Lithium batteries contain high density which is ideal when powering electronics and applications that require a large amount of power over an extended time. However, the battery chemistry is unstable. These batteries can experience overcharging, over-discharging, and discharging that occurs too fast. When the following actions occur, the battery can become damaged, have a diminished lifespan, or even create a safety hazard.
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Protection boards for lithium batteries offer monitoring protection. Low-voltage lithium batteries require a protection board. When using high-voltage lithium batteries, a battery management system (BMS) is typically chosen since these systems contain more functions for monitoring the state of the battery pack.
Main Parts of a Protection Board
A protection board consists of integrated circuits (ICs), metal-oxide semiconductors (MOS) switches, capacitors, resistors, negative temperature coefficient thermistors (NTCs), positive temperature coefficient thermistors (PTCs), memory, ID, and other auxiliary devices. You can find protection boards as standard catalog items from a supplier, as you would get them customized based on the battery pack size, voltage, amp-hour ratings, chemistries, and any added functions.
Most ICs are currently manufactured in the Asia region, specifically Taiwan, as the following are commonly used around the world:
You can also obtain custom-built protection boards with your custom battery packs. This arrangement is ideal since the battery manufacturer will have a greater understanding of the protection needs of the custom pack that they design for the customer. So, the protection board would cater to these design requirements.
Custom battery pack with protection board.
For some battery packs, other types of features are desired, such as cell balancing and fuel gauging. When additional functions are added, it is recommended to obtain a BMS that can be tailored for both the device and the battery pack.
Functions of a Protection Board
The main function of the protection board is to monitor the state of charge (SoC), temperature, voltage, current, and state of health (SoH) of the battery pack. The MOS is controlled by the control IC. The MOS is always turned on during normal functions. If the battery pack goes outside normal thresholds that are programmed into the circuit board, the IC instructs the MOS to switch to the off position. This function helps to protect the battery pack. Let's take a deeper look at how the protection board functions when there is overcharging, over-discharging, or a short circuit.
Overcharge Protection
The battery pack will experience normal charging when connected to the charger. As the voltage rises, the IC will monitor to see if the charge state of the battery pack goes over the normal charging limit of 4.4v. If this issue occurs, the 3rd output voltage pin disconnects, and the switch tube becomes closed. Both the protection circuit and the battery cell remain in this disconnected state until the voltage load is discharged indirectly. Once the voltage reaches normal levels, the output voltage pin and the overcharge control tube is turned on. Normal charging can be done to the battery pack again.
Over-Discharge Protection
Lithium batteries have a discharge limit of 2.3v. Going below this rating can damage the battery cell. While the pack is going through normal discharging while in use with the connected device, the IC monitors the discharge rate using resistance. If the battery pack goes beyond the discharge threshold, the IC disconnects the 1st output voltage pin as the switch tube becomes closed. The battery cell's discharge circuit is cut off and the cells will no longer discharge. When proper charging voltage is applied to the cells, the outage voltage pin is turned back on, and the switch tube is reconnected. The battery cells can now receive a charge from a charger.
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Over Current and Short-Circuit Protection
Some devices may pull out too much of a charge in too fast of a short time span. To protect the battery cell and MOS tube, the protection board enacts discharge protection to the cell, turning off the pins and disconnecting the switch tubes.
The short circuit protection function is similar to the over-current protection function. The two electronic switches are the equivalent of two resistors that contain a very small amount of resistance. If the load voltage reaches over 300mA immediately, the voltage pin is turned off and the switch tube is disconnected. This feature helps protect the battery cell.
Certification of Protection Boards
All lithium battery cells, BMS, and protection boards undergo certification. UN/DOT 38.3.5 involves the shipping and transportation of lithium batteries. Other certifications include the IEC as well as UL , UL , and UL . These certifications will be based on whether the battery cell and protection circuit are shipped separately if they are shipped with the device in the same package and if the battery pack is installed in the device when it is shipped. Certification may also be specific to an industry. There are certifications for batteries installed in light vehicles, wheelchairs, household devices, and electric bikes.
Keep in mind that the certification only applies to the battery pack and protection board sent for testing. If any changes are made to the battery cells or to the protection board, they must go back through recertification due to those changes. The certification also does not pass from one manufacturing company to the next if you decide to switch suppliers. The battery cell and protection board have to again undergo certification.
Summary
All lithium batteries must have a protection board or BMS connected to the battery cells. The customer must also obtain certification for the cell and BMS system. Keep in mind that customizations can be performed to the protection board so that it can properly monitor the features of the battery cell.
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.
Capacity Management
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.
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