Battery Specification Guide (WIP)

Owned by Eric Zhao
Last updated: Jan 06, 2022
5 min read

List of common battery specifications and the meaning behind them (will be updating slowly, honestly sort of a notes dump right now).

If you come across any other terms or see any mistakes, feel free to edit this page.

TODO:

  • Add more terms, organize page better

  • Potentially move some information into separate pages so it’s not just a note dump

Table of Contents

Energy Density

Some different specifications useful for comparing the energy density of various types of batteries against each other.

  • Specific energy (gravimetric energy density): Wh/kg (Energy per unit mass)

    • This is the most relevant specification, as battery mass is usually the main limiting factor, and we want to pack as much capacity into the allowed mass of batteries

  • Energy density (volumetric energy density): Wh/L (Energy per unit volume)

    • Less relevant compared to specific energy, unless there are concerns about space

Specific power

Indicates the loading capability of the battery.

  • Can be indicated in W (power), but can also be indicated by the charge/discharge ratings (which are in amperes at a specific voltage level)

C-Rating

A battery’s capacity “decreases” when it’s discharged faster, as the internal resistance of a battery increases with discharge current, causing energy to be lost as heat.

The C-rating relates a battery’s capacity to its discharge current. Nominal capacities listed on datasheets will always have a respective standard discharge current listed as a C-rating (1C, 0.2C, etc.). The C-rate essentially specifies the speed at which a battery is charged or discharged.

  • (1/#)C

    • # specifies how long the discharge current will take to fully discharge a battery (in hours).

    • 1C specifies a current which will fully discharge the battery in 1 hour.

      • For a 1Ah capacity rated cell, this would mean a discharge current of 1A.

    • 0.2C specifies a current which will fully discharge a battery in 5 hours (1/5).

      • For a 1Ah capacity rate cell, this would mean a discharge current of 0.2A.

  • Cell capacity ratings are always linked with a standard discharge current by the C-rating

    • For a 3500mAh rated battery with a standard discharge current of 0.2C:

      • To get a full 100% use of the 3500mAh stored you need to discharge the battery at 0.7A.

      • Discharging the battery at a higher current will result in lower capacity readings.

      • Discharging the battery at a lower current will result in higher capacity readings.

Voltage Ratings

Batteries don’t have a constant voltage. As a battery is discharged, their voltage will decrease relative to the amount of charge left in the cell, similarly to the graph below:

  • Nominal voltage

    • The average voltage that a cell outputs as it’s discharged.

    • Roughly equivalent to the relatively horizontal part of a discharge curve.

    • A fully charged battery will have a higher voltage, and a drained battery will have lower voltage than the nominal voltage

  • Max (charge) voltage

    • The voltage of the cell when it’s fully charged.

    • Typically the voltage used for constant voltage charging.

    • Do not charge batteries above this voltage, causes permanent damage, thermal runaway.

  • Cut-off voltage

    • The minimum allowable voltage, voltage at which the battery is considered fully drained.

    • Do not use batteries below this voltage, can cause permanent damage.

  • Battery pack voltage range

    • Range of voltages that a battery pack may have during operation depending its state of charge.

    • Maximum voltage = # cells in series * max voltage (specified for the individual cell)

    • Minimum voltage = # cells in series * min voltage (specified for the individual cell)

Capacity Ratings

Specifies the number of Amp-hours available when the battery is discharged at a specific discharge rate (C-rate). For example, if a battery is rated for 5Ah with a discharge rate of 0.2C, you can expect the battery to last 5 hours discharging a continuous current of 1A.

  • Nominal capacity

    • The amount of capacity the cell has if you were to discharge it at its standard discharge rate (specific C-rate).

    • The capacity you should expect when ordered fresh from a manufacturer.

    • Discharging the cell above the specified C-rate will result in a reduced capacity, and vice versa for discharging it at a lower C-rate.

  • Minimum capacity

    • The lowest capacity a cell should have when you purchase it fresh from the manufacturer

  • Nominal energy

    • The amount of energy in Watt-hours available if the battery is discharged at the standard discharge rate.

    • Can be calculated by multiplying the capacity (Ah) by the nominal voltage (V).

    • A better way to compare the “capacity” of different batteries (especially if they have different nominal voltage ratings).

Charge Ratings

  • Standard

  • Max continuous charge current

  • Max peak charge current

Discharge Ratings

  • Standard discharge current

  • Max continuous discharge current

  • Max peak discharge current

Cycle Life

  •  

State-of-health (SoH)

As batteries decrease in capacity as they’re discharged and charged, SoH is an indicator of the general condition of a battery and its ability to deliver the specified performance compared with a fresh battery. This is a subjective measurement that needs to be interpreted by a variety of different indicators from a battery.

There are three main SoH indicators:

  1. Capacity, the ability to store energy

  2. Internal resistance, the capability to deliver current

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

SoH is sometimes divided into two types:

  • 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

State-of-charge (SoC)

SoC reflects the battery charge level, commonly seen as the percentage sign in the top corner of your phones. These readings may be misleading as a new battery and a faded battery may both show a SoC of 100% when fully charged, even though the faded battery is holding a smaller capacity.

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

    • ASoC = Remaining Capacity / Design Capacity

    • Absolute state of charge will eventually not be able to reach 100% as a battery’s capacity decreases from its set design capacity

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

    • RSoC = Remaining Capacity / Faded Cell Capacity

    • A fully charged battery would show a RSoC of 100% no matter what the full capacity is

State-of-function (SoF)

SoF reflects a battery’s readiness in terms of usable energy by comparing SoC relative to the available capacity

 

 

 

 

References