[BELL RINGING]

The BQ77915 is the primary protector for lithium ion batteries, providing over-voltage, under-voltage, over current charge and discharge, temperature, and open wire protections. It also has a hibernate mode and cell balancing. In this video, we will cover the cell balancing feature.

The BQ77915 uses passive cell balancing. Current which would normally go into the cell will be directed around the cell. Power is dissipated in resistors. For balancing current within the range supported by the device, internal balancing can be used where current is handled by the internal FET. Current is adjusted with the Rin of the filter circuit. As balancing current is increased by using smaller resistors, the filter effectiveness is decreased.

When a higher-balanced current or better filtering is desired, an external FET can be controlled by the internal FET. As the filter resistance is increased for better filtering, the internal balance current is decreased. The pins pull together and approximately half the cell voltage is available to turn on the gate of the external FET. Balanced current is selected by adjusting the RCB resistor.

When the conditions to balance are met, the input pins of the cells to be balanced are pulled together. The device uses around a 90% duty cycle during balancing, turning it off to check the cell voltages. When adjacent cells need to be balanced, the part will also duty cycle between odd and even cells, with a 50% duty cycle, and a balanced duration of 521 milliseconds.

Cell balancing is controlled by the CBI input pin. When CBI is pulled down, balancing will be enabled. When conditions for balance are met, balancing will begin after enable. In a simple design, balancing might always be enabled by pulling the CBI pin to ground. Cell balancing requires current. The state comparator must detect current flow before balancing will be enabled. Current flow is indicated to an upper device through the CBO pin when the balancing is enabled.

Ideally, when you charge a set of series cells, they charge with a constant current, constant voltage profile, and the voltages of the individual cells match throughout their range. But practically, there will be variations in the voltage due to differences in the state of charge and cell capacity. Since the BQ77915 is a protector, it is not able to track the stated charge of the individual cells like an impedance track gauge, therefore it uses voltage-based balancing.

The BQ77915 has multiple thresholds for balancing. To avoid balancing cells when voltage difference is large due to state of charge mismatch, the part has a VSTART threshold below which balancing will not occur. It also has voltage hysteresis and step options, which will become apparent in later slides. Please note that the configuration options are factory programmed for a given part number and cannot be changed on the board.

As subvoltages rise through the start threshold, high and low thresholds are set by the device, separated by the voltage specified in the VSTEP setting. As cell voltages cross the lower threshold, the device does not start balancing. When the cells are separated enough so that one or more cells is below the lower cell balance threshold and one or more cells is above the upper threshold, the cells above the upper threshold will balance. In this illustration, cell 3 will start to balance.

When all the cells are above the lower threshold, both thresholds will increment by a value of VSTEP. In this example, balancing will stop, since there are no longer cells above the high and below the low threshold. At this point, the charge current is still high, and as the cells rise, once again, since there are cells above and below the two thresholds, balancing resumes on those above the high threshold. This widely-separated voltage balancing mode operates when there is current and there are no faults.

Once the cells reach the VFC threshold, which is the hysteresis level below the overvoltage threshold, the part enters into a second mode of balancing. In this mode, the requirement to have cells below the lower cell balance threshold is dropped. All cells above the VFC threshold are balanced as long as the charge current remains sufficient.

When current drops below the state comparator hysteresis level, the balancing will stop. This may be from the charger terminating, or it may be from the threshold crossing. The third balance mode is an overvoltage. When a cell is pushed above the overvoltage threshold, the fault will stop charge current. If cell balancing is enabled, the overvoltage cells will balance. The overvoltage cells will balance until they drop below VFC, or the overvoltage hysteresis.

In a stack design, the bottom device senses current. The bottom device's OCDP pin is configured for the device to look at the current for a balancing decision. Balancing is indicated for the upper device by the CBO pin, controlling the upper part CBI pin. The upper device's OCDP pin is set to indicate that it is an upper part and to ignore current.

Another aspect of cell balancing is the supply current. The device includes a charge pump to provide adequate drive for the internal balance FETs when cells 3, 4, or 5 balance. The value is shown on the data sheet for ICC when balancing cells 3, 4, or 5. Thank you for your interest in the BQ77915. Look for more information in the product folder on ti.com.