BQ25606: Icharge less than configured

Hey Stephanie,

Based on what you are telling me, the charger is in the CV mode of charging.

Although the bq25606 does not show it in it's datasheet, you can look at the bq25601 datasheet. Please refer to Figure 11. Battery Charging Profile to understand why you are seeing less current.

Essentially, the ICHG programmed current only appears in CC mode when the BAT pin voltage is below 4.20V. In CV mode, the charging current is dictated by the voltage different between the regulation voltage of the charger @ 4.20V and the actual battery cell voltage divided by both the R of the line to the battery and the R of the internal cell ESR.

Regards,

Joel H

The voltage before you apply charge is not the problem here. There are a few things that happen in conjunction with each other:

1) Battery relaxation. When a battery is no longer getting any current, the battery voltage tends to sag/relax down. This is typical with Li-ion. So the voltage you see when unplugged is likely a relaxed battery voltage. Once it charges again, it will rise up.

2) Resistance; trace/line and internal cell impedance. In general, the more resistance you have in the path of the battery/cell, the higher the voltage you will see on the BAT pin of the charger when we are charging. As you push current into the cell, the subsequent voltage on the BAT pin will be pushed higher by an amount equal to the trace resistance + the internal cell ESR multiplied by the charging current in CC mode. In other words, VBAT(charger) = V_CELL(battery) + I_CHG*(R_CELL + R_LINE).

R_CELL is defined by both battery temperature and cell age. So older or colder batteries tend to have higher internal ESR.
R_LINE is defined by the resistance between the cell and the actual charger. Depending on how your test setup is, this could be wires from the battery itself to the PCB plus the PCB trace resistance. In general, thinner traces and longer wires lead to higher resistance here.


Regards,
Joel H