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Laptop Battery & Power Supply news you can use – battery maintenance and battery life

June 7th, 2011 at 1:46 am

Lead-based Batteries

Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable battery for commercial use. Today, the flooded lead acid is common in automobiles, golf cars, forklifts and uninterruptible power supplies (UPS).

The first lead acids were flooded, and during the mid 1970s, the sealed or maintenance-free version appeared. The liquid electrolyte is transformed into moistened separators and the sealed enclosure fitted with safety valves to control venting of gas during charge and discharge. The sealed lead acid has the advantage that it can operate in any position.

Maintenance-free Lead Acid

Driven by the advantage of being maintenance-free, two types emerged: the sealed lead acid (SLA), also known as gel cell, and the valve-regulated lead acid (VRLA). The systems are similar and no scientific convention exists as to what constitutes an SLA and a VRLA. (Engineers may argue that the term “sealed lead acid” is a misnomer because no lead acid battery can be totally sealed.)

We identify SLA as having a capacity range up to 30Ah. Typical uses are personal UPS for PC backup, small emergency lighting units, ventilators for healthcare patients and wheelchairs. Known for its economical price, dependable service and minimal maintenance, the SLA is the preferred choice for biomedical and healthcare instruments in hospitals and retirement homes. The VRLA battery is larger and mostly used for stationary applications. Capacities range from 30Ah to several thousand Ah and make up larger UPS systems. These may be cellular repeater towers, cable distribution centers, Internet hubs and utilities, as well as power backup for banks, hospitals, airports and military installations.

Unlike the flooded lead acid, both the SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging causes gassing and water depletion. Consequently, the SLA and VRLA can never be charged to their full potential.

Applying the right voltage limit when charging lead acid systems is critical and any voltage level is a compromise. A low voltage may shelter the battery but this causes poor performance and a buildup of sulfation on the negative plate. A high voltage limit improves performance but it promotes grid corrosion on the positive plate. The corrosion is permanent and cannot be reversed. Temperature changes the voltage threshold.

Lead acid does not lend itself to fast charging and a fully saturated charge requires 14 to16 hours. The battery must always be stored at full state-of-charge. Dwelling on low charge causes sulfation, a condition that robs the battery of performance. The addition of carbon on the negative electrode helps to alleviate some of these problems but lowers the specific energy.

Lead acid is not subject to memory, but correct charge and float voltages are important to achieve a long life. Charge retention is best among rechargeable batteries, and while NiCd loses approximately 40 percent of its stored energy in three months, lead acid self-discharges the same amount in one year.

Lead acid batteries are inexpensive on cost-per-watt but are less suitable for repeated deep cycling. A full discharge causes strain and each discharge/charge cycle permanently robs the battery of a small amount of capacity. This loss is tiny while the battery is in good operating condition; however, the fading becomes more acute once the performance drops below 80 percent of its nominal capacity. This wear-down characteristic also applies to all batteries in various degrees.

Depending on the depth of discharge and operating temperature, lead acid for deep-cycle applications provides 200 to 300 discharge/charge cycles. The primary reasons for its relatively short cycle life are grid corrosion on the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at higher operating temperatures and are permanent.

The optimum operating temperature for a VRLA battery is 25°C (77°F). As a guideline, every 8°C (15°F) rise above this temperature cuts battery life in half. A lead acid that would last for 10 years at 25°C would only be good for five years if continuously operated at 33°C (95°F). The same battery would endure a little more than one year at a temperature of 42°C (107°F).

Lead acid batteries are rated at a 5-hour (0.2C) and 20-hour (0.05C) discharge, and the battery performs best when discharged slowly. The capacity readings are notably higher at a slow discharge than with a fast rate. Lead acid can, however, deliver high pulse currents of several C if done for only a few seconds. This makes the lead acid well suited as a starter battery, also known asstarter-light-ignition (SLI). The high lead content and the sulfuric acid make lead acid environmentally unfriendly. The following section looks at the different architectures and explains why one battery type does not fit all.

Starter and Deep-cycle Batteries

The starter battery is designed to crank an engine with a momentary high power burst; the deep-cycle battery, on the other hand, is built to provide continuous power for a wheelchair or golf car. From the outside both batteries look alike; however, there are fundamental differences in design. While the starter battery is made for high peak power and does not like deep cycling, the deep-cycle battery has a moderate power output but permits cycling. Let’s examine the architectural difference between these batteries further.

Starter batteries have a CCA rating imprinted in amperes. CCA refers to cold cranking amps, which represents the amount of current a battery can deliver at cold temperature. SAE J537 specifies 30 seconds of discharge at –18°C (0°F) at the rated CCA ampere without dropping below 7.2 volts. (SAE stands for Society of Automotive Engineers.)

Starter batteries have a very low internal resistance, and the manufacturer achieves this by adding extra plates for maximum surface area (Figure 1). The plates are thin and the lead is applied in a sponge-like form that has the appearance of fine foam. This method extends the surface area of the plates to achieve low resistance and maximum power. Plate thickness isless important here because the discharge is short and the battery is recharged while driving;the emphasis is on power rather than capacity.

