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Battery Definitions and what they mean
Dec 13, 2018

Understand the differences in chemistries and ratings and how they apply.


Batteries are specified by three main characteristics: chemistry, voltage and specific energy (capacity). A starter battery also provides cold cranking amps (CCA), which relates to the ability to provide high current at cold temperatures.




The most common battery chemistries are lead, nickel and lithium, and each system needs a designated charger. Charging a battery on a charger designed for a different chemistry may appear to work at first but might fail to terminate the charge correctly. Observe the chemistry when shipping and disposing of batteries as each chemistry has a different regulatory requirement.




Batteries are marked with nominal voltage; however, the open circuit voltage (OCV) on a fully charged battery is 5–7 percent higher. Chemistry and the number of cells connected in series provide the OCV. The closed circuit voltage (CCV) is the operating voltage. Always check for the correct nominal voltage before connecting a battery.




Capacity represents specific energy in ampere-hours (Ah). Ah is the discharge current a battery can deliver over time. You can install a battery with a higher Ah than specified and get a longer runtime; you can also use a slightly smaller pack and expect a shorter runtime. Chargers have some tolerance as to Ah rating (with same voltage and chemistry); a larger battery will simply take longer to charge than a smaller pack, but the Ah discrepancy should not exceed 25 percent. European starter batteries are marked in Ah; North America uses Reserve Capacity (RC). RC reflects the discharge time in minutes at a 25A discharge. (See BU-904: How to Measure Capacity.)



Cold cranking amps (CCA)

Starter batteries, also known as SLI (starter light ignition) are marked with CCA. The number indicates the current in ampere that the battery can deliver at –18°C (0°F). American and European norms differ slightly. (See BU:902a: How to measure CCA; see BU:1102: Abbreviation under CCA)



Specific energy, energy density

Specific energy, or gravimetric energy density, defines battery capacity in weight (Wh/kg); energy density, or volumetric energy density, reflects volume in liters (Wh/l). Products requiring long runtimes at moderate load are optimized for high specific energy; the ability to deliver high current loads can be ignored.



Specific power

Specific power, or gravimetric power density, indicates loading capability. Batteries for power tools are made for high specific power and come with reduced specific energy (capacity). Figure 1 illustrates the relationship between specific energy (water in bottle) and specific power (spout opening).


Figure 1: Relationship between specific energy and specific power.

The water in the bottle represents specific energy (capacity); the spout pouring the water govern specific power (loading).


AA battery can have high specific energy but poor specific power as is the case with the alkaline battery, or low specific energy but high specific power as with the supercapacitor.



The C-rate specifies the speed a battery is charged or discharged. At 1C, the battery charges and discharges at a current that is on par with the marked Ah rating. At 0.5C, the current is half and the time is doubled, and at 0.1C the current is one-tenth and the time is 10-fold. ( See BU-402, What is C-rate? )




A load defines the current that is drawn from the battery. Internal battery resistance and depleting state-of-charge (SoC) cause the voltage to drop under load, triggering end of discharge. Power relates to current delivery measured in watts (W); energy is the physical work over time measured in watt-hours (Wh).



Watts and Volt-amps (VA)

Watt is real power that is being metered; VA is the apparent power  that is affected by a reactive load. On a purely resistive load, watt and VA readings are alike; a reactive load such as an inductive motor or fluorescent light causes a phase shift between voltage and current that lowers the power factor (pf) from the ideal one (1) to 0.7 or lower. The sizing of electrical wiring and the circuit breakers must be based on VA power. (See also BU-902: How to Measure Internal Resistance.)



State-of-health (SoH)

The three main state-of-health indicators of a battery are:


Capacity, the ability to store energy

Internal resistance, the capability to deliver current, and

Self-discharge, reflecting mechanical integrity and stress-related conditions

Li-ion reveals SoH in capacity. Internals resistance and self-discharge stay low under normal circumstances. SoH is commonly hidden form the user in consumer products; only state-of-charge (SoC) is provided.  (See BU-901: Fundamentals in Battery Testing)

SoH is sometimes divided into:


Absolute state-of-health (ASoH), the ability to store the specified energy when the battery is new

Relative state-of-health (RSoH), available storage capability when battery is broken in

Note: Unless otherwise mentioned, RSoH refers to SoH.



State-of-charge (SoC)

SoC reflects the battery charge level; a reading battery user is most familiar with. The SoC fuel gauge can create a false sense of security as a good and faded battery show 100 percent when fully charged.  

SoC is sometimes divided into:


Absolute state-of-charge (ASoC), the ability to take the specified charge when the battery is new

Relative state-of-health (RSoC), available charge level taking capacity fade into account.

Note: Unless otherwise mentioned, RSoC refers to SoC.




State-of-function (SoF)

SoF reflects battery readiness in terms of usable energy by observing state-of-charge in relation to the available capacity. This can be shown with the tri-state fuel gauge in which the usable capacity is reflected as stored energy in the form of charge (RSoH); the part that can be filled as empty and the unusable part that cannot be restored as dud. SoF can also be presented with the fishbowl icon for a battery evaluation at a glance. Tri-state fuel gauges are seldom used in fear of elevated warranty claims. Some devices offer an access code for service personnel to read SoF.


Figure 2 summarizes battery state-of-health and state-of-charger graphically.





SoH State-of-health. Generic term for battery health. Capacity is leading health indicator.

ASoH Absolute state-of-health of a new battery.

RSoH Relative state-of-health relating to available capacity

SoC State-of-charge. Generic term for charge level.

ASoC   Absolute state-of-charge of a new battery

RSoC Relative state-of-charge; charge level with capacity fade.