A common difficulty with portable equipment is the gradual decline in battery performance after the first year of service. Although fully charged, the battery eventually regresses to a point where the available energy is less than half of its original capacity.
Rechargeable batteries are known to cause more concern, grief and frustration than any other component of a portable device. Given its relatively short lifespan, the battery is also one of the most expensive and least reliable parts. In many ways, a battery exhibits human-like characteristics: it needs good nutrition, prefers moderate room temperature and with the nickel-based system, requires regular exercise to prevent the phenomenon called 'memory'.
How to restore and prolong nickel-based batteries
When nickel-based batteries are mentioned, the word 'memory' comes to mind. Memory was originally derived from 'cyclic memory', meaning that a nickel-cadmium (NiCd) battery could remember how much energy was required and would provide similar amounts on subsequent discharges. Improvements in battery technology have virtually eliminated this phenomenon. The modern term of 'memory' is a crystalline formation that robs the battery of its capacity. Applying one or several full discharge cycles can commonly reverse this effect.
The active cadmium material of a NiCd battery is present in finely-divided crystals. In a good cell, these crystals remain small, obtaining maximum surface area. Memory causes the crystals to grow, reducing the surface area. In advanced stages, the sharp edges of the crystals may penetrate the separator, initiating high self-discharge or an electrical short.
The effect of crystalline formation is most visible if a NiCd battery is left in the charger for days, or if repeatedly recharged without a periodic full discharge. Since most applications do not use up all energy before recharge, a periodic discharge to 1 V/cell (known as exercise) is essential to prevent memory.
All NiCd batteries in regular use and on standby mode (sitting in a charger for operational readiness) should be exercised once per month. Between these monthly exercise cycles, no further service is needed and the battery can be used with any desired user pattern without memory concern.
If no exercise is applied to a NiCd for three months or more, the crystals ingrain themselves, making them more difficult to break up. In such a case, exercise may no longer be effective in restoring a battery and reconditioning is required. Recondition is a secondary discharge that slowly removes the remaining battery energy by draining the cells to virtually zero volts. NiCd batteries can tolerate a small amount of cell reversal. During deep discharge, caution must be applied to stay within the allowable current limit to minimise cell reversal.
When nickel-metal hydride (NiMH) was introduced in the early 1990s, there was much publicity about its memory-free status. Today we know that NiMH also suffers from memory, but to a lesser extent than NiCd. No scientific research is available that specifies optimal maintenance. Applying a full discharge once every three months appears right. Because of the shorter service life, over-exercising of NiMH is not recommended.
Simple guidelines
* Do not leave a nickel-based battery in a charger for more than a day with the 'ready' light on. It is better to remove the battery from the charger and applying a charge before use.
* Apply periodic discharge cycles; running the battery down in the equipment may do this also.
* It is not necessary to discharge the battery before each charge. This would put undue stress on the battery.
* Avoid elevated temperature - the battery should cool off and remain at ambient temperature after full-charge.
* Use high quality chargers.
The effect of zapping
Remote control (RC) racing enthusiasts have experimented with all imaginable methods to maximise battery performance. One technique that seems to work is zapping the cells with a very high pulse current. Zapping is said to increase the cell voltage by 20 to 40 mV under a 30 A load. According to experts, the voltage gain is stable; only a small drop is observed with usage and age.
During the race, the motor draws 30 A from a 7,2 V battery. This calculates to over 200 W or close to a quarter HP of power. The race lasts about four minutes.
According to experts, zapping works best with NiCd cells. NiMH cells have been tried but the results are inconclusive. Zapping is done with a 47 000 µF capacitor charged to 90 V. Best results are achieved if the battery is cycled twice after treatment, then zapped again. Once in service, zapping no longer improves the cell's performance. Neither does zapping regenerate a cell that has become weak.
Companies specialising in zapping batteries use top quality Japanese-made NiCd cells. The cells are normally sub-C sized and are handpicked at the factory. Specially labelled, the cells arrive in discharged state with open cell voltages of 1,11 to 1,12 V. If below 1,06 V, the cell is suspect and zapping does not work well.
