The customer assumes that, “Naturally, if two or more of these brand new batteries show low readings, it can only be the analyzer’s fault.”
Battery analyzers play a critical role in identifying non-performing batteries, new or old. Conventional wisdom says that a new battery always performs flawlessly. Yet many users realize that a fresh battery may not always meet the manufacturer's specifications. Weak batteries can be identified and primed. If the capacity does not improve, the packs can often be returned to the vendor for warranty replacement. Whole batches of new batteries have been sent back because of unacceptable performance. Had these batteries been released without prior inspection, the whole system would have been jeopardized, resulting in unpredictable performance and frequent down time.
In addition to getting new batteries field-ready, battery analyzers perform the important function of weeding out the deadwood in a battery fleet. Weak batteries can often hide among their peers. However, when the system is put to the test in an emergency, these non-performers become a real nuisance.
Organizations tend to postpone battery maintenance until a crisis situation develops. One fire brigade using two-way radios experienced chronic communication problems, especially during emergency calls which lasted longer than two hours. Although their radios functioned in the receive mode, they were not able to transmit and firefighters were left unaware that their calls did not get through.
The fire brigade acquired a Cadex battery analyzer and all batteries were serviced through exercise and recondition methods. Those batteries that did not recover to a preset target capacity were replaced.
Shortly thereafter, the firefighters were summoned to a ten-hour call that demanded heavy radio traffic. To their astonishment, none of the two-way radios failed. The success of this flawless operation was credited to the excellent performance of their batteries. The following day, the Captain of the fire brigade personally contacted the manufacturer of the battery analyzer and enthusiastically endorsed the use of the device.
Batteries placed on prolonged standby commonly fail. Such was the case when a Cadex representative was allowed to view the State Emergency Management Facility of a large US city. In the fortified underground bunker, over one thousand batteries were kept in chargers. The green lights glowed, indicating that the batteries where ready at a moment’s notice. The officer in charge stood tall and confidently said, “We are prepared for any emergency”.
The representative then asked the officer to hand over a battery from the charger to check the state-of-health (SoH). Within seconds, the battery analyzer detected a fail condition. In an effort to make good, the officer grabbed another battery from the charger bank but, it failed too. Subsequent batteries tested also failed.
Scenarios such as these are common but such flaws do not get rectified quickly. Political hurdles and lack of funding are often to blame. In the meantime, all the officer can do is pray that no emergency occurs.
Eventually, a new set of batteries is installed and the system returns to full operational readiness. However, the same scenario will reoccur, unless a program is implemented to exercise the batteries on a regular basis. Advanced battery analyzers, such as the Cadex 7000 Series, apply a conditioning discharge every 30 days to prevent the memory phenomenon on nickel-based batteries. [16.1]
The military also relies heavily on batteries. Defense organizations take great pride in employing the highest quality and best performing equipment. When it comes to rechargeable batteries, however, there are exceptions. The battery often escapes the scrutiny of a full military inspection and only its visual appearance is checked. Maintenance requirements are frequently ignored. Little effort is made in keeping track of the battery’s state of health, cycle count and age. Eventually, weak batteries get mixed with new ones and the system becomes unreliable. This results in soldiers carrying rocks instead of batteries. A battery analyzer, when used correctly, keeps deadwood out of the arsenal.
The task of keeping a battery fleet at an acceptable capacity level has been simplified with battery analyzers that offer target capacity selection. This novel feature works on the basis that all batteries must pass a user-defined performance test. Batteries that fall short are restored with the recondition cycle. If they fail to recover, the packs are replaced.
The target capacity setting of a battery analyzer can be compared to a student entry-exam for college. Assuming that the passing mark is 80 percent, the students who do not obtain that level are given the opportunity to take a refresher course and are thereafter permitted to rewrite the exam. In our analogy, the refresher course is the recondition cycle that is applied to nickel-based batteries. If the passing mark is set to 90 percent, for example, fewer but higher qualified students are admitted.
A practical target capacity setting for batteries in public safety is 80 percent. Increasing the capacity requirement to 90 percent will provide an extra 10 percentage points of available energy. However, higher settings will yield fewer batteries since more packs will fail as they age.
Many organizations allocate the top performing batteries for critical applications and assign the lower performers for lighter duties. This makes full use of the available resources without affecting reliability.
Some battery analyzers display both the reserve capacity (motor fuel left in the tank before refill) and the full-charge capacity (full tank) of the batteries serviced. The user is then able to calculate how much energy was consumed during the day by subtracting the reserve from the full-charge capacity. To ensure a reasonable safety margin after a routine day, the reserve capacity should be about 20 percent. If less reserve capacity is available, the target capacity should be set higher. By allowing reasonable reserve capacity, unexpected downtime in an emergency or on extra-strenuous field activities can be eliminated.