LTC4162-L Battery Detection Issue on Solar

Hi There-

We've been testing the LTC4162-L charger, and have come across an unusual situation related to battery detect I was hoping someone could help with. We had months of solid solar charge performance, but began to see issues when we started to work with telemetry and thermistors on the pack itself. We didn't want to delay development to work through those issues, so we went back to disabling telemetry and populating two 100k fixed resistors to ensure the thermistor always read 23*C. However we have not been able to return to reliable charging.

We are having problems initiating a charge in some situations. We are using a 7V, ~4W solar panel to charge a 1S 8P manufactured pack of 18650 Li-Ion batteries (via 24" 22AWG battery wires) using the LT4162-L. I've read several of the earlier forum posts but am still stumped.

We have a lab-bench case that typically works. When I use a PSU set to 9.3V with a 5 ohm resistor in series with the +V line, it generally works. Sometimes I can get a failure if I slowly increase the voltage on the dial. The register readouts are as follows:

  • Start system with nothing plugged in at the input. 
    • Config Reg:0x0001, CHEM_CELLS_REG:0x01f0
    • chrgr_state:0x0100, chrg_status:0x0000, sys_status:0x00A1
  • Plug in 9.3V power supply w. 5 ohm R for input
    • chrgr_state:0x0800, chrg_status:0x0000, sys_status:0x0067 for 1 second
    • CHEM_CELLS_REG:0x11f1
    • chrgr_state:0x0040, chrg_status:0x0008, sys_status:0x0147
    • CHEM_CELLS_REG:0x11f1

This success produces the following scope shot which I believe is the passing battery detection phase where ch.1 (yellow) is input voltage to the charger, ch.2 (blue) is A/C coupled battery voltage, and ch.3 (pink) is current into Vin. DC battery voltage is 3.9V

9.3V PSU with 5ohm series R

When I try to charge from our solar panel, typically the system doesn't seem to break out of battery detection.

  • Start system with nothing plugged in at the input. 
    • Config Reg:0x0001, CHEM_CELLS_REG:0x01f0
    • chrgr_state:0x0100, chrg_status:0x0000, sys_status:0x00A1
  • Plug in solar panel in sun through window, charging does not begin.
    • chrgr_state:0x0800, chrg_status:0x0000, sys_status:0x0067

This failure produces the following scope shot which I believe is the failing battery detection phase where ch.1 (yellow) is input voltage to the charger, ch.2 (blue) is A/C coupled battery voltage, and ch.3 (pink) is current into Vin. DC battery voltage is 3.9V.

Solar panel as input

I have also tried to disable the MPPT feature based on other posts I've read here, with that still not working on the solar charge case.

Would you have any ideas of what could be going wrong here? Thanks so much for any help!

 

 



removed docs
[edited by: DYoung at 2:56 PM (GMT -4) on 14 Jun 2021]
  • Quick update with more data taken today:

    • The above failure goes away when MPPT is turned back on (CONFIG_BITS_REG = 0x0002).
    • If MPPT is turned off (CONFIG_BITS_REG = 0x0000), the system will charge fine for 30 seconds, but then fail battery detection as shown above. We are looking into our other code, but could there be anything internal to the chip that occurs ~30 seconds after the config register is set?
    • If our debugger (STlink2) is connected over SWD to be able to monitor registers, the system can charge until the moment the MPPT setting is turned off.

    The above tests were completed after dark, so the solar panel could not be used. Instead I used a bench supply set at 8.3V with a 50 ohm resistor in-line with the +V connection.

    Thanks so much for any suggestions you may have!

  • 0
    •  Analog Employees 
    on Jun 14, 2021 8:19 AM in reply to DYoung

    Hi DYoung,

    Hope you are doing well.

    - This is the case for a solar panel since an MPPT algorithm will ensure to deliver the maximum power delivered from the solar panel.

    -The failing of battery may be a result of the MPPT turning off that will result to voltage at VIN falling below BATSENS+. As for the battery checking, refer to the attached diagram as the 30 second delay is a feature of the 4162-L charging state diagram.

    Regards,

    Siglo

  • Thanks for your reply!

    We found that with MPPT off and the uvlo register set at it's default value of 4.5V, in low light situations the LTC4162 would increase charge current quickly and cause the input voltage to fall ~0.5V below the 4.5V uvlo threshold. We assume that the IC didn't like having its input voltage taken below the 4.5V input voltage minimum

    We were able to get it to work by either turning MPPT on, or increasing the uvlo register level to something like 4.8V to give the control system a chance to reduce input current before the input voltage fell below the minimum of 4.5V.

    Thanks again for your help,

    Dave