Basic Battery Concepts

Cell: The basic building block of any battery is the cell, the unit in which the chemical reactions of charging and discharging occur. All lead-acid cells nominally produce 2 V; a 12 V battery has six cells connected in series.

Battery: A battery is a generic term for a collection of one or more cells in a single case. This can be confusing, as we are used to thinking of a car battery (12 V with six cells) or a golf-cart battery (6 V with three cells, but as large and heavy as a large car battery). In most PV applications, batteries have one to six cells. The cells may be of any size, but the term remains the same. In fact, the larger the capacity of each cell, the fewer the cells in the battery case.

Flooded: The most common batteries in off-grid residential systems are flooded lead-acid. Flooded refers to an internal structure that uses a liquid sulfuric acid and water electrolyte to submerge suspended lead plates.

Sealed: Sealed batteries, better known as valve-regulated lead-acid (VRLA) batteries, surround the lead plates with an electrolyte that is either gelled (gel batteries) or absorbed within a fiberglass mat (absorbed glass mat or AGM batteries). Opinions vary in the industry as to which sealed type performs better in off-grid use.

Shallow: A car battery is the perfect example of a shallow-cycle battery. It has to supply high current to start a stiff engine in below-zero weather, so it has multiple thin plates to maximize surface area for more starting current. This is why automotive batteries are rated in “cold-cranking amps.” Even a cold start, however, only discharges a few percent of the battery’s capacity; it then is immediately recharged by the car’s alternator.

Deep: A deep-cycle battery has fewer but much heavier plates, as it is designed to be deeply discharged and recharged multiple times without damage. Deep-cycle batteries are rated in amp-hours (Ah), a measure of the battery’s ability to deliver current over an extended time.

Series: A series connection adds voltage by connecting individual cells positive-to-negative. Six 2 V cells in a battery are connected in series to make 12 V. Either four 6 V batteries or twelve 2 V batteries connected in series makes a 24 V battery string.

Parallel: Parallel connects strings of the same voltage together, positive-to-positive and negative-to-negative, to increase the battery bank’s capacity. For example, wiring two 12 V, 220 Ah batteries in parallel will make a 12 V, 440 Ah bank.

Bank: Multiple batteries connected together, in series and/or parallel, are referred to as a “bank” instead of individual batteries.

Amp-Hour: Any cell or battery has a specified capacity, described as the amp-hour capacity (Ah) of the battery. This is a common term for comparing types and sizes of batteries.

Kilowatt-Hour: A more useful term is the kilowatt-hour capacity (kWh) of a battery bank: this is the amp-hour capacity multiplied by the bank’s nominal voltage.

State of Charge (SOC) and Depth of Discharge (DOD): These are the terms for how charged or discharged a cell or battery is, usually expressed in percent. The two always add up to 100%: a cell that has a 70% SOC has a 30% DOD.

Days of Autonomy: An off-grid RE system is sized so that the total amount of daily charging energy from all sources—PV, wind, hydro or generator—exceeds the home’s total average daily load. The major role of the battery bank is to store energy between charging periods. Days of autonomy refers to the theoretical number of days that a battery could supply the total average daily load without recharging, usually down to a minimum threshold of about 80% DOD (20% SOC).

C/Rate: This ratio is used to quantify charge and discharge rates. It refers to the rated capacity of a cell or battery divided by the number of hours to either fully discharge or charge it. For example, a common golf cart battery has a capacity of 220 Ah. If a 22 A load is placed on the battery, it is being discharged at a C/10 rate (220 ÷ 22 = 10). If the battery is then recharged by a PV array producing 11A, it’s being charged at a C/20 rate. A 1,000 Ah battery would need to be charged at 50 A to achieve the same C/20 rate.