The Solar Stik - The New Generator

A Battery is the “heart” of any DC-based electrical system. While the Solar Stik™ System is engineered to work with nearly any 12 Volt battery technology, it will usually best operated with lead-acid batteries.

The battery for any Solar Stik™ System will require proper use and care, so it is important for the operator to fully understand lead-acid batteries. The following section is an easy-to-understand introduction to the most commonly-used battery technology... the 12 Volt Lead-acid Battery:

Lead-acid is the oldest rechargeable battery technology in existence. Invented by the French physician Gaston Planté in 1859, lead-acid was the first rechargeable battery to be used in commercial applications. 150 years later, we still have no real cost-effective alternatives for cars, boats, RVs, wheelchairs, scooters, golf carts and UPS systems. The lead-acid battery is still the most widely used 12 Volt power storage device today.

A lead-acid battery is an electrical storage device that uses a chemical reaction to store and release energy. It uses a combination of lead plates and an electrolyte to convert “electrical energy” into “potential chemical energy” and back again.

There are many newer battery technologies available in the marketplace, however, lead-acid technologies are the most-understood, and widely accepted as the “standard” by which all other batteries are measured. Newer technologies often have operational constraints like maximum & minimum operating temperatures and special charging requirements that make them less versatile and usable for the average consumer in everyday applications.

* Deep-cycle - The deep-cycle battery is designed for maximum energy storage capacity and high cycle count (long life). This is achieved by installing thick “lead plates” with limited surface area. Typical applications are boats, golf carts, wheelchairs, solar applications, RVs, and Un-interruptable Power Supplies (UPS).

* Engine Starting - Starter batteries are made for maximum power output, usually rated in CCA (Cold-Cranking Amps). The battery manufacturer obtains this by adding multiple “lead plates” to obtain larger surface area for maximum conductivity. Typical applications are vehicles & motorcycles.

* Electrolyte - a diluted solution of sulfuric acid and water (also known as “battery acid”).

Battery “cells” are the most basic individual component of a battery. A cell is simply a container in which electrolyte and lead plates can interact. The electrolyte is usually a 35% sulfuric acid and 65% water solution. The lead plates are usually treated with lead oxide and powdered sulfates to give them their positive and negative properties.

Individual cells are then placed together in a single case and connected in series. Since each cell produces a certain voltage (also known as “electro-motive force”), they can be configured to provide a cumulative voltage necessary to perform certain functions (such as 12 Volts, 24 Volts, or even 48 Volts).

The voltage rating of a lead-acid battery is directly related to the number of cells that have been wired in series. A standard lead-acid battery cell normally has a voltage of around 2 Volts, so a 12 Volt battery usually consists of 6 cells wired in series (6 cells multiplied by 2 Volts = 12 Volts).

* Flooded Lead-acid - The oldest of the lead-acid battery types, flooded-cell batteries can be either the sealed or open variety. Sealed flooded cells are frequently found as starter batteries in cars. Their electrolyte cannot be replenished, so when enough electrolyte has evaporated due to charging, age, or just ambient heat, the battery has to be replaced. Deep-cycle open flooded cells usually have removable caps that allow you to replace any electrolyte that has evaporated over time.

* Sealed Lead-acid - In the mid 1970s, a "maintenance-free" lead-acid battery was developed that can operate in any position. The liquid electrolyte is gelled into moistened lead plate-separators, which allow the case to be sealed. Safety valves allow venting during charge, discharge and atmospheric pressure changes. Driven by different market needs, two lead-acid systems emerged: The small sealed lead-acid (SLA), also known under the brand name of Gel-cell, and the larger Valve-regulated-lead-acid (VRLA). Both batteries are similar. Engineers may argue that the word 'sealed lead-acid' is a misnomer because no rechargeable battery can be totally sealed.

VRLA batteries remain under constant pressure of 1-4 psi. (This pressure helps the recombination process during charging under which 99+% of the Hydrogen and Oxygen generated during charging are turned back into water.)

