Battery labels are full of numbers, and they are not interchangeable. A phone with 5,000mAh does not hold more energy than a 1kWh home battery. Once you know which number is which, comparing batteries becomes straightforward.
Voltage (V): the push
Voltage is the electrical pressure a battery provides. It is fixed by the chemistry: 1.5V for alkaline, 1.2V for NiMH, 3.7V for a lithium-ion cell, 12V nominal for a car battery.
Nominal voltage is a label figure, not a constant. A 3.7V lithium cell sits near 4.2V when full and around 3.0V when flat. A '12V' car battery rests around 12.6V when fully charged.
Amp-hours (Ah) and milliamp-hours (mAh): the amount of charge
An amp-hour is a quantity of charge. One Ah means 1 amp flowing for 1 hour, or 0.5 amps for 2 hours. Milliamp-hours are simply one thousandth of an amp-hour, which is why small batteries use the smaller unit.
Capacity is affected by how fast you drain it. A cell rated 2,000mAh at a gentle drain will deliver less than that at a high current, and much less in the cold.
Watt-hours (Wh): the actual energy
Watt-hours are what you actually care about, because they combine both numbers: volts multiplied by amp-hours. This is the only fair way to compare batteries of different voltages.
| Battery | Voltage | Capacity | Energy |
|---|---|---|---|
| AA alkaline | 1.5 V | 2.9 Ah | about 4.4 Wh |
| Phone battery | 3.7 V | 5 Ah | 18.5 Wh |
| Tool battery pack | 18 V | 5 Ah | 90 Wh |
| Car battery | 12 V | 70 Ah | 840 Wh |
| Home battery | nominal, varies | 100 Ah equivalent | 5,000 Wh (5 kWh) |
Formula: volts x amp-hours = watt-hours. To go the other way, divide watt-hours by volts.
Phones versus power stations: why 20,000mAh is not 20Ah of home power
A 20,000mAh power bank is rated at the internal cell voltage of about 3.7V, so it holds roughly 74Wh. Printed 'usable' figures at 5V are lower still because of conversion losses.
A 1kWh portable power station holds 1,000Wh. That is more than thirteen times the energy in that power bank, even though the mAh numbers look comparable. Always compare watt-hours.
C-rate: how hard you are pushing it
A 1C rate means draining the battery in one hour. A 10Ah battery at 10A is 1C; at 5A it is 0.5C. Higher C-rates deliver energy faster but waste more as heat and shorten life.
This is why an e-bike pack and a home battery of the same capacity are not interchangeable. One is built for a high sustained current, the other for a slow daily cycle.
Series and parallel in one minute
Cells in series add voltage, capacity stays the same. Cells in parallel add capacity, voltage stays the same. A pack marked 4S2P is four cells in series, twice over.
Power tools use this to reach 18V from 3.6V cells. The total watt-hours stay the sum of every cell, which is the honest measure of how much work a pack can do.
The short version
- Voltage is the push, amp-hours are the charge, watt-hours are the energy.
- Watt-hours are the only fair way to compare batteries of different voltages.
- Capacity drops with fast discharge and cold temperatures.
- Series adds volts, parallel adds capacity, total energy adds either way.