Miscellaneous/Ask Isidor 031217
Isidor Buchmann, 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. He can be reached by E Mail at: Isidor.Buchmann@cadex.com
(April 2004) Q: How Can Lead-acid batteries be restored and have prolonged life?
The sealed Lead-acid battery is designed with a low over-Voltage potential to prohibit the battery from reaching its gas-generating state during charge. This prevents water depletion of the sealed system. Consequently, these batteries will never get fully charged and some sulfation will develop over time.
Finding the ideal charge Voltage threshold is critical and any level is a compromise. A Voltage limit above 2.40 Volts per cell produces good battery performance but shortens the service life due to grid corrosion on the positive plate. The corrosion is permanent. A Voltage below the 2.40V/cell threshold strains the battery less but the capacity is low and sulfation sets in over time on the negative plate.
Driven by diverse applications, two sealed Lead-acid types have emerged. They are the sealed lead-acid (SLA), and the valve regulated Lead-acid (VRLA). Technically, both batteries are the same. Engineers may argue that the word ‘sealed Lead-acid’ is a misnomer because no Lead-acid battery can be totally sealed.
Programmable battery analyzer services 36V packs
The Cadex C7400ER is the most powerful battery analyzer in its series. Each of the four stations can be programmed to charge and discharge at 6A, adjustable in 25mA increments. The analyzer accommodates nickel, lead and lithium-based batteries with Voltages from 1.2-36V (28.8 for nickel-based). Batteries interface with custom and universal SnapLock™ adapters that automatically configure the analyzer to the correct settings. The optional BatteryShop™ software transfers operation to a PC and allows 120 analyzers fully extended. +
The SLA has a typical capacity range of 0.2Ah to 30Ah and powers personal UPS units, local emergency lighting and wheelchairs. The VRLA battery is used for large stationary applications for power backup. We are looking at methods to restore and prolong these two battery systems separately.
The sealed Lead-acid (SLA)
SLA batteries with mild sulfation can be restored but the work is time consuming and the results are mixed. Reasonably good results are achieved by applying a charge on top of a charge. This is done by fully charging an SLA battery, then removing it for a 24 to 48 hour rest period and applying a charge again The process is repeated several times and the capacity is checked with a final full discharge and recharge.
Another method of improving performance is by applying an equalizing charge, in which the charge Voltage threshold is increased by about 100mV, typically from 2.40V to 2.50V. This procedure should last no longer than one to two hours and must be carried out at moderate room temperature. A careless equalize charge could cause the cells to heat up and induce venting due to excessive pressure. Observe the battery during the service.
The cylindrical SLA, made by Hawker and sold under the Cyclone name, requires slightly higher Voltages to reverse sulfation. An adjustable power supply works best for the service. Set the current limit to the lowest practical setting and observe the battery Voltage and temperature during charge. Initially, the cell Voltage may rise to 5V, absorbing only a small amount of current. In about two hours, the small charging current converts the large sulfate crystals back into active material. The internal cell resistance decreases and the cell starts to clamp the Voltage. At around 2.30V, the cell accepts charge. If the sulfation is advanced, this remedy does not work and the cell needs replacing.
Sealed Lead-acid batteries are commonly rated at a 20-hour discharge. Even at such a slow rate, a capacity of 100% is difficult to achieve. For practical reasons, most battery analyzers use a 5-hour discharge when servicing these batteries. This produces 80% to 90% of the rated capacity. SLA batteries are normally overrated and manufacturers are aware of this practice.
Cycling an SLA on a battery analyzer may provide capacity readings that decrease with each additional cycle. A battery may start off at a marginal 88%, then go to 86%, 84% and 83%. This phenomenon can be corrected by increasing the charge Voltage threshold from 2.40V to 2.45V and perhaps even 2.50V. Always consider the manufacturer’s recommended settings. Cyclone batteries require slightly higher Voltage settings than the plastic version.
Avoid setting the charge Voltage threshold too high. In an extreme case, the limiting Voltage may never be reached, especially when charging at elevated temperatures. The battery continues charging at full current and the pack gets hot. Heat lowers the battery Voltage and works against a further Voltage raise. If no temperature sensing is available to terminate the charge, a thermal runaway can be the result.
The recovery rate of SLA batteries is a low 15%. Other than reverse sulfation, there is little one can do to improve SLA. Because the SLA has a relatively short cycle life, many fail due to wear-out.
Valve regulated Lead-acid (VRLA)
The charge Voltage setting on VRLA is generally lower than SLA. Heat is a killer of VRLA. Many stationary batteries are kept in shelters with no air conditioning. Every 8°C (15°F) rise in temperature cuts the battery life in half. A VRLA battery, which would last for 10 years at 25°C (77°F), will only be good for 5 years if operated at 33°C (95°F). Once damaged by heat, no remedy exists to improve capacity.
The cell Voltages of a VRLA battery must be harmonized as close as possible. Applying an equalizing charge every 6 months brings all cells to similar Voltage levels. This is done by increasing the cell voltage to 2.50V/cell for about 2 hours. During the service, the battery must be kept cool and careful observation is needed. Limit cell venting. Most VRLA vent at 0.3 Bar (5 psi). Not only does escaping hydrogen deplete the electrolyte, it is highly flammable.
Water permeation, or loss of electrolyte, is a concern with sealed Lead-acid batteries. Adding water may help to restore capacity but a long-term fix is uncertain. The battery becomes unreliable and requires high maintenance.
Always store Lead-acid in a charged condition. Never let the open cell Voltage drop much below 2.10V. Apply a topping charge every six months or when recommended.
· Avoid repeated deep discharges. Charge more often or use a larger battery.
Prevent sulfation and grid corrosion by choosing the correct charge and float Voltages.
Avoid operating Lead-acid at elevated ambient temperatures.
(October ,2003)Recycling batteries
Modern batteries are often promoted on their environmental qualities. Lithium-based batteries fall into this category. While Nickel-cadmium presents an environmental problem on careless disposal, this chemistry continues to hold an important position among rechargeable batteries. Power tools are almost exclusively powered by Nickel-cadmium. Lead-acid batteries continue to service designated market niches and these batteries also need to be disposed of in a proper manner. Lithium-ion would simply be too fragile to replace many of these older, but environmentally unfriendly, battery chemistries.
Our quest for portability and mobility is steadily growing, so is the demand for batteries. Where will the mountains of batteries go when spent? The answer is recycling.
The lead acid battery has led the way in recycling. The automotive industry should be given credit in organizing ways to dispose of spent car batteries. In the USA, 98% of all lead acid batteries are recycled. In comparison, only one in six households in North America recycle batteries.
Careless disposal of Nickel-cadmium is hazardous to the environment. If used in landfills, the cadmium will eventually dissolve itself and the toxic substance can seep into the water supply, causing serious health problems. Our oceans are already beginning to show traces of cadmium (along with aspirin, penicillin and antidepressants) but the source of the contamination is unknown.
Although Nickel-metal-hydride is considered environmentally friendly, this chemistry is also being recycled. The main derivative is nickel, which is considered semi-toxic. Nickel-metal-hydride also contains electrolyte that, in large amounts, is hazardous. If no disposal service is available in an area, individual Nickel-metal-hydride batteries can be discarded with other household wastes. If ten or more batteries are accumulated, the user should consider disposing of these packs in a secure waste landfill.
Lithium (metal) batteries contain no toxic metals, however, there is the possibility of fire if the metallic lithium is exposed to moisture while the cells are corroding. Most lithium batteries are non-rechargeable and are used in cameras, hearing aids and defense applications. For proper disposal, the batteries must first be fully discharged to consume the metallic lithium content.