Batteries come in all shapes and sizes and there could be as many types as there are species of dog. Rather than giving batteries unique names as we do with pets, we distinguish batteries by chemistry, voltage, size, specific energy (capacity), specific power, (delivery of power) and more. A battery can operate as a single cell to power a cellular phone, or be connected in series to deliver several hundred volts to serve a UPS (uninterruptible power supply system) and the electric powertrain of a vehicle. Some batteries have high capacity but cannot deliver much power, while a starter/engine battery has a relatively low capacity but can crank the engine with 300A.
The largest battery systems are used for grid storage to store and delivery energy derived from renewable power sources such as wind turbines and solar systems. A 30-megawatt (MW) wind farm uses a storage battery of about 15MW. This is the equivalent of 20,000 starter batteries and costs about $10 million. One mega-watt feeds 50 houses or a super Walmart store. Let’s now examine each of the battery characteristics further.
The most common chemistries are lead, nickel and lithium. Each system requires its own charging algorithm. Unless provisions are made to change the charge setting, different battery chemistries cannot be interchanged in the same charger. Also observe the chemistry when shipping and disposing of batteries; each type has a different regulatory requirement.
Voltage describes the nominal open circuit voltage (OCV), which varies with chemistry and number of cells connected in series. Always observe the correct voltage when connecting to a load or a charger. Do not proceed if the voltage does not agree.
Capacity represents the specific energy in ampere-hours (Ah). Manufacturers often overrate a battery by giving a higher Ah rating than it can provide. You can use a battery with different Ah (but correct voltage), provided the rating is high enough. Chargers have some tolerance to batteries with different Ah ratings. A larger battery will take longer to charge than a small one.
Cold cranking amps (CCA)
CCA specifies the ability to draw high load current at –18°C (0°F) on starter batteries. Different norms specify dissimilar load durations and end voltages.
Specific energy and energy density
Specific energy or gravimetric energy density defines the battery capacity in weight (Wh/kg); energy density or volumetric energy density is given in size (Wh/l). A battery can have a high specific energy but poor specific power (load capability), as is the case in an alkaline battery. Alternatively, a battery may have a low specific energy but can deliver high specific power, as is possible with the supercapacitor. Specific energy is synonymous with battery capacity and runtime.
Specific power or gravimetric power density indicates the loading capability, or the amount of current the battery can provide. Batteries for power tools exhibit high specific power but have reduced specific energy (capacity). Specific power is synonymous with low internal resistance and the delivery of power.
C-rates specify charge and discharge currents. At 1C, the battery charges and discharges at a current that is par with the marked Ah rating; at 0.5C the current is half, and at 0.1C it is one tenth. On charge, 1C charges a good battery in about one hour; 0.5C takes 2 hours and 0.1C 10 to 14 hours.
Also known as electromotive force (EMF), the load draws energy from the battery. Internal battery resistance and depleting state-of-charge cause the voltage to drop.
Watts and Volt-amps (VA)
Power drawn from a battery is expressed in watts (W) or volt-amps (VA). Watt is the real power that is being metered; VA is the apparent power that determines the wiring sizing and the circuit breakers. On a purely resistive load, watt and VA readings are alike; a reactive load such as an inductive motor or florescent light causes a drop in the power factor (pf) from the ideal one (1) to 0.7 or lower. For example, a pf of 0.7 has a power efficiency of 70.