Basics
A battery works because two different electrode materials have different chemical tendencies. The electrodes are separated by an electrolyte that conducts ions but is arranged so that electrons must travel through the external circuit. When the circuit is complete, the chemical reaction pushes electrons through the load.
The main parts of a cell
- Negative electrode: supplies electrons to the external circuit during discharge.
- Positive electrode: accepts electrons from the external circuit during discharge.
- Electrolyte: allows ions to move inside the cell.
- Separator: keeps the electrodes from touching while permitting ionic conduction.
- Case and terminals: contain the chemistry and provide safe electrical connections.
Voltage, current, capacity, and energy
Voltage
Voltage is measured in volts. It represents a difference in electric potential between two points. A typical alkaline cell is labeled 1.5 V, a nickel-metal hydride cell about 1.2 V, a lead-acid cell about 2 V, and many lithium-ion cells about 3.6 or 3.7 V. These are nominal values; the actual voltage changes during use and charging.
Current
Current is measured in amperes. The circuit and load determine how much current flows. A battery must be capable of supplying that current without excessive voltage drop or heating.
Capacity
Capacity is commonly stated in ampere-hours (Ah) or milliampere-hours (mAh). A 2 Ah cell might ideally supply 2 A for one hour, 1 A for two hours, or 0.2 A for ten hours. Real results differ because capacity depends on discharge rate, temperature, cutoff voltage, age, and chemistry.
Energy
Series and parallel connections
| Connection | Voltage | Capacity | Purpose |
|---|---|---|---|
| Series | Adds | Remains approximately that of one cell | Obtain a higher voltage |
| Parallel | Remains approximately that of one cell | Adds | Obtain more capacity and current capability |
Internal resistance
Every battery behaves as though a small resistance were hidden inside it. When current flows, some voltage is lost inside the battery and some energy becomes heat.
A battery with low internal resistance can supply a larger burst of current. As many batteries age or become discharged, their effective internal resistance rises. This is why a weak battery may still measure a reasonable voltage with no load, yet collapse when placed in the equipment.
Simple testing
- Inspect the cell for swelling, leakage, corrosion, dents, or heat damage.
- Set the meter to an appropriate DC-voltage range.
- Measure the open-circuit voltage with the red lead on positive and black on negative.
- When appropriate, test under a known safe load and observe the voltage drop.
- Compare the result with the equipment manual or battery manufacturer's discharge data.
Charging basics
A charger must suit the battery chemistry and cell count. Lead-acid, nickel-based, and lithium-ion batteries require different charge methods and different end-of-charge decisions. Some packs also require balancing, thermal monitoring, current limiting, and protective switching. A power supply set to the battery's printed voltage is not automatically a safe charger.
Care and service life
- Avoid temperatures outside the manufacturer's stated range.
- Do not leave cells in equipment where leakage could cause damage.
- Replace weak cells as a matched set when the equipment uses a series string.
- Keep contacts clean and dry.
- Store batteries so their terminals cannot be shorted by metal objects.
- Recycle or dispose of batteries through an appropriate local program.