We seem to always want a battery that has the biggest "wallop." But at what cost? For what size? And at what weight? Let's start by considering specifications and terminology.
Nominal cell voltage: The manufacturer's reported voltage. Usually, this is the average of the voltages as the cell discharges through its useful life.
Terminal voltage: The instantaneous voltage across the battery terminals (not across the load) with a load applied. This varies with the state of charge and discharge/charge current history and environment.
Open-circuit voltage: The voltage across the battery terminals with no load applied. The open-circuit voltage is highest with the "freshest" battery just after being charged (as appropriate) by the most optimum charging method. Some battery chemistries require a charge/discharge cycle or two to attain their peak value.
Charge voltage: The voltage that the battery is charged to when charged to full capacity (see also Charging techniques below).
Float voltage: A steady-state voltage applied to a battery after being charged to 100 percent of its state-of-charge to maintain its capacity. This is employed to compensate for self-discharge of the battery.
Cut-off voltage: The minimum allowable terminal voltage. This is the voltage that the manufacturer considers to be the empty state of the battery. Operating the battery below this value is not recommended, unless it has been designed to handle these conditions, such as deep-discharge battery types.
Charging techniques: There are multiple specifications with regard to charging rechargeable batteries, mainly based on the specific chemistry. Some are based on a simple voltage-limited, constant current technique; some employ a simple current-limited, constant voltage approach; and the rest involve combinations and twists in multiple-stage charging. This is a large topic that we will be considering in more detail in a future post.
Depth of Discharge (DOD): The amount of battery capacity that has been discharged, expressed as a percentage of maximum capacity. A discharge to at least 80% DOD is referred to as a deep discharge.
State of charge (SOC): An expression of the present battery capacity as a percentage of its maximum capacity. SOC is generally calculated by integrating the load current over time (see also "Fuel gauge or gas gauge below). Smart batteries (as discussed above) typically use this approach, which offers a more accurate technique than simply monitoring the terminal voltage alone.
Fuel gauge or gas gauge: This refers to a device or circuit and technique for measuring the continuous operation of the battery so as to determine its state of charge (SOC). The best gauges (effectively) monitor terminal voltage and current, multiply these two, integrate over time, and compare against the reference energy. Smart batteries use various tricks to produce a reasonable result (we will consider this in more detail in a future post).
Charge current (recommended): The ideal current at which the battery is initially
charged (to roughly 70 percent state of charge) under a constant-current scheme before transitioning
into constant-voltage charging. However, this specification may change meaning for the different charging schemes (more on this in a future post).
Time durability or service life: The length of time a battery remains serviceable and viable. During this time, rechargeable batteries can be successfully recharged, while non-rechargeable batteries have a state-of-charge above the manufacturer's specification. Part of this specification is shelf life, which is the length of time the battery can remain unused (no load) while slowly degrading its capacity via self-discharge to the manufacturer's rated percentage. In some cases, service life may be specified as an available number of charge/discharge cycles, or it may be defined as the length of time before the battery needs to be replaced in a given application. Minimum service life occurs for the harshest environments, maximum loads, and poorest recharging. Maximum service life occurs with optimum environmental conditions and lightest loads, or at the shelf life in the limit case.
General durability: Robustness against abuse from factors such as environment and electrical treatment.
Reliability and mean time between failures (MTBF): See Lies, Damned Lies, and Statistics (The Statistical Treatment of Battery Failures by Jim McDowall for a good introduction to this topic.
Cold cranking amps (CCA): This is an industry rating that measures the cranking power a battery has available to start a car's cold engine. Battery Council International (BCI) defines this as "The number of amps a lead acid battery at 0 degrees F can deliver for 30 seconds and maintain at least 1.2 volts per cell."
Cranking amp (CA): Similar to cold cranking amps (as discussed above), cranking amps is a measure of the number of amps a lead acid battery at 32 degrees F can deliver for 30 seconds and maintain at least 1.2 volts per cell. (CA ratings are more commonly used in climates where temperatures rarely drop to 0 degrees F.)
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