There are no apparent side-effects to zapping but the battery manufacturers remain non-committal. No scientific explanation is available and only little is known on the longevity of the cells after treatment.
How to prolong lithium-based batteries
Battery research is focusing heavily on lithium chemistries, so much so that one could presume that all future batteries will be lithium systems. In many ways, the Lithium-ion (Li-ion) is superior to nickel and lead-based chemistries.
A Li-ion battery provides 300 to 500 discharge/charge cycles or two to three years of service from the time of manufacturing. The loss of battery capacity occurs gradually and often without the knowledge of the user. There are no remedies to restore Li-ion batteries when worn out.
Li-ion prefers a partial, rather than a full discharge. Avoid depleting the battery fully too frequently. Instead, charge more often or use a larger battery. There is no memory to worry about.
Although lithium-ion is memory-free in terms of performance deterioration, engineers often refer to 'digital memory' on batteries with fuel gauges: small discharges with subsequent re-charges do not allow the calibration needed to track the chemical battery's state-of-charge with the fuel gauge. A forced full discharge with recharge every 30 charges, or so, will correct this problem. Letting the battery run down in the equipment to the cut-off point will do this. If not done, the fuel gauge becomes increasingly less accurate.
The aspect of ageing is an issue that is often ignored. A time clock starts ticking as soon as the battery leaves the factory. The electrolyte slowly 'eats up' the positive plate, causing the internal resistance to increase. Eventually, the cell resistance reaches a point where the battery can no longer deliver energy, although the battery may still contain charge.
The speed at which Li-ion ages, is governed by temperature and state-of-charge. The most harmful combination is full charge and high temperature. If possible, store the battery in a cool place at a 40% charge level. Table 1 illustrates the capacity loss as a function temperature and charge level.
Simple guidelines
* Avoid full frequent discharges; recharge Li-ion more often. There is no memory to worry about.
* Although memory-free, apply a deliberate full discharge once every 30 days on batteries with fuel gauge to calibrate the battery. If not done, the fuel gauge will become increasingly less accurate.
* Keep the Li-ion battery cool. Never freeze the battery. Avoid a hot car.
* For prolonged storage, keep the battery at 40% charge level.
* Avoid purchasing spare Li-ion batteries for later use. Observe manufacturing date. Do not buy old stock, even if sold at clearance prices.
How to restore and prolong lead acid batteries
The sealed lead acid battery, known as valve regulated lead acid (VRLA), is designed with a low over-voltage potential. This is done to prevent water depletion. Consequently, these systems never get fully charged and some sulphation will develop over time.
Finding the ideal charge voltage limit is critical. Any voltage level is a compromise. A high voltage limit produces good battery performance but shortens the service life due to grid corrosion on the positive plate. The corrosion is permanent. A low voltage protects the battery and allows charging under a higher temperature but is subject to sulphation on the negative plate.
Restoring a sulphated battery is difficult and time-consuming. One method that provides reasonably good results is applying a charge on top of a charge. This is done by fully charging a battery, then removing it for a 24 to 48 hour rest period and applying a charge again. This process is repeated several times and the capacity is checked again with a full discharge. The lead acid battery is able to accept some overcharge but too much causes corrosion and loss of electrolyte.
Applying an over-voltage charge of up to 2,50 V/cell for one to two hours can also reverse sulphation. During treatment, the battery must be kept cool and careful observation is needed. Prevent venting. Most plastic VRLA batteries vent at 34 kPa (5 psi). Not only do escaping gases deplete the electrolyte, they are highly flammable.
Sealed lead acid batteries are also available in cylindrical form. The Cyclon by Hawker resembles an oversized D sized cell. If sulphated, applying an elevated charge voltage commonly reactivates the cell. Initially, the cell voltage may rise to 5 V, absorbing only a small amount of current. In about two hours, the small charging current converts the large sulphate crystals back into active material. The internal cell resistance decreases and the charge voltage normalises. When within 2,10 V to 2,40 V, the cell starts to accept normal charge. If the sulphation is advanced, this remedy does not work and the cell needs replacing.
