In order for a cell or battery to be able to deliver electrical current to an external circuit, a potential difference must exist between the positive and negative electrodes. The potential difference (usually measured in volts) is commonly referred to as the voltage of the cell or battery. A single lead-acid cell can develop a maximum potential difference of about 2 V under load. A completely discharged lead-acid cell has a potential difference of about 1.75 V, depending on the rate of discharge.

Capacity and Battery Ratings

In general terms, the capacity of a cell/battery is the amount of charge available expressed in ampere-hours (Ah). An ampere is the unit of measurement used for electrical current and is defined as a coulomb of charge passing through an electrical conductor in one second. The capacity of a cell or battery is related to the quantity of active materials in it, and the amount of electrolyte and the surface area of the plates. The capacity of a battery/cell is measured by discharging at a constant current until it reaches its terminal voltage (usually about 1.75 volts). This is usually done at a constant temperature, under standard conditions of 25C (77F). The capacity is calculated by multiplying the discharge current value by the time required to reach terminal voltage.

The most common term used to describe a battery's ability to deliver current is its rated capacity. Manufacturers frequently specify the rated capacity of their batteries in amperehours at a specific discharge rate. For example, this means that a lead-acid battery rated for 200 Ah (for a 10-hour rate) will deliver 20 amperes of current for 10 hours under standard temperature conditions (25C or 77F). Alternatively, a discharge rate may be specified by its charge rate or Crate, which is expressed as a multiple of the rated capacity of the cell or battery. For example, a battery may have a rating of 200 Ah at a C/10 discharge rate. The discharge rate is determined by the equation below:

C/10 rate (amperes) = 200 Ah/10 h = 20 amperes.

Battery capacity varies with the discharge rate. The higher the discharge rate, the lower the cell capacity. Lower discharge rates result in higher capacity. Manufacturer's literature on batteries will normally specify several discharge rates (in amperes) along with the associated discharge time (in hours). The capacity of the battery for each of these various discharge rates can be calculated as discussed above.

The rated capacity for lead-acid batteries is usually specified at the 8-, 10-, or 20-hour rates (C/8, C/10, C/20). UPS batteries are rated at 8-hour capacities and telecommunications batteries are rated at 10-hour capacities.

Series and Parallel Connections

Cells and batteries may be connected in series, parallel, or combinations of both. Cells or batteries connected in series have the positive terminal of one cell or battery connected to the negative terminal of another cell or battery. This has the effect of increasing the overall voltage but the overall capacity remains the same. For example, the 12-V lead-acid automobile battery contains 6 cells connected in series with each cell having a potential difference of about 2 V. Another example of cells or batteries connected in series is shown in Figure 2.

Cells or batteries connected in parallel have their like terminals connected together. The overall voltage remains the same but the capacity is increased. For example, if two 12-V automotive batteries were connected in parallel, the overall voltage for the batteries would still be 12 V. However, the connected batteries would have twice the capacity of a single 12-V battery. Another example of cells or batteries connected in parallel is shown in Figure 3.

Figure 2. Cells connected in series.

Figure 3. Cells connected in parallel.

Batteries may also be connected in a series/parallel combination. Batteries are added in series until the desired voltage is obtained, and in parallel until the battery bank meets capacity requirements. Only like cells or batteries should be connected together. Connecting cells or batteries of different rating or manufacturer may produce undesirable or even dangerous results.