Starter battery Figure 1: Starter battery

The starter battery has many thin plates in parallel to achieve low resistance with high surface area. The starter battery does not allow deep cycling.

Courtesy of Cadex

Deep-cycle lead acid batteries for golf cars, scooters and wheelchairs are built for maximum capacity and high cycle count. The manufacturer achieves this by making the lead plates thick (Figure 2). Although the battery is designed for cycling, full discharges still induce stress, and the cycle count depends on the depth-of-discharge (DoD). Deep-cycle batteries are marked in Ah or minute of runtime.

Deep-cycle battery Figure 2: Deep-cycle battery

The deep-cycle battery has thick plates for improved cycling abilities. The deep-cycle battery generally allows about 300 cycles.

Courtesy of Cadex

A starter battery cannot be swapped with a deep-cycle battery and vice versa. While an inventive senior may be tempted to install a starter battery instead of the more expensive deep-cycle on his wheelchair to save money, the starter battery won’t last because the thin sponge-like plates would quickly dissolve with repeated deep cycling. There are combination starter/deep-cycle batteries available for trucks, buses, public safety and military vehicles, but these units are big and heavy. As a simple guideline, the heavier the battery is, the more lead it contains, and the longer it will last. Table 3 compares the typical life of starter and deep-cycle batteries when deep-cycled.

Depth of Discharge Starter Battery Deep-cycle Battery
100%

50%

30%

12–15 cycles

100–120 cycles

130–150 cycles

150–200 cycles

400–500 cycles

1,000 and more cycles

Table 3: Cycle performance of starter and deep-cycle batteries. Starter batteries and deep-cycle batteries have their unique purposes and cannot be interchanged.

Absorbent Glass Mat (AGM)

AGM is an improved lead acid battery with higher performance than the regular flooded type. Instead of submerging the plates into liquid electrolyte, the electrolyte is absorbed in a mat of fine glass fibers. This makes the battery spill-proof, allowing shipment without hazardous material restrictions. The plates can be made flat like the standard flooded lead acid and placed in a rectangular case, or wound into a conventional cylindrical cell.

AGM has very low internal resistance, is capable of delivering high currents and offers long service even if occasionally deep-cycled. AGM has a lower weight and provides better electrical reliability than the flooded lead acid type. It also stands up well to high and low temperatures and has a low self-discharge. Other advantages over regular lead acid are a better specific power rating (high load current) and faster charge times (up to five times faster). The negatives are slightly lower specific energy and higher manufacturing costs.

AGM batteries are commonly built to size and are found in high-end vehicles to run power-hungry accessories such as heated seats, steering wheels, mirrors and windshields. Starter batteries also power navigation systems, traction and stability control, as well as premium stereos. NASCAR and other auto racing leagues choose AGM products because they are vibration resistant. Start-stop batteries are almost exclusively AGM. The classic flooded type is not robust enough and repeated micro cycling would induce capacity fade.

AGM is the preferred battery for upscale motorcycles. It reduces acid spilling in an accident, lowers weight for the same performance and allows installation at odd angles. Because of good performance at cold temperatures, AGM batteries are also used for marine, motor home and robotic applications.

As with all gelled and sealed units, AGM batteries are sensitive to overcharging. These batteries can be charged to 2.40V/cell (and higher) without problem; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell), and a direct replacement with a sealed unit could spell trouble by exposing the battery to undue overcharge on a long drive.

AGM and other gelled electrolyte batteries do not like heat and should be installed away from the engine compartment. Manufacturers recommend halting charge if the battery core reaches 49°C (120°F). While regular lead acid batteries need a topping charge every six months to prevent the buildup of sulfation, AGM batteries are less prone to this and can sit in storage for longer before a charge becomes necessary.

Ever since Cadillac introduced the electric starter motor in 1912, lead acid remained the natural choice of battery to crank the engine. Lead is toxic and environmentalists would like to replace it with another chemistry. Europe succeeded in keeping nickel-cadmium batteries out of consumer products, and authorities try to do the same with the starter battery. The choice is lithium-ion, but at a price tag of $3,000, it won’t fly. Regulators hope that advancements being made in the electric powertrain will lower the cost, but such a large price reduction to match lead acid may not be possible. Lead acid will continue to be the battery of choice to crank the engines.

In summary, Table 4 spells out the advantages and limitations of common lead acid batteries in use today. The table does not include the new chemistries discussed above or those under development.

Advantages Inexpensive and simple to manufacture; lowest cost per watt-hour

Mature and well-understood technology; provides dependable service

Low self-discharge; lowest among rechargeable batteries

High specific power, capable of high discharge currents

Limitations Low specific energy; poor weight-to-energy ratio

Slow charge; fully saturated charge takes 14 hours

Must always be stored in charged condition

Limited cycle life; repeated deep-cycling reduces battery life

Flooded version requires watering

Not environmentally friendly

Transportation restrictions on the flooded type

Table 4: Advantages and limitations of lead acid batteries. Flooded and dry systems are similar.

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