I'm designing a laptop that has the unique feature of using LiFePO4 batteries (8x 18650 cells in series). To charge them, I chose the LTC4162-F charger coupled with a LTC6803-4 cell stack monitor. Both are watched/controlled by a NXP LPC11U24 MCU via I2C and SPI, respectively. The idea is to automatically bypass undervolted cells when discharging and overvolted cells when charging, if that makes sense.
I'm now done with bringup of Rev 1 of this board and working on Rev 2, fixing a bunch of mistakes. Also, so far I didn't manage to get the charger to actually charge, which leads to my central question for this post. I can talk to the charger over I2C fine, and the chemistry (4) and cell count (8) readouts are OK. Also the VIN and VBAT measurements seem OK. Wall power VIN is 32V. There are no errors in the status/state registers. I also tried to switch off en_jeita, setting the charge current to 1 (register 0x1a) and setting the recharge threshold to 32765 (register 0x2e), with no change.
The problem is that the charger_state seems stuck in battery detection state (0x800 / 2048) and charge_status = 0 (not charging) when the 8 battery cells are connected.
When the batteries are not connected, but there is wall power, there is a periodical spike (roughly every 5 seconds) on the BATFET pin that looks like this:
When the batteries are connected, and no wall power is connected, the BATFET level is constant at 29V (!) even if the battery level is at 25.79V. INFET is at 0V.
I suspect a soldering issue as I had the wrong version of LTC4162 fitted before and exchanged it with hot air, but I wanted to confirm the overall concept and schematics before doing more pointless experimentation.
These are the full schematics of the project, the charger and balancer section is on the last page:
The board is a six layer board. Here are the layers sorted by stack up (plus silkscreen first):
For orientation, here is the location of the two relevant chips (southeast corner of the board):
Any hints concerning schematics or PCB problems are appreciated.
Lukas F. Hartmann / MNT Research