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 VIN pin
(using the VINDPM feature), BAT pin (as part of
normal termination), and TS pin (through the
battery temperature monitoring feature through
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 ensure that the
shunt for the 10kΩ resistor that simulates the NTC
thermistor is in place.
When using a source meter that
can source and sink current as your battery
simulator, TI highly recommends adding a large
(≥1000μF) 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 a 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 BQ2569X/X-Q1EVM.