When applying over-voltage, current-limiting must be applied. Always set the limit to the lowest practical setting on the power supply and observe the battery voltage and temperature during charge. Improving the capacity of an older lead acid battery by cycling is mostly in vain. Such a battery may simply be worn out and cycling wears it further down. The lead acid battery is not affected by memory.
VRLA batteries are commonly rated at a 20-hour discharge. Even at such a slow rate, a capacity of 100% is difficult to obtain. For practical reasons, most battery analysers use a 5-hour discharge when servicing these batteries. This typically produces 80 to 90% of the rated capacity. VRLA cells are normally overrated and manufacturers are aware of this practice.
Simple guidelines
* Always store lead-acid charged. Never let the open cell voltage drop below 2,10 V. Apply a topping charge every six months or when recommended.
* Avoid repeated deep discharges. Charge more often or use a larger battery.
* Prevent sulphation and grid corrosion by choosing the correct charge and float voltages.
Battery recovery rate
Restoring batteries by applying controlled discharge/charge cycles varies with chemistry type, cycle count, maintenance practices and age of the battery. The best results are achieved with NiCd. Typically 50 to 70% of discarded NiCd batteries can be restored when using the exercise and recondition methods of a Cadex battery analyser or equivalent.
Not all batteries respond well to exercise and recondition. An older battery may show low and inconsistent capacity readings. Another battery may get worse with each advancing cycle. An analogy can be made to a frail old man for whom exercise is harmful. Such a condition suggests battery replacement.
Some older NiCd batteries recover to near original capacity when serviced. Caution should be applied when rehiring these old-timers because of possible high self-discharge. If in doubt, measure the self-discharge. A 10% self-discharge in the first 24 hours after charging is normal. Discard the battery if the self-discharge approaches 30%.
The recovery rate of NiMH is about 40%. The lower yield is in part due the reduced cycle life. Some batteries may exhibit irreversible heat damage suffered by incorrect charging. Elevated operating and storage temperatures also contribute to permanent capacity loss.
Lithium-based batteries have a defined age limit. Once the anticipated cycles have been delivered, no method exists to restore them. The main reason for failure is high internal resistance caused by oxidation. Operating the battery at elevated temperatures will momentarily improve the performance. However, the high internal resistance will revert to its former state when the temperature normalises.
Many Li-ion batteries for cellphones are being discarded under the warranty return policy. Dealers have confirmed that 80 to 90% of these batteries can be repaired with a battery analyser. Because no equipment is on hand, the batteries are often sent back to the manufacturers or are discarded without attempting to restore them.
Some Li-ion batteries fall asleep if discharged below 2,5 V/cell. The internal safety circuit opens and the charger can no longer service the battery. Advanced battery analysers feature a boost function to activate the protection circuit enabling a recharge. If the cell voltage has fallen below 1,5 V/cell and has remained in that state for a few days, a recharge should be avoided because of safety concerns.
The recovery rate for lead acid batteries is a low 15%. The reasons for the low yield may be due to incorrect charging methods, high cycle count, operating at elevated temperatures and old age.
The question is often asked whether a restored battery will work as well as a new one. The breakdown of the crystalline formation on NiCd can be considered a full restoration. However, the battery will revert back to its former state if the required maintenance is denied. If the separator is damaged by excess heat or is marred by uncontrolled crystalline formation, that part of the battery will not improve.
Battery test equipment
Battery analysers have become an important tool to test, exercise and restore batteries. The Cadex 7400 (Figure 1), for example, accommodates NiCd, NiMH, Li-ion/polymer and lead acid batteries and is programmable to a wide range of voltage and current settings. A quick-test program measures battery state-of-health in three minutes and a boost program reactivates dead batteries. There is even a program to measure the battery self-discharge.
Battery analysers are capable of solving a multitude of battery problems. Regular exercise doubles the service life of NiCd and reduces replacement costs. Unserviceable batteries are weeded out before they cause problems. Most importantly, battery analysers improve battery reliability, an issue that is of significance in critical mission applications.
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