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Batteries and cell characteristics

Batteries are cheap, small and relatively safe way of having portable energy source. There are many types of batteries with different characteristics and uses. Probably many of you heard terms battery and cell referring to the same. But from technical perspective these are different things – cell is a single unit which houses a single chemical reaction and produces electricity, while battery is a pack of cells.

Nominal cell voltage and battery voltage

For instance single AA battery can be called a cell because this is a single unit where single chemical reaction generates voltage of 1.5V, but car lead-acid battery have six cells producing 2.0V each while all battery produces 12V. So first important thing to remember that single cell can produce specific nominal cell voltage that depends on chemical reaction, while battery voltage is a combined voltage of multiple cells connected in series (or parallel).

Cell capacity

So we know that battery characteristics directly depend on characteristics of cells inside. Second important parameter of batteries is cell capacity which is measured in amperes-hours. On smaller batteries there are mA/h used as capacity is relatively small like comparing to car batteries. So what is a cell capacity? This is certain amount of current supplied before cell voltage drops below predefined threshold – or simply speaking amount of energy released before battery dies or discharges. If we take simple AA battery of 1.5V which is rated about 1000mA/h, so this means that battery can supply continuous 1A current for one hour before it dies and voltage drops to like 0.9V (Alkaline battery). Usually battery capacity is tested for longer time. Like in 20 hour duration. If battery is rated 1A/h, so it should provide constant 50mA current for twenty hours. If battery is drained more quickly, then you can assume that capacity is less than label shows.

Power density of cell

This parameter tends to be very important today as all electronic equipment shrinks; power demand grows because of growing functionality. Batteries have to integrate in smaller devices and provide more power. This is real headache for scientists how to put more energy in to less space. So cells are characterized with another parameter – power density, what means capacity per unit weight. Cheapest batteries like carbon-zinc have lowest power density of all cell types while higher power density belongs to Lithium ion-polymer batteries.

Discharge curve of battery cell

Capacity and power density can’t give clear view about battery quality. Fact is that when battery discharges its voltage drops over the cells life. Different types of cells have their own discharge curves. Like alkaline batteries have linear drop of voltage over all discharge time. By having linear characteristics are easy to determine how much battery is discharged, but some devices require constant voltage to operate normally. So other batteries have sharp dropping characteristic. This means, that voltage drops at some point when it is discharged. And it is hard to determine now much battery is discharged.

Internal resistance of cell

Resistance is present everywhere where electric current flows. Battery cells aren’t exception. Every cell has its own internal resistance. When current is drawn out of the cell, some voltage drops across this internal resistance. So every battery cell can be modeled as ideal voltage source and resistor connected in series. Internal resistance is important parameter because it determines the maximum rate at which power can be drawn out of cell. The higher current flows through resistor, more heat is produced due resistance. If this current is too big, then it can melt insulation, wires and other. So the same is with batteries. For instance lead-acid car batteries have low internal resistance, so they are ideal for releasing huge amounts of current during starting motor. In other hand if alkaline batteries are used in flash camera, it takes time to charge flash capacitor. It cannot charge instantly, as it would result in heat. And of course any kind of batteries don like fast discharge due to chemical reactions. Short circuit of batteries can damage batteries while some types may even explode.

Rechargeability of cells

So there are two types of batteries: rechargeable and not. Both of them are used today because of difference in cost, technology and purposes. As all batteries are made of toxic chemicals, using rechargeable cells is more plausible, but using rechargeable batteries is not always an issue. Some non rechargeable batteries still have higher power densities than rechargeable can give. And of course cost. Rechargeable batteries are more expensive but seem that this difference is shrinking.

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