When discussing batteries, it’s important to distinguish between power and capacity. A battery rated at twelve volts but less than 20ah may not provide sufficient power to drive certain devices. However, it’s crucial to understand that 20ah refers to the battery’s capacity, not its power. Capacity indicates how long the battery can supply power at a constant current, while power measures the rate of energy transfer.
The term “20ah” signifies that if the battery outputs 20 amps of current, it can sustain this for one hour. Alternatively, if the output current is reduced to 2 amps, the battery can last for 10 hours. The product of current and time equals 20, representing the total charge the battery can deliver.
Devices requiring a high-capacity battery of 20ah indicate they draw a significant operating current. Smaller capacity batteries may not be able to handle this high current demand. However, if a smaller battery can withstand the device’s operating current, it can still power the device. For instance, connecting multiple small batteries in series to drive an electric bicycle may not be feasible due to insufficient capacity.
Battery capacity is measured in ampere-hours (ah) or milliampere-hours (mah), representing the time a battery can discharge at a specific current. In contrast, power is measured in watts (w), calculated using the formula P=UI, where U is voltage and I is current.
To illustrate, a 12V battery with a capacity of 100ah can supply 100 amperes of current for one hour. The specific power delivered depends on the battery’s voltage. For example, a 12V battery can operate a 1200-watt appliance for one hour with a working current of 100 amperes or for 10 hours with a current of 10 amperes.
Regular No. 5 dry batteries have a voltage of 1.5V and an internal resistance of about 1 ohm. As they discharge, the voltage decreases. For instance, a discharge current of 500mA results in a voltage drop of 0.5V, reducing the output voltage to 1V. Higher discharge currents reduce the total discharge capacity and shorten the battery’s lifespan.
Let’s delve deeper into the units:
- Power (P) is measured in watts (W), calculated using P=UI.
- Battery capacity is measured in milliampere-hours (mAh).
These two units are distinct. Electrical energy is measured in joules (J), and power is the rate of energy transfer over time, given by W=Pt. One joule equals one volt-ampere-second (1J=1VAs).
To determine the electrical energy stored in a battery, multiply its voltage by its capacity. The unit V*mAh can be converted to joules:
1VmAh = 1V * 10^-3A * 3600s = 3.6VA*s = 3.6J
A battery is an energy storage device, not a power source. Its rated capacity, measured in watt-hours (Wh), is obtained by multiplying its voltage by its capacity. For example, a “12V12AH battery” refers to its capacity, not its power. If the discharge current is 1 ampere, it can last for 12 hours. If the discharge current is 0.1 ampere, it can last for 120 hours, and so on.
Batteries also have a rated working current, which should not be exceeded to prolong their lifespan. Connecting a 300-watt lamp to a battery requires a discharge current of 25 amperes (I=P/U=300/12=25A). If this exceeds the battery’s rated current, it cannot be used. If it can be used, the approximate usable time is 0.48 hours (12ah/25a=0.48h), although losses in the inverter, battery, and circuit may reduce this time.
In summary:
- mAh is a unit of battery capacity, also known as milliampere-hour.
- Coulomb is a unit of electricity, representing the amount of charge transported in one second with a current of 1 ampere.
- Power is calculated using the formula P=UI, where P is in watts, U is in volts, and I is in amperes.
To calculate the usage duration of a battery:
- Determine the current drawn by the electrical device.
- Divide the battery’s capacity by the device’s current to obtain the usable duration.
For example, a 3000mAh battery with a rated voltage of 5V powering a 2.5W device at 5V has a theoretical usage duration of 6 hours (3000mAh/500mA=6h). Alternatively, using the Coulomb method, the charge quantity is 10800 coulombs (3A3600s=10800c), equivalent to 54000 joules of electrical energy (10800c5v=54000w). Dividing this by the device’s power consumption of 2.5W yields a usage duration of 6 hours (54000w/2.5w=21600s/3600s=6h).