Unlike the flooded lead-acid battery, both SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge because excess charging would cause gassing and water depletion. Gel batteries feature an electrolyte that has been immobilized using a gelling agent like fumed silica. Consequently, these batteries can never be charged to their full potential.

* Absorbed Glass Mat Batteries (AGM) - The AGM is a newer type sealed lead-acid that uses absorbed glass mats between the plates. It is sealed, maintenance-free and the plates are rigidly mounted to withstand extensive shock and vibration. AGM batteries feature a thin fiberglass felt that holds the electrolyte in place like a sponge. Nearly all AGM batteries are recombinant, meaning they can recombine 99% of the oxygen and hydrogen, and there is almost zero water is loss. The charging voltages are the same as for other lead-acid batteries. Even under severe overcharge conditions, hydrogen emission is below the 4% specified for aircraft and enclosed spaces. The low self-discharge of 1-3% per month allows long storage before recharging. The AGM costs twice that of the flooded version of the same capacity and are usually found in applications where high performance is demanded.

Like other VRLA batteries, the AGM lead-acid battery remains under constant pressure of 1-4 psi. (This pressure helps the recombination process during charging under which 99+% of the Hydrogen and Oxygen generated during charging are turned back into water.)

Neither AGM or Gel cells will leak if inverted, pierced, etc. and will continue to operate even under water.

The secret of any battery’s “runtime” lies in the battery plate’s “Capacity”.

The service life of a lead-acid battery can, in part, be measured by the thickness of the positive plates. The thicker the plates, the longer the life will be and the more energy storage one can expect. During charging and discharging, the lead on the plates gets gradually eaten away and the sediment falls to the bottom. The weight of a battery is a good indication of the lead content and the life expectancy. The plates of automotive starter batteries are about 0.040" (1mm) thick, while the typical golf cart battery will have plates that are between 0.07-0.11" (1.8- 2.8mm) thick. Forklift batteries may have plates that exceed 0.250" (6mm).

Most industrial flooded deep-cycle batteries use lead-antimony plates. Antimony is an alloy that stiffens the lead plate and helps prevent failure due to plate structure failure. This improves the plate life but increases gassing and water loss. Antimony is not necessary in AGM batteries due to the rigid construction of the overall battery.

As a rule of thumb, the heavier the battery, the more lead it contains and the longer it will last.

Battery Capacity and deep-cycling are less important for automotive because the battery is being recharged while driving. If continuously cycled, the thin lead plates of the starter battery would wear-down rather quickly.

When the positive and negative lead plates are submersed in the electrolyte, a chemical reaction occurs. This reaction produces electrons that flow between the lead plates. The amount of “push” or “force” of the electrons moving between the plates is known as the “voltage”.

The process can be summarized as a "hand-off" of sulfate in the battery cell between the water and the lead plates during charging and discharging.

As the battery is discharged, sulfate in the electrolyte combines with the lead plates of the battery to form lead sulfate. As the plates accumulate the sulfate, the electrolyte becomes more like water and less like sulfuric acid.

The reverse occurs as the battery is charged. As charging current flows through the battery, the battery plates revert back to their original condition and the electrolyte reverts back to its original sulfuric acid content.

When describing the electrolyte (battery acid) of a lead-acid battery, the term “specific gravity” is often used. It is the density of the liquid electrolyte compared to the density of water at a specific temperature and pressure. During a battery’s “discharge” process, the electrolyte transfers its sulfuric content to the lead plates, which results in a release of energy. As the electrolyte becomes more like water, its specific gravity also gets closer to that of water, indicating that the battery has been dis-charged. Because the specific gravity of the electrolyte is measurable on a scale, it can be used to determine the state of a battery’s charge and health.

The “Voltage” of a battery is a direct indication of its state of charge. It indicates the amount of electro-motive force (or the amount of “push”) that moves electrons from positive to negative fields.

Voltage is a function of the specific gravity of the electrolyte at the place in the battery where the chemical reaction occurs. This chemical reaction takes place inside the pores of the active material on the lead plates.

