Detecting a variety of fault conditions and protecting the battery from damage during the charging and discharging process is the main functionality of a Battery Management System (BMS). Operating a battery outside of its specifications causes damages to the battery cells and leads to a failure of the battery. This is not only causing maintenance efforts this is also a major cost factor. Batteries need to be closely monitored during charging and discharging.
Discharging protection mainly focuses on Battery over-discharge protection (under-voltage protection). The deep discharge protection is when the battery is disconnected form supplying a device with energy once its voltage reaches a lower critical value. Lithium batteries are completely empty when discharged to 2.5 V/cell. Discharging a lithium cell this low is stressful to the cell and reduces cell lifetime. A good battery protection circuit will also provide over-discharge protection.
Discharging protection also includes monitoring the following functions:
- Reverse polarity protection (Battery Swap)
- Short circuit protection (eFuse)
- Current inrush protection
Charging protection mainly focuses on overcharge protection (over-voltage protection). Overcharge protection is when the battery is disconnected from the charger once its voltage reaches an upper critical value. Lithium batteries, for example, can be safely charged to 4.1 V or 4.2 V/cell, but not higher. Overcharging causes damage to the battery and leads to the risk of positive electrode decomposition and/or thermal runaway. That creating a safety hazard such as fire danger. A battery protection circuit should be used to prevent this.
- Higher performance with lower RDS(on)
- Wider safe operating area (SOA)
- Cheaper solutions with a more compact bill of material and more effective parallelization solutions
- Short circuit protection with higher peak current rates
- Turn-on and turn-off solutions tailored to applications needs
- Up to 250 V MOSFET protection solutions (including single- and multi-module)
Protection solution for single-module batteries
Single-module batteries are typical for applications with voltage range not exceeding. the 150 V, such as battery-powered tools, vacuum cleaners, multicopter, robots, e-scooters, e-bikes, low voltage telecom, and server UPSs.
Discover the best-fit products for your design for single-module batteries.
Protection solution for multi-module batteries
Multi-module batteries are typical for applications with high-voltage batteries. including automotive, e-forklifts, e-boats, residential and utility size energy storage systems and UPSs.
Discover the best-fit products for your design for multi-module batteries.
Battery protection topology
The common source battery protection topology is when the FETs are assembled in back-to-back format with a common source as shown in the picture:
Advantages: The advantage of the common source topology is the battery is disconnected in both discharging and charging modes. Furthermore, both of the FETs can be connected to the same gate driver since they share the same source
Disadvantages: The two FETs in series mean that the heat losses are doubled.
Split side protection is when the discharge and charge protection MOSFETs are on two different circuit paths as shown in the picture: The split topology is popular in applications like power tools where:
- The load has no to little reverse currents
- the battery is dismountable
Advantages: The split topology has lower energy losses and is cheaper for applications where the discharge and charge rates are different. For example in power tools, the discharge current can be few folds higher than the charging current. As such the charging protection FETs can be less in count than the discharge protection FETs
Disadvantages: The multipath of the split topology means that the different battery poles are only protected for either charging or discharging. Misuse of the battery can bypass the protections.
High side protection is when the protection MOSFET are positioned on the high-side of the circuit, as shown in the pictures:
Advantages: The high side architecture guarantees that the battery is disconnected at the high side. As such an architecture fits well into not low voltage applications (>50V)
Disadvantages: The FETs on the high side would require a charge pump to drive the gate
Low side protection is when the protection MOSFET is positioned on the low-side of the circuit, as shown in pictures.
Disadvantages: The load is not directly connected to GND because the FET is in between load and GND. This topology is only used in applications where load can be “floating”.
In this topology, both high sides and low sides are disconnected. Such a topology would be seen in larger batteries such as automotive applications and eForklifts.
Advantages: The two-sided architecture adds extra safety in disconnecting large batteries. For example in eForklifts, the individual packs (e.g: 48V) form a larger battery pack (e.g. 400V). During maintenance, for example, all battery packs get disconnected to turn the battery from high voltage (400V) to a low voltage system (48V). The high and low side redundancy guarantees that all packs are disconnected. Thus increase safety.
Disadvantages: The double-sided topology is almost twice as an expense as the single-sided topology. Furthermore, the energy dissipation is double of the single-sided topology.