Crash Course: Battery-Gobbledygook

Whether cylindric, pristmatic or as pouchbag cell: Everybody knows her from daily use- the battery. Unchanged in shape since decades, today’s batteries interior has changed rapidly within the last years. Electrochemistry placed within the battery can nowadays range from good old lead acid battery being on the market as long as since 1881 and still in heavy use as car starter, light and ignition (SLI) battery, over nickel metal hydride batteries entering the market in the 1990′s up to the family of modern lithium ion batteries being designated to propulse our automotive future.

The properties of these batteries usually are described by abbrevations. However- gobbledygook was yesterday. Within this crash course we will give a short introduction to what we feel the most important terms:

SOC means “State of Charge”. It describes the equivalent of a fuel gauge in percent. A SOC of 100% describes a fully charged battery, a SOC of 0% a fully discharged battery.

SOH means “State of Health”. It describes the condition of an already used, somewhat degraded battery in a sense of capability to comply with the requested performance parameters compared to the performance parameters provided by a new battery. It therefore is a measure of battery degradation.

SOF means “State of Fitness”. It describes the capability of a battery to perform according to an actually requested level of performance. It is usually affected by factors as SOC, SOH and battery temperature.

BMS means “Battery Management System”. A BMS is an electronic device designated for protection of the cells within a battery pack from exceeding charging and discharging currents, from too high or too low battery temperatures and from operating the battery pack at too high or too low voltages. Frequently it also balances the individual cells within the battery pack and provides information on battery state using a data interface.

DOD means “Depth of Discharge”. It describes more or less the contrary of SOC in terms of battery charge. Frequently of practical use is the term “maximum DOD”, which describes the maximum capacity recommended for discharge from fully charged state in relation to the maximum capacity availible for discharging. This is of practical relevance, because maximum DOD strongly affects the expected lifetime of a battery.

LFP means “Lithium Ferrum Phosphate”. It describes a type of electrochemistry and a battery type from the lithium ion battery family. Batteries with LFP electrochemistry are more durable than many other types of the lithium ion battery family and have better safety margins. LFP batteries are among the favorites for use within electric cars. They often can endure 2000-5000 full cycles and more.

LTO means “Lithium Titanium Oxide”. It describes another type of electrochemistry and a battery type from the lithium ion battery family. LTO battery cells are even more durable than LFP batteries at an even increased level of safety. They often can endure 10000 full cycles and more.

Balancing is called a process applied during recharging a battery pack consisting of more than one cells arranged in series, especially with lithium ion battery cells. Within the balancing process, the state of charge (SOC) of the individual battery cells are equalized (“balanced”). This is necessary, as the single battery cells could, due to manufacturing tolerances, different self discharge rates, temperature gradients and some more factors within the battery pack, degradate with different speed, featuring different SOC’s for each cell and limiting the overall pack capacity towards the lowest SOC of all cells. This effects are compensated by bringing the SOC of all cells to a common level as a result of the balancing process.

“C”, frequently “1C” oder “0.2C”, the so-called discharge rate, is a term for describing the recharging- and discharging process within a battery specifically, i.e. independently of the batterie’s absolute capacity in Ah and the flowing currents in Amps. A discharge rate of “1C” implies, that a fully charged battery is discharged by continuous current completely within one hour. A discharge rate of “2C” correspends with a complete discharge process within half an hour; a discharge rate of “10C” would correspond with the complete discharge of a fully charged battery within six minutes, and so on.

End of charge voltage: The end of charge voltage designates the maximum voltage being applied on a battery cell during charging process. Its value is recommended by the battery manufacturer and should not be exceeded due to risk of battery destruction and self ignition.

End of discharge voltage: The end of discharge voltage designates the minimum voltage being allowed on a battery cell during discharging process. It’s value is recommended by the battery manufacturer and should not be exceeded due to risk of increased battery degradation / destruction within a process called deep discharge.

battery cycle: cycles are a result of charging and discharging of a battery cell. A full cycle is performed, whenever a capacity equal to the capacity of the battery cell has been turned over once. The DOD performed within the charging and discharging cycles is considered within this calculation as shown within the following example:
A full cycle has been perfomed whenever…

-the battery cell has been empty, then fully charged and then fully discharged, or

-the battery cell has been empty, then fully charged twice and discharged two times towards a SOC of 50%, or

-the battery cell has been charged two times from a SOC=30% towards a SOC=80% and discharged back to SOC=30%

The information on cycles endured is important for life estimation resp. life expectancy of a battery cell or a battery pack.