The leads and cables to the various power supplies,
batteries and loads have resistance. The current meters also have series
resistance. The charger dynamically reduces charge current depending on the
voltage sensed at the VBUS pin (using the VINDPM feature), BAT pin (as part of
normal termination), and TS pin (through the battery temperature monitoring
feature via battery thermistor). Therefore, voltmeters must be used to measure
the voltage as close to the IC pins as possible instead of relying on the
digital readouts of the power supply. If a battery thermistor is not available,
then that shunts JP11 and JP13 in place.
When using a source meter that can source and sink current as your battery
simulator, TI highly recommends adding a large (>=1000+ μF) capacitor at the
EVM BATTERY and GND connector to prevent oscillations at the BAT pin due to
mismatched impedances of the charger output and source meter input within their
respective regulation loop bandwidths. Configuring the source meter for 4-wire
sensing eliminates the need for a separate voltmeter to measure the voltage at
the BAT pin. When using 4-wire sensing, always make sure that the sensing leads
are properly connected to prevent accidental overvoltage by the power
leads.
For precise measurements of input and output current, especially near
termination, the current meter in series with the battery or battery simulator
must not be set to auto-range and needs to be removed entirely. An alternate
method for measuring charge current is to either use an oscilloscope with hall
effect current probe or by a differential voltage measurement across the
relevant sensing resistors populated on the BQ25638xEVM.