If the battery has just been charged, the electrolyte in the pores of the battery’s lead plates is very rich in sulfuric acid, and as a result, the battery’s voltage will be high, perhaps 13 to 14 Volts. As the battery rests following charge, voltage slowly drops and stabilizes as the electrolyte stabilizes its mixture between the plates.

A similar change in battery voltage occurs during discharge. While a fully charged battery may read 12.68 Volts, the voltage will drop and then stabilize at a somewhat lower value as a load is applied to the battery. The change in voltage occurs even though the state of charge of the battery has not significantly changed. This is due to the local electrolyte in the pores of the plates becoming less rich in sulfuric acid as the battery supplies current. As the battery discharges, electrolyte more like sulfuric acid enters the pores while electrolyte more like water exits the pores. As discharge continues, the electrolyte in the pores eventually stabilizes at a specific gravity somewhat lower than the average value in the battery, producing the slightly lower battery voltage.

The operational characteristics of the lead-acid battery can best be explained in “Capacity” and “Cold Cranking Amps” (CCA).

Capacity is the amount of energy a battery can store. The definition of Capacity is usually given in Amp-hours (AH), and it specifies the amount of current (Amps) it can provide over a period of one hour, rendering the battery “discharged”.

Cold Cranking Amps (CCA) for the amount of energy a battery can deliver. Cold Cranking Amps (CCA) is the maximum amount of current (Amps) that a battery can deliver at 0 ° F for 30 seconds and not drop below 7.2 volts. So a high CCA battery rating is good especially in cold weather. Starting batteries are often rated in CCA and are designed to deliver a short-duration burst of power (like for starting a vehicle).

For typical 12 Volt batteries, the Amp-hour rating is determined at what is termed a 20 hour rate. That is, a constant current is consumed from the battery that will cause battery voltage to drop to 10.50 volts in 20 hours.

For example, a 105 Amp-Hour battery can deliver 5.25 amps for 20 hours before the battery voltage drops to 10.50 volts, at which point the battery is dead. The same battery will also deliver approximately 10.5 amps for 10 hours, and so on.

The formulas for determining that power available from a lead-acid battery are not concrete. The actual available Amp-hours from a particular battery will be somewhat more if less current is delivered over a longer period, and somewhat less if more current is delivered over a shorter period.

As with most chemical reactions, there is always a variance in the exact amount of any assigned value (like Amp-hours).

If you are trying to determine the exact amount of power that is available from a battery, the number will always be a little bit different as they are affected by factors such as temperature, rate of discharge, battery health & age, etc.

For example, if a battery is rated at 105 Amp-hours, theoretically one can draw 100 Amps for a period of one hour. It is unreasonable to expect the same results after 50 discharge & recharge cycles. As the battery cycles increase, you will notice a decrease in the amount of power that is available, even though you may have fully recharged the battery to 13 Volts every time.

As a battery ages, Capacity and CCA do not age at the same pace. The CCA tends to stay high through most of the battery's life, and then drops quickly towards the end. If you own and drive a car, you’ve probably experienced this when all of a sudden the car won't start in the morning. In comparison, Capacity decreases gradually. A new battery is designed to deliver 100% of its rated Capacity. As the battery ages, the Capacity steadily drops and it should be replaced when its ability to store power falls below 70% of its original rating.

Let's look at Capacity and CCA and how aging affects them with graphic illustrations.

Figure 1 – CAPACITY: The following illustrations show two fully-charged lead-acid batteries, one that has a high Capacity, and one that has aged. The build-up of so-called "rock content" as part of aging robs the battery of usable energy storage although it may still provide good cranking power.

Figure 2 – CCA: The following illustration shows a battery with high and low CCA by simulating free-flowing and restricted taps.

* Low charge: A low charge results in a weak drive of current output

Low Charge

* Low Capacity: A low capacity battery will likely have good conductivity and strong torque. The voltage checks out fine and everything appears normal except the short runtime. Knowing the capacity on an aging deep cycle battery is very important because it's the best indication when a battery should be replaced.

