Don’t know your volt from your amp? Here’s the BatteryIQ quick guide to lithium battery terminology.
Amp
Amp or Ampere is a unit of current. Electrical power transmission can be thought of as similar to water going through a hosepipe; an amp is similar to the rate of flow; how much water goes through the hosepipe per second.
Balancing / Cell Balancing / Battery Balancing
This is a mandatory function of a BMS to keep all cells charged to an equal voltage. Lithium cells are all microscopically different and each cell has a tendency to absorb charge, or to discharge, at a different rate. Over time a cell could approach an unsafe voltage. The BMS actively counteracts the tendency of cells to go out of balance. It does this by diverting charge from one cell to another. This can happen during Charge, Idle or Discharge.
Battery
A battery is a grouped package of one or more cells fitted with a safety circuit (see BMS). A battery will typically have some external protective cover so that the cell electrodes and wiring are covered.
Battery Management System / BMS
Also know as a BMS, this it a circuit that protects the battery from any unsafe conditions. It also provides Balancing functionality. See Dumb (BMS) and Intelligent (BMS).
Cell
This is a single energy storage unit. Lithium cells are available in a range of standard metal cylindrical sizes (18650, 20700, 21700, etc.) as well as lots of flat varieties for us in laptops or mobile phones. A Battery is comprised of multiple Cells plus management electronics.
Charging (a battery)
This is where energy & charge is being added to a battery by an external charger or by energy recovery from a motor. The BMS will Balance the charge across the cells. Each cell has a maximum safe voltage, usually around 4.25 volts depending on chemistry. The battery has a safe voltage which will be lower than the sum of its cells, e.g. a ten cell battery might have a charge cut-off of just over 42 volts. The BMS can also stop charging at a lower voltage, which might be useful to extend the overall life of the battery.
Cell End-Of-Life
If a cell is discharged to an extremely low level, or its chemistry breaks down due to age, it can stop accepting charge at all. Cell death will be a mostly open-circuit failure – it won’t cause a Short-Circuit. The BMS will detect this because the cell will fail in an Over-Voltage condition when charge is added and Under-Voltage during discharge.
Dendrite
A dendrite is an unwanted crystalline growth within a cell. It can be caused by a number of factors, such as low quality cell chemistry, low temperature use, low temperature charging, Undercharge conditions (particularly at low temperature). Dendrites can grow within a cell where they eventually cause a cell Short-Circuit. Dendrite growth can be detected by measuring charge leakage in a cell; if a cell appears to ‘lose’ charge relative to its peer cells then it may be suffering from dendrite growth.
Discharging (a battery)
This is the condition where energy & charge is being taken from the battery. Batteries are rated for a maximum average discharge current (e.g. 8 amps), a peak short-term discharge (e.g. 20 amps) and a minimum discharge voltage (e.g. 2.9 volts, depending on specific chemistry). The BMS will monitor all these data points to ensure the cell doesn’t broach its operating range.
Dumb (BMS)
Most battery BMS units are ‘dumb’; they can detect extreme unwanted cell parameters and battery conditions and may disconnect the battery from external circuitry but they don’t transmit this data in any way. They also cannot detect low level problems that are indicative of future problems, e.g. charge loss due to Dendrite growth.
Energy
See Joule
Energy Recovery System
Some mobility systems can recover energy and push it back into their batteries. For example, instead of slowing an electrical vehicle by using the brakes, the vehicle electronics can use the motor as a generator, which has the effect of converting movement back into energy.
Idle / Self-Discharge
This is where the battery pack is either turned off or disconnected. Lithium cells suffer from slight internal self-discharge. This means that if left for a number of months, they will discharge themselves to a very low level. If left for long enough, and particularly at cold temperatures, one or more cells could fail.
Over-Voltage
This is the condition where a Cell or Battery has been charged too much. The BMS will stop more charge being added to the Battery or Cell if this condition is detected.
Over-Current
This is the condition where a Cell or Battery is being discharged at all high a rate. This is a very dangerous condition as high current flow will cause heating to the Cells as well as wiring.
Over-Temperature
This is the condition where a Cell or Battery is too hot. A battery should not be charged or discharged if over-temperature so the BMS will disconnect the output terminals.
Joule
This is a measure of electrical energy delivery. Electrical energy can be thought of as similar to a river flowing; energy or joules is similar to how much water has been delivered to the bottom of a waterfall over a period of time, e.g. ‘one thousand gallons’.
Intelligent (BMS)
BatteryIQ BMS units are Intelligent. They provide all the safety cut-off features of a BMS and will also transmit this data to external readers. BatteryIQ BMS units feature data logging so even if not being actively read they will store critical battery readings and make this available for reading when connected. They also scan continuously for signs of cell degradation such as Dendrite growth.
Nominal power (of motor)
Nominal power is the manufacturer’s rating of how much the motor can deliver for an extended period without overheating or damaging the bearings. Overheating is caused by the motor being unable to dissipate excess heat. If the temperature gets too high part of the motor wiring will melt. Motors can withstand a higher power for a short period and manufacturers tend to quote a fairly conservative / small value for nominal power for a given physical motor design. Electrically assisted pedal cycle (EAPC) motor systems in the UK & EU have a limit of 250 watts nominal; all systems can and do exceed this rating for short periods.
Power
See Watt.
Short Circuit
A short-circuit is where electricity flows (often rapidly) between two points in an electrical circuit where it shouldn’t. i.e. a ‘short-circuit’ is the electricity jumping quickly through a shorter, direct route. Short-circuits can happen in wiring, in circuit boards and in cell internal chemistry. A short-circuit in a cell is a result of cell physical catastrophic failure and may lead to rapid heating or a fire. Cell short-circuit can be caused by physical damage (being squashed or cut) or by Dendrite formation.
Under-Voltage
Where a Cell or Battery has been discharged too much and the voltage is detected as too low. This might be 2.5 volts for a cell or 26 volts for a battery made of ten cells. The BMS will stop more charge being added to the Battery or Cell if this condition is detected.
Volt
Volt or voltage is a unit of electrical potential energy. Electrical power transmission can be thought of as similar to water going through a hosepipe; a volt is a bit like the pressure of the water, that’s pushing the water through the hosepipe. The bigger the hosepipe, the more water (charge) will flow.
Watt
A watt is a measure of power; power is how much energy is delivered over a period of time. One Watt is defined as one amp flowing across one volt. So a 36 volt battery pushing 10 amps delivers 360 watts.
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