Realized thermal management is entirely dependent upon the size and cost of the battery pack and performance objectives, design criteria of the BMS, and product unit, which may include consideration of targeted geographic region (e.g. Alaska versus Hawaii). Regardless of the heater type, it is generally more effective to draw energy from an external AC power source, or an alternative resident battery purposed to operate the heater when needed. However, if the electric heater has a modest current draw, energy from the primary battery pack can be siphoned to heat itself. If a thermal hydraulic system is implemented, then an electric heater is used to heat the coolant which is pumped and distributed throughout the pack assembly.
BMS design engineers undoubtedly have tricks of their design trade to trickle heat energy into the pack. For example, various power electronics inside the BMS dedicated to capacity management can be turned on. While not as efficient as direct heating, it can be leveraged regardless. Cooling is particularly vital to minimize the performance loss of a lithium-ion battery pack. For example, perhaps a given battery operates optimally at 20°C; if the pack temperature increases to 30°C, its performance efficiency could be reduced by as much as 20%. If the pack is continuously charged and recharged at 45°C (113°F), the performance loss can rise to a hefty 50%. Battery life can also suffer from premature aging and degradation if continually exposed to excessive heat generation, particularly during fast charging and discharging cycles. Cooling is usually achieved by two methods, passive or active, and both techniques may be employed. Passive cooling relies on movement of air flow to cool the battery. In the case of an electric vehicle, this implies that it is simply moving down the road. However, it may be more sophisticated than it appears, as air speed sensors could be integrated to strategically auto-adjust deflective air dams to maximize air flow. Implementation of an active temperature-controlled fan can help at low speeds or when the vehicle has stopped, but all this can do is merely equalize the pack with the surrounding ambient temperature. In the event of a scorching hot day, this could increase the initial pack temperature. Thermal hydraulic active cooling can be designed as a complementary system, and typically utilizes ethylene-glycol coolant with a specified mixture ratio, circulated via an electric motor-driven pump through pipes/hoses, distribution manifolds, a cross-flow heat exchanger (radiator), and cooling plate resident against the battery pack assembly. A BMS monitors the temperatures across the pack, and open and closes various valves to maintain the temperature of the overall battery within a narrow temperature range to ensure optimal battery performance.
Maximizing a battery pack capacity is arguably one of the most vital battery performance features that a BMS provides. If this maintenance is not performed, a battery pack may eventually render itself useless. The root of the issue is that a battery pack “stack” (series array of cells) is not perfectly equal and intrinsically has slightly different leakage or self-discharge rates. Leakage is not a manufacturer defect but a battery chemistry characteristic, though it may be statistically impacted from minute manufacturing process variations. Initially a battery pack may have well-matched cells, but over time, the cell-to-cell similarity further degrades, not just due to self-discharge, but also impacted from charge/discharge cycling, elevated temperature, and general calendar aging. With that understood, recall earlier the discussion that lithium-ion cells perform superbly, but can be rather unforgiving if operated outside a tight SOA. We learned previously about required electrical protection because lithium-ion cells do not deal well with over-charging. Once fully charged, they cannot accept any more current, and any additional energy pushed into it gets transmuted in heat, with voltage potentially rising quickly, possibly to dangerous levels. It is not a healthy situation for the cell and can cause permanent damage and unsafe operating conditions if it continues.
The battery pack series cell array is what determines the overall pack voltage, and mismatch between adjacent cells creates a dilemma when attempting to charge up any stack. Figure 3 shows why this is so. If one has a perfectly balanced set of cells, all is fine as each will charge up in equal fashion, and the charging current can be cut off when the upper 4.0 voltage cut-off threshold is reached. However, in the unbalanced scenario, the top cell will reach its charge limit early, and the charging current needs to be terminated for the leg before other underlying cells have been charged to full capacity.