Low Capacity

* Mismatched Batteries: Batteries do not age at an equal pace. Like the links of a chain, the battery with the lowest capacity will affect the runtime.

The “Conversion Efficiency” of a battery denotes how well it converts an electrical charge into chemical energy and back again. The higher this factor, the less energy is converted into heat and the faster a battery can be charged without overheating (all other things being equal). The lower the internal resistance of a battery, the better its conversion efficiency.

Storage batteries are not 100% efficient at converting “charging Amp-hours” back onto “stored Amp-hours”. One of the main reasons why lead-acid batteries dominate the energy storage markets is that the conversion efficiency of lead-acid cells is 85%-95%, and is often much higher than other types of rechargeable technologies. At 90% efficiency, it will take approximately 110 amp-hours of charge to replace 100 Amp-hours of consumed capacity from a lead-acid battery

Sulfation of lead-acid batteries starts when the electrolyte’s specific gravity falls below 1.225, or when voltage measures less than 12.4 (in a 12 Volt battery) or 6.2 (6 Volt battery). Sulfation is a salt-like substance that forms on the battery plate surface, and it can harden on the battery plates if left long enough, reducing and eventually blocking chemical reaction between the lead plate and the electroyte. It disables the ability of the battery to generate its rated voltage and amperage. Sulfation is the main reason a significant portion of lead-acid batteries don't reach their intended life span.

As the chemical reaction occurs between the lead plates and the electrolyte, a layer of sulfation forms on the plates. This buildup acts as a barrier on the lead plates and inhibits their ability to store and dispense energy. An equalizing charge raises the battery voltage for several hours above the battery’s rated voltage. Although beneficial in reversing sulfation, the side effects are elevated temperature, gassing and loss of electrolyte if the service is not administered correctly. The equalization step should be a last resort to break up the sulfate layers using a controlled overcharge. Because, the process will cause the battery electrolyte to boil and gas, so it should be only done with strict supervision of the battery and with the proper precautions.

Batteries start to gas when you attempt to charge them faster than they can absorb the energy. The excess energy is turned into heat, which then causes the electrolyte to boil and evaporate. The evaporated electrolyte can be replenished in batteries with removable caps such as most flooded deep-cycle batteries, however, many car batteries are sealed and thus need to be replaced when their electrolyte evaporates over time.

Since AGM and Gel cells are always sealed, it is very important to guarantee they are not overcharged. The only way to ensure this is to use a temperature-compensated charging system. Such chargers use a temperature probe on the battery to ensure that it does not overheat. If a charger has temperature compensation, it will detect and react to the battery accordingly. As the battery heats up due to a fast (high-current) recharge, the charging current is reduced to prevent thermal runaway, a very dangerous condition.

This is a very dangerous condition that can occur if batteries are charged too fast. The byproducts of Gassing are Oxygen and Hydrogen. As the battery heats up, the gassing rate increases as well and it becomes increasingly likely that the Hydrogen around it will explode. The danger posed by high Hydrogen concentrations is one of the reasons that many regulatory agencies require that batteries be installed in well-ventilated areas.

The self-discharge rate is a measure of how much batteries discharge on their own. The self-discharge rate is governed by the construction of the battery and the properties of the components used inside the cell (alloy of the lead, sulfuric concentrations of the electrloyte, etc.).

For instance, flooded cells typically use lead alloyed with Antimony (Antimony stiffens the plates) to increase their mechanical strength. However, the Antimony also increases the self-discharge rate to between 8 - 40% per month. This is why flooded lead-acid batteries should be in use often or left on a trickle-charger.

The lead found in Gel and AGM batteries does not require a lot of mechanical strength since it is immobilized by the gel or fiberglass. Thus, it is typically alloyed with Calcium to reduce Gassing and Self-Discharge. The self-discharge of Gel and AGM batteries is only 2 - 10% per month and thus these batteries need less maintenance to keep them happy.

