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Deep cycle battery versus a starting battery

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This page will give you some basic information about Lead Acid batteries and the difference between starter batteries and deep cycle batteries.

Lead-based Batteries

Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable battery for commercial use. Despite its advanced age, the lead chemistry continues to be in wide use today, and there are good reasons for its popularity; lead acid is dependable and inexpensive on cost-per-watt base. There are few other batteries that deliver bulk power as cheaply as lead acid, and this makes the battery cost-effective for automobiles, golf cars, forklifts, marine and uninterruptible power supplies (UPS).

But lead acid has disadvantages; it is heavy and is less durable than nickel- and lithium-based systems when deep-cycled. A full discharge causes strain and each discharge/charge cycle permanently robs the battery of a small amount of capacity. This loss is small while the battery is in good operating condition, but the fading increases once the performance drops to half the nominal capacity. This wear-down characteristic applies to all batteries in various degrees.

Depending on the depth of discharge, 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 elevated operating temperatures and high-current discharges..

Charging a lead acid battery is simple but the correct voltage limits must be observed, and here there are compromises. Choosing a low voltage limit shelters the battery but this produces poor performance and causes a build-up of sulfation on the negative plate. A high voltage limit improves performance but form grid corrosion on the positive plate. While sulfation can be reversed by using the Battery Extra, corrosion is permanent.

Lead acid does not lend itself to fast charging and with most types, a full charge takes 14 to16 hours. The battery must always be stored at full state-of-charge. Low charge causes sulfation, a condition that robs the battery of performance. Adding carbon on the negative electrode reduces this problem but this lowers the specific energy..

Lead acid has a moderate life span and is not subject to memory as nickel-based systems are. Charge retention is best among rechargeable batteries. 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 work well at cold temperatures and is superior to lithium-ion when operating in subzero conditions.



Sealed Lead Acid (SLA)

The first sealed, or maintenance-free, lead acid emerge in the mid-1970s. The engineers argued that the term "sealed lead acid" is a misnomer because no lead acid battery can be totally sealed. This is true and battery designers added a valve to control venting of gases during stressful charge and rapid discharge. Rather than submerging the plates in a liquid, the electrolyte is impregnated into a moistened separator, a design that resembles nickel- and lithium-bases system. This enables the battery to operate in any physical orientation without leakage.

The sealed battery contains less electrolyte than the flooded type, hence the term "acid-starved." Perhaps the most significant advantage of the sealed lead acid is the ability to combine oxygen and hydrogen to create water and prevent water loss. The recombination occurs at a moderate pressure of 0.14 bar (2psi). The valve serves as safety vent if gases build-up during over-overcharge or stressful discharge. Repeated venting would lead to an eventual dry out.

Driven by these advantages, several types of sealed lead acid have emerged and the most common are known as valve-regulated lead acid (VRLA), and absorbent glass matt (AGM). Smaller packs with capacities of up to 30Ah are called SLA (sealed lead acid). Packaged in a plastic container, these batteries are used for small UPS, emergency lighting, ventilators for healthcare and wheelchairs. Because of economical price, dependable service and low maintenance, the SLA remains the preferred choice for biomedical and healthcare in hospitals and retirement homes.

The AGM is a newer design and suspends the electrolyte in a specially designed glass mat. This offers several advantages to lead acid systems, including faster charging and instant high load currents on demand. AGM works best as a mid-range battery with capacities of 30 to 200Ah and is less suited for large systems, Typical uses are for micro electric cars, as well as marine, motorhomes (RV), golf buggies, small solar systems that need some cycling.

With cycling and age, the capacity of AGM fades gradually; AGM is more expensive than flooded.

Unlike the flooded, the sealed lead acid battery is designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging causes gassing, venting and subsequent water depletion and dry out. Consequently, they cannot be charged to their full potential and the charge voltage limit must be set lower than that of a flooded. The float charge on full charge must also be lowered. In respect to charging, the AGM are not direct replacements to the flooded type. If no designated charger is available with lower voltage settings, disconnect the charger after 24 hours of charge. This prevents gassing due to a float voltage that is set too high..

The optimum operating temperature for a VRLA battery is 25°C (77°F); every 8°C (15°F) rise above this temperature threshold cuts battery life in half. Lead acid batteries are rated at a 5-hour (0.2C) and 20-hour (0.05C) discharge. The battery performs best when discharged slowly and the capacity readings are notably higher at a slow discharge 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 as starter-light-ignition (SLI). The high lead content and the sulfuric acid make lead acid environmentally unfriendly.

The following paragraphs look at the different architectures within the lead acid family and explain 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 is less important here because the discharge is short and the battery is recharged while driving; the emphasis is on power rather than capacity.


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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 solar battery systems, fork trucks, 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.

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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. Below we compares the typical life of starter and deep-cycle batteries when deep-cycled.


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Lead is toxic and environmentalists would like to replace the lead acid battery with another chemistry. Europe succeeded in keeping nickel-cadmium batteries out of consumer products, and authorities will try to do it with the starter battery. The choices are NiMH and lithium-ion, but with the price tag for Li-ion, this will not fly. In addition, Li-ion has poor performance at sub-freezing temperature. Regulators hope that advancements in the electric powertrain will lower the cost, but a large price reduction to match the low-cost lead acid may not be possible. Lead acid will continue to be the battery of choice to crank the engines

Below is a list of the advantages and limitations of common lead acid batteries in use today.

Advantages
Inexpensive and simple to manufacture. Low cost per watt-hour.Low self-discharge; lowest among rechargeable batteries. High specific power, capable of high discharge currents. Good low and high temperature performance.

Limitations
Low specific energy; poor weight-to-energy ratio. Slow charge; fully saturated charge takes 14 hours. Must be stored in charged condition to prevent sulfation. Limited cycle life; repeated deep-cycling reduces battery life. Flooded version requires watering. Transportation restrictions on the flooded type. Not environmentally friendly


With thanks to the battery universty



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