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19 October 2014

Important Facts about Primary Battery

There has been growth in the use of the rechargeable (secondary) batteries but the primary (non-rechargeable) batteries are also very important. They play a very important role in the market as they are used in the applications such as remote controls, wrist watches, children’s toys and the electric keys. They are also very useful in a situation where charging is not possible or impractical such as rescue missions, military combat, and the forest fire services. Other very important applications of the primary batteries are transmitters for bird tracking, intelligent drill bits for mining, light beacons, remote reporter stations, pace makers for heart patients and the tire pressure gauges in vehicles. The long storage times, operational readiness and the high specific energy makes it suited for these types of applications. The battery can be carried easily to the remote places and used immediately even after long storage. Most of the primary batteries are environmentally friendly, cheaper and readily available.

The leclanche’s battery which is also termed as the carbon-zinc is the least expensive battery and will always come with consumer devices when the batteries are included. They are general purpose batteries and are mainly used in applications having low power drain such as the flashlights, remote controls, wall clocks and children’s’ toys. The most common primary battery for consumers is the alkaline-manganese. It was invented in 1949 by Lewis Urry while he was working in the Eveready Battery Company Laboratory in Parma, Ohio. It delivers more energy while at higher load currents compared to carbon-zinc. Besides, it does not leak when it is depleted like for the case of the carbon-zinc. However, it is more expensive.

Primary batteries happen to be among the batteries with the highest densities. Even though the secondary batteries have improved, the regular household alkaline will provide 50 % more energy compared to lithium-ion. The primary battery which is the most energy-dense is the lithium battery which is made for the military combat and the film cameras. It usually holds more than three times the energy of the lithium ion. It comes in various blends which include lithium oxygen; lithium-sulfur dioxide, lithium-metal, lithium-thionyl chloride, lithium manganese dioxide and others.

The specific energy indicates the amount of energy that a battery can hold. However, this will not guarantee delivery. The primary batteries have higher internal resistance and this limits the discharge to light loads such as the flashlights, the portable entertainment devices, and remote controls. The digital cameras are borderline and the power drill on the alkaline is usually unthinkable.

The manufacturers of the primary battery will specify the specific energy while the ability to deliver power is not published. Most of the secondary batteries are rated at discharge current of about 1C while the capacity of the primary batteries will be measured by discharging them at a very low current of about 25mA or fraction of 1C. Moreover, the batteries are allowed to go down to very low voltage of 0.8 volts per cell. This provides impressive readings on the paper but the results will be poor under a demanding load.

The main reason for the sharp drop in the performance is due to the higher internal resistance in the primary battery. This causes a drop in voltage under the load. The high resistance increases further while the battery depletes on discharge. As the battery on the digital camera goes flat, the precious capacity is left behind. When alkaline is spent, it can power a kitchen clock for two years. Figure 2 shows largest discrepancy between “actual” and the “Rated” on alkaline. A long life alkaline will deliver better results.
It can be expensive to use primary batteries and its ability to recharge raises the cost of power by roughly thirty folds over the secondary batteries. The pricing will be more acute when the packs are being replaced after each mission regardless of the length of service. Discarding the partially used batteries is very common in critical missions and the fleet applications. It is safer and more convenient to issue the troops with fresh packs rather than estimate the remaining state of charge. According to one of the US army general, about half of the batteries that are discarded have 50% of the energy left.

Estimating the state of charge in the battery will help but the instruments to use are inaccurate and expensive. One of the common methods is measuring of the open circuit voltage and reading of the internal resistance by applying some load and recording the voltage drop. When there is a large voltage differential, it will relate to the rising resistance which is a hint to the end of its life. An accurate method is to count the out-flowing energy (coulomb counting) but this method requires expensive circuitry due to inherent inaccuracies and high cost, the fuel gauges are not always used on the primary batteries.

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