Lead-acid batteries are reasonably forgiving in temperature extremes, as we are all familiar with our cars’ batteries. Part of this tolerance is credited to the sluggishness of the lead-acid system. Some battery types (like the AGM) permit freezing and low level charging, yet others (like the flooded cell) sustain damage and deliver reduced capacity and a short service life under the same conditions.

The optimum operating temperature for the lead-acid battery is 25°C (77°F). As a guideline, every 8°C (15°F) rise in temperature will cut the battery life in half. A VRLA, 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). Theoretically the same battery would last a little more than one year at a desert temperature of 42°C (107°F)

Over time, excessive heat degrades lead-acid batteries. The warmer the cells, the shorter the life is. Elevated temperatures cannot always be prevented, especially during fast charging, but efforts must be made to keep this time brief. While 45°C (113°F) is acceptable if kept short, at 50°C (122°F) and above, the battery starts to suffer. Note that the cells inside the battery case are always a few degrees warmer than the temperature of the housing.

* Charging Current: All batteries have a “maximum current” at which they can be safely charged. High charging current means less time is necessary to complete the recharging process, however, a maximum value can also shorten battery life. Cases of extreme over-current could result in a hazardous condition due to battery overheating and thermal runaway.

* Charging Voltage: A lead-acid battery is charged by applying a voltage across its positive & negative terminals that is higher than the voltage it already has across them. The greater the difference between the applied voltage and the battery voltage, the greater the charging current that will flow, and the quicker the battery will be charged.

* Charging Time: The charge time of a sealed lead-acid battery is 12-16 hours (up to 36 hours for larger capacity batteries). With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less. It takes 3 to 5 times as long to recharge a lead-acid battery to the same level as it does to discharge it.

* Battery Chargers: Most garage and consumer (automotive) type battery chargers are bulk charge only, and have little (if any) voltage regulation. They are fine for a quick boost to low batteries, but not to leave connected for long periods. If extended connection is done, damage to the battery usually occurs.

* Regulated Chargers: A voltage-regulated charge control is designed supply a constantly-regulated voltage to connected batteries. If these are set to the correct voltages for your batteries, they will keep the batteries charged without damage. These are sometimes called "taper charge".

* Battery Charge Controllers: A charge controller is a regulator that goes between a charging source (like solar panels) and the storage batteries. Regulators for solar systems are designed to keep the batteries charged at peak without overcharging.

Three-stage Charging - The Best Charge Method for Deep-cycle Lead-acid Batteries

Whatever the charging source (AC, solar, or even a vehicle’s alternator), a multi-stage, or “three-stage” charging process is the best method to recharge a lead-acid battery. This method takes all three parameters, and sequentially applies each one at a specific rates and duration. The three stages are:

* Bulk Charge: The charging device applies a constant current charge, raising the cell voltage to a preset voltage. Since the charge rate depends on the difference between the charging voltage and the battery voltage, the first stage of three-stage charging is to keep this voltage difference constant so that, as the battery voltage rises, the charging voltage rises and the charging current remains constant.

This stage typically takes the battery to about three-quarters of full charge, and at a rate that usually does not exceed 25% of the battery’s Amp-hour capacity.

* Acceptance Charge: In this stage, the charge current is gradually reduced as the cell is being saturated. Charging current is reduced to about half that of the Bulk charge rate. Since battery charging is an electro-chemical process that has (in the case of deep cycle batteries) very long reaction times, the charging voltage is now reduced for a few hours to enable the “charge”, that is now concentrated in and around the plates, to become homogeneously distributed through the plates and the electrolyte.

* Float Charge: The final stage is the “float” charge, which compensates for the battery’s self-discharge. Following the Acceptance (absorption) stage, the charging voltage drops once again to a level that eventually counterbalances the battery’s internal losses. Depending on ambient temperature and battery type, will usually be from 13.2-13.3 Volts (AGM/Gel) to 13.6-13.8 Volts (flooded cell).

