Hosts and batteries

Most fuel gauges support either a two-wire SMbus (system-management bus), such as an I2C (inter-integrated circuit), or a one-wire HDQ (high-speed-DQ) interface for communication with the host device, which can be a portable device or a charger. Several Maxim gauge ICs support the proprietary Maxim 1-Wire interface. You can use this interface to program the gauge IC during manufacturing and communicate many parameters with the portable host device and charger. Most gauges that support SMbus communication also support the SBS (Smart Battery System) 1.1 list of standard battery parameters (Reference 9). The low signal reference for these digital-communication interfaces carries the return current for the battery. Be careful that the voltage drop between the gas-gauge reference to signal ground and the host-system ground is not excessive at high battery current. Digital signals may be unable to achieve a valid low at either the gas gauge or the host system during high-battery-current situations. This inability can be due to battery-to-host system-contact resistance, wire resistance, shunt resistors, or even PCB-trace resistance. Watch out for pulse-current situations, such as inrush current during battery connection, start-up current for host devices, or high charger current. These conditions may cause communication dropouts due to signal-ground lift.

Cell balancing

Manufacturers recommend cell balancing, either in the fuel gauge or in the protection IC, for 3 and 4S lithiumion batteries and require it for 5S and larger batteries, and many gauge ICs have this feature built in. Cell balancing is necessary because the capacity of the individual cells can diverge as the battery cycles through charge and discharge. This situation is especially true if the battery often deeply discharges.

The simplest cell-balancing method, passive balancing, shunts current around each fully charged cell in the series stack until all cells in the stack have the same capacity. Fuel gauges that keep track of the relative capacity of each cell in the stack perform this task on each charge cycle. The Linear Technology LTC6802-1 is a cell-monitoring IC that implements this technique.

TIs bq78PL114 and some O2Micro products implement a more complex cell-balancing technique, active balancing. This method controls small switching power supplies at each cell. These circuits pump current into the cell to balance it with the others in the stack. Control and circuit design for this method is fairly complex, but it optimizes charger energy and minimizes charge time.

Connecting the gauge

The cell array, or core pack, of a high-S- and P-count battery can be complex. To ensure that the fuel gauge maintains an accurate available-capacity measurement, you must carefully wire the gauge voltage and current sense to the core pack. Also, many gas gauges require a first-connection orderusually from the lowest to the highest voltageduring manufacturing to prevent damage to the IC.

When designing the battery, ensure that little current flows in the voltagesense connections between the gauge IC and the core pack. This requirement usually calls for a separate sense wire, or Kelvin connection, between the cells positive connection and the gauge IC. Also, be sure to follow the layout guidelines for the gauge IC you use, especially between the current shunt and the gauge IC.compaq nc8000 battery,Dell d620 battery,Dell inspiron 1300 battery