Battery balancing

), and can be exacerbated by the cumulative effect of parasitic loads, such as the cell monitoring circuitry often found in a battery management system (BMS).

To ensure safe operation and prevent hazardous conditions, the Battery Management System (BMS) continuously tracks critical parameters like temperature and voltage at the individual cell level, detecting any potential deviations that could lead to failure.

Compounding the danger is that many lithium cell chemistries include hydrocarbon chemicals[citation needed] (the exact nature of which is typically proprietary), and these are flammable.

A full BMS might include active balancing as well as temperature monitoring, charging, and other features to maximize the life of a battery pack.

Passive balancing is inherently wasteful, with some of the pack's energy spent as heat for the sake of equalizing the state of charge between cells.

Despite the obvious advantages, the additional cost and complexity of an active balancing topology can be substantial, and doesn't always make sense depending on the application.

Another variant sometimes used on electric bicycle battery packs uses a multi pin connector with a resistor and diode in series on each node: as the drops are known the charger then applies either a suitable discharge current or charges the weak cells until they all read the same loaded terminal voltage.

This has the advantage of reducing pack weight slightly and lowering parasitic draw, as well as permitting multi-point balancing.

Examples of complex circuit boards of lithium-ion batteries that include BMS sub-circuitry. [ image reference needed ]
A simplified circuit design of an active balancer acting on two of several cells in series. In two switching operations, energy can be transferred from the battery "Cell n" via a field effect transistor , "FET n", into the coil "L n" (the loop in red, labeled 1). In the second part of the balancing process (illustrated by the loop in green, 2), the energy in coil "L n" is diverted to "Cell n-1" via diode "D n-1", resulting in charge being added to "Cell n-1". [ image reference needed ]
A DeWalt 20V Max power tool battery (model referred to as 18V XR in Europe), showing its contacts and their labels. [ image reference needed ] The C1–C4 contacts are connected to the individual cells, [ image reference needed ] which allows for battery balancing by the battery charger . [ dubious – discuss ]
Cell balancing in a 5-cell battery, where the fifth cell has a lower capacity. Legend: A , Case where cells are unbalanced; B , Case where cells 3 and 5 and significantly discharged; C , Case where cells 2 and 5 and significantly over-charged; D , Case where battery is being charged with a passive balancer working on cells 2 and 5. [ citation needed ]