Batteries may usually be left ‘floating’ indefinitely. During the float stage, a battery will typically be 95%-97% charged.

To improve charge performance of lead-acid batteries at colder temperatures and avoid thermal runaway during heat spells, controlling the charging voltage is important. Implementing such a measure can prolong battery life by up to 15%. Higher temperatures require slightly lower charging voltages and vice versa. Three-stage chargers that have a battery temperature sensor are excellent for applications where exposure to large temperature fluctuations are expected.

As covered earlier, individual lead-acid cells are wired in series to achieve a desired voltage (as in a 12 Volt battery).

Correct settings of the voltage limits (maximum voltage) for the cells are critical. In a lead-acid cell, the range is usually from 2.30 Volts to 2.45 Volts. Setting the voltage limit is a compromise. On one end, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate. A continually over-saturated condition at the other end, however, would cause grid corrosion on the positive plate. It also promotes gassing, which results in venting and loss of electrolyte.

The battery cannot remain at the peak voltage for too long; the maximum allowable time is 48 hours. When reaching full charge, the voltage must be lowered to maintain the individual battery cells at between 2.25 and 2.27 Volts. Manufacturers of large lead-acid batteries recommend a float charge of 2.25 Volts at 25°C per cell.

Car batteries and valve-regulated-lead-acid batteries (VRLA) are typically charged to between 2.26 and 2.36 Volts per cell. At 2.37 Volts, most lead-acid batteries start to gas, causing loss of electrolyte and possible temperature increases. The exceptions are small sealed lead acid batteries (SLA), which can be charged to 2.50V/cell without adverse side effect.

Large VRLA batteries are often charged with a float-charge current to 2.25 Volts per cell. A full charge may take several days. Note that the current in “float-charge” mode gradually increases as the battery ages in standby mode. The reasons are typcally electrical cell leakages and a reduction in the battery's chemical efficiency.

Aging affects each battery cell differently. Since the cells are connected in series, controlling the individual cell voltages during charge is virtually impossible. Even if the correct overall voltage is applied, a weak cell will generate its own voltage level and intensify the condition further.

Lead-acid batteries must always be stored in a charged state. A topping charge should be applied every six months to avoid the voltage from dropping below 2.10 Volts per cell on an SLA. Prolonged storage below the critical voltage causes sulfation, a condition that is difficult to reverse.

While dwelling on float-charge, an external load can be connected to a lead-acid battery. In such a case, the battery acts as a buffer. The Solar Stik™ System works this way. During off-peak periods, the batteries get fully charged. On peak power demand, the load exceeds the net supply provided by the solar array and the battery supplies the extra energy. A car battery works in a similar way.

When configuring a battery as a buffer, make certain that the battery has the opportunity to fully charge between loads. The net charge must be greater than what is drawn from the battery.

Battery testing can be done in several ways, and the method is often chosen according to the battery type and tools that are available. The most popular methods include measurement of specific gravity, and battery voltage.

The state-of-charge of a lead-acid battery can be estimated by measuring the open (no load) battery terminal voltage using a digital voltmeter. Prior to measuring, the battery must have rested for 4-8 hours after charge or discharge and resided at a steady room temperature. With these conditions met, voltage measurements provide an amazingly accurate state-of-charge for lead acid batteries.

Specific gravity applies to wet cells with removable caps, giving access to the electrolyte. To measure specific gravity, buy a temperature compensating hydrometer at an auto parts store or tool supply.

“Load-testing” is another method of testing a battery. Load-testing removes and measures the Amps from a battery (similar to starting an engine). Some battery companies label their battery with the Amp load for testing. This number is usually 1/2 of the CCA rating. For instance, a 500 CCA battery would load test at 250 amps for 15 seconds. A load test can only be performed if the battery is at or near a full charge. Some electronic load testers apply a 100 Amp load for 10 seconds, and then display battery voltage. This number is compared to a chart on the tester, based on CCA rating to determine battery condition.

