Can lithium-ion batteries be stacked?

Rationale

[

edit

]

The individual cells in a battery pack naturally have somewhat different capacities, and so, over the course of charge and discharge cycles, may be at a different state of charge (SOC). Variations in capacity are due to manufacturing variances, assembly variances (e.g., cells from one production run mixed with others), cell aging, impurities, or environmental exposure (e.g., some cells may be subject to additional heat from nearby sources like motors, electronics, etc.), 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).

Balancing a multi-cell pack helps to maximize capacity and service life of the pack by working to maintain equivalent state-of-charge of every cell, to the degree possible given their different capacities, over the widest possible range. Balancing is only necessary for packs that contain more than one cell in series. Parallel cells will naturally balance since they are directly connected to each other, but groups of parallel wired cells, wired in series (parallel-series wiring) must be balanced between cell groups.

Implications for safety

[

edit

]

To prevent undesirable, and often unsafe conditions, the battery management system must monitor the condition of individual cells for operational characteristics such as temperature, voltage, and sometimes current drawn—although the latter is often only measured per-pack rather than per-cell, perhaps with one-shot protection at the cell level against abnormally high current (such as in a short, or other failure condition.)

Under normal operation, discharging must stop when any cell first runs out of charge even though other cells may still hold significant charge. Likewise, charging must stop when any cell reaches its maximum safe charging voltage. Failure to do either may cause permanent damage to the cells, or in extreme cases, may drive cells into reverse polarity, cause internal gassing, thermal runaway, or other catastrophic failures. If the cells are not balanced, such that the high and low cutoff are at least aligned with the state of the lowest capacity cell, the energy that can be taken from and returned to the battery will be limited.

Because lithium chemistries often permit flexible membrane structures, lithium cells can be deployed in flexible though sealed bags, which permits higher packing densities within a battery pack. When a lithium cell is mistreated, some of the breakdown products (usually of electrolyte chemicals or additives) outgas. Such cells will become 'puffy' and are very much on the way to failure. In sealed lithium-ion cylindrical-format batteries, the same outgassing has caused rather large pressures (800+ psi has been reported[citation needed]); such cells can explode if not provided with a pressure relief mechanism. 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. Therefore, in addition to the risk of cell mistreatment potentially causing an explosion, a simple non-explosive leak can cause a fire.

Most battery chemistries have less dramatic, and less dangerous, failure modes. The chemicals in most batteries are often toxic to some degree, but are rarely explosive or flammable[citation needed]; many are corrosive, which accounts for advice to avoid leaving batteries inside equipment for long periods as the batteries may leak and damage the equipment. Lead acid batteries are an exception, for charging them generates hydrogen gas, which can explode if exposed to an ignition source (e.g., a lit cigarette ) and such an explosion will spray sulfuric acid in all directions. Since this is corrosive and potentially blinding, this is a particular danger.

Technology

[

edit

]

Balancing can be active or passive.[3] The term battery regulator typically refers only to devices that perform passive balancing.

A full BMS might include active balancing as well as temperature monitoring, charging, and other features to maximize the life of a battery pack.[4]

Battery balancing can be performed by DC-DC converters, in one of three topologies:

• Cell-to-battery
• Battery-to-cell
• Bidirectional

Typically, the power handled by each DC-DC converter is a few orders of magnitude lower than the power handled by the battery pack as a whole.

Passive balancing

[

edit

]

In passive balancing, energy is drawn from the most charged cell and dissipated as heat, usually through resistors.

Passive balancing equalizes the state of charge at some fixed point—usually either "top balanced", with all cells reaching 100% SOC at the same time; or "bottom balanced", with all cells reaching minimum SOC at the same time. This can be accomplished by bleeding energy from the cells with higher state of charge (e.g., a controlled short through a resistor or transistor), or shunting energy through a path in parallel with a cell during the charge cycle so that less of the (typically regulated constant) current is consumed by the cell. 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. The build-up of waste heat may also limit the rate at which balancing can occur.