batteries with a balancer
Monitor A BMS may monitor the state of the battery as represented by various items, such as: •
Voltage: total voltage, voltages of individual cells, or voltage of •
Temperature: average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cells • Coolant flow: for liquid cooled batteries •
Current: current in or out of the battery • Health of individual cells •
State of balance of cells
Electric vehicle systems: energy recovery • The BMS will also control the recharging of the battery by redirecting the recovered energy (i.e., from
regenerative braking) back into the battery pack (typically composed of a number of battery modules, each composed of a number of cells). Battery thermal management systems can be either passive or active, and the cooling medium can either be air, liquid, or some form of phase change. Air cooling is advantageous in its simplicity. Such systems can be passive, relying only on the convection of the surrounding air, or active, using fans for airflow. Commercially, the Honda Insight and Toyota Prius both use active air cooling of their battery systems. The major disadvantage of air cooling is its inefficiency. Large amounts of power must be used to operate the cooling mechanism, far more than active liquid cooling. The additional components of the cooling mechanism also add weight to the BMS, reducing the efficiency of batteries used for transportation. Liquid cooling has a higher natural cooling potential than air cooling as liquid coolants tend to have higher thermal conductivities than air. The batteries can either be directly submerged in the coolant or the coolant can flow through the BMS without directly contacting the battery. Indirect cooling has the potential to create large thermal gradients across the BMS due to the increased length of the cooling channels. This can be reduced by pumping the coolant faster through the system, creating a tradeoff between pumping speed and thermal consistency. Low-voltage centralized BMSes mostly do not have any internal communications. Distributed or modular BMSes must use some low-level internal cell–controller (modular architecture) or controller–controller (distributed architecture) communication. These types of communications are difficult, especially for high-voltage systems. The problem is the voltage shift between cells.The first cell ground signal may be hundreds of volts higher than the other cell ground signal. Apart from software protocols, there are two known ways of hardware communication for voltage shifting systems,
optical-isolator and
wireless communication. Another restriction for internal communications is the maximum number of cells. For modular architecture, most hardware is limited to a maximum of 255 nodes. For high-voltage systems the seeking time of all cells is another restriction, limiting minimum bus speeds and losing some hardware options. The cost of modular systems is important, because it may be comparable to the cell price. Combination of hardware and software restrictions results in a few options for internal communication: • Isolated serial communications • Wireless serial communications To bypass power limitations of existing USB cables due to heat from electric current, communication protocols implemented in
mobile phone chargers for negotiating an elevated voltage have been developed, the most widely used of which are
Qualcomm Quick Charge and
MediaTek Pump Express. "
VOOC" by Oppo (also branded as "Dash Charge" with "OnePlus") increases the current instead of voltage with the aim to reduce heat produced in the device from internally converting an elevated voltage down to the battery's terminal charging voltage, which however makes it incompatible with existing USB cables and relies on special high-current USB cables with accordingly thicker copper wires. More recently, the
USB Power Delivery standard aims for a universal negotiation protocol across devices of up to 240 watts.
Protection A BMS may protect its battery by preventing it from operating outside its
safe operating area, such as: • Over-charging • Over-discharging • Over-current during charging • Over-current during discharging • Over-voltage during charging, especially important for
lead–acid,
Li-ion, and
LiFePO4 cells • Under-voltage during discharging, especially important for Li-ion, and LiFePO4 cells • Over-temperature • Under-temperature • Over-pressure (
NiMH batteries) • Ground fault or leakage current detection (system monitoring that the high voltage battery is electrically disconnected from any conductive object touchable to use like vehicle body) The BMS may prevent operation outside the battery's safe operating area by: • Including an internal
switch (such as a
relay or
MOSFET) which is opened if the battery is operated outside its safe operating area • Asking the devices to reduce or even stop using or charging the battery. • Actively controlling the environment, such as through heaters, fans, air conditioning or liquid cooling • Reduce processor speed to reduce heat.
Battery connection to load circuit A BMS may also feature a precharge system allowing a safe way to connect the battery to different loads and eliminating the excessive inrush currents to load capacitors. The connection to loads is normally controlled through electromagnetic relays called contactors. The precharge circuit can be either power resistors connected in series with the loads until the capacitors are charged. Alternatively, a
switched mode power supply connected in parallel to loads can be used to charge the voltage of the load circuit up to a level close enough to the battery voltage in to allow closing the contactors between the battery and load circuit. A BMS may have a circuit that can check whether a relay is already closed before recharging (due to welding for example) to prevent inrush currents from occurring.
Balancing In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same voltage or State of Charge, through balancing. The BMS can balance the cells by: • Dissipating
energy from the most charged cells by connecting them to a
load (such as through passive
regulators) • Shuffling energy from the most charged cells to the least charged cells (
balancers) • Reducing the charging current to a sufficiently low level that will not damage fully charged cells, while less charged cells may continue to charge (does not apply to Lithium chemistry cells) Some chargers accomplish the balance by charging each cell independently. This is often performed by the BMS and not the charger (which typically provides only the bulk charge current, and does not interact with the pack at the cell-group level), e.g.,
e-bike and
hoverboard chargers. In this method, the BMS will request a lower charge current (such as EV batteries), or will shut-off the charging input (typical in portable electronics) through the use of
transistor circuitry while balancing is in effect (to prevent over-charging cells). == Topologies ==