Battery manufacturers define the “end-of-life” of a battery when it can no longer hold a proper charge (for example, a cell has shorted out) or when the available battery capacity is 70% or less than what the battery was rated for. The life of Lead-acid batteries is usually limited by several factors:

* Cycle Life is a measure of how many charge and discharge cycles a battery can take before its lead-plate grids/plates are expected to collapse and short out. The greater the average depth-of-discharge, the shorter the cycle life.

* Age also affects batteries as the chemistry inside them attacks the lead plates. The healthier the "living conditions" of the batteries, the longer they will serve you. Lead-acid batteries like to be kept at a full charge in a cool place. Only buy recently manufactured batteries, so learn to decipher the date code stamped on every battery... (inquire with the manufacturer). The longer the battery has sat in a store, the less time it will serve you! Since lead-acid batteries will not freeze if fully charged, you can store them in the cold during winter to maximize their life.

* Construction - has a big role in battery life too, some designs are better at preserving batteries than others and the suitability of a design for a given application plays a role also. For example, flooded lead-acid cells will typically fare worse than their VRLA cousins in operations that involve a lot of jerky motion - the immobilized plates in VRLA cells will be stressed less than suspended plates in cheap flooded cells.

* Plate Thickness - the thicker the plates, the more abuse, charge and discharge cycles they can take. Thicker plates will also survive any equalization treatments for sulphation better. The heavier the battery for a given group size, the thicker the plates are, so you can use weight as one guide to buying lead-acid batteries.

* Sulfation - is a constant threat to batteries that are not fully re-charged. A layer of lead sulfate can form in lead-acid cells and inhibit the electro-chemical reaction that allows you to charge/discharge batteries. Many batteries can be saved from the recycling heap if they are Equalized. Additionally, there are devices for removing hard sulfation, but the best practice is preventing formation by proper battery care and recharging after a discharge cycle.

The design life of a battery depends in part on its construction, its type, the thickness of the plates, its charging profiles, etc. All these factors come together to determine just how long your battery may ultimately serve you.

Below is a summary of the strength and limitations of today's popular battery systems. Although energy density is paramount, other important attributes are service life, load characteristics, maintenance requirements, self-discharge costs and safety. Nickel-cadmium is the first rechargeable battery in small format and forms a standard against which other chemistries are commonly compared. The trend is towards lithium-based systems.

* Nickel-cadmium - mature but has moderate energy density. Nickel-cadmium is used where long life, high discharge rate and extended temperature range is important. Main applications are two-way radios, biomedical equipment and power tools. Nickel-cadmium contains toxic metals.

* Nickel-metal-hydride - has a higher energy density compared to nickel-cadmium at the expense of reduced cycle life. There are no toxic metals. Applications include mobile phones and laptop computers. NiMH is viewed as steppingstone to lithium-based systems.

* Lithium-ion - fastest growing battery system; offers high-energy density and low weight. Protection circuits are needed to limit voltage and current for safety reasons. Applications include notebook computers and cell phones. High current versions are available for power tools and medical devices.

* Lead-acid - most economical for larger power applications where weight is of little concern. Lead-acid is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS systems. Lead acid is inexpensive and rugged. It serves a unique niche that would be hard to replace with other systems.

* Mature, reliable and well-understood technology - when used correctly, lead-acid is durable and provides dependable service.

* Low energy density - poor weight-to-energy ratio limits use to stationary and wheeled applications.

* Cannot be stored in a discharged condition - the cell voltage should never drop below 2.10V.

* Allows only a limited number of full discharge cycles - well suited for standby applications that require only occasional deep discharges.

* Lead content and electrolyte make the battery environmentally unfriendly. Proper disposal is to recycle the battery.

* Transportation restrictions on flooded lead acid - there are environmental concerns regarding spillage.

* The electrolyte of lead-acid batteries is hazardous to your health and may produce burns and other permanent damage if you come into contact with it.

The group size will merely indicate the approximate exterior dimensions (including terminals) and voltage of the battery in question. However, the exact dimensions can only be directly obtained from each manufacturer.