LTC4015 does not detect a lead acid battery to charge

Hello Zack.

This last month I repeated an order of battery chargers with the LTC4015, 150 pcs.

I still have the same problem of before: 5% of boards do not detect the battery to charge.

The battery to charge is a lead-acid, 24V (12 cells). 36AH.

To test the single board, I am using ALWAYS the same battery and the same load (a 10ohm resistor).

I set up the same condition for all the boards to avoid doubts.

With 143 boards everything works fine and as expected.

With 7 boards the charge doesn’t start.

I investigate a lot and:

Vin = 28.8V; Vbatt =25.4V.

If I switch off the power supply (Vin), the battery powers the load, Vsys = 25.4V, and I can read the right amperes sourced by the battery (I monitor at the display of my control panel). That means the telemetry is working, and the current sensor is OK. The battery discarges regularly and for sure the inrush current control is working.

When I switch on the power supply, Vsys = 28.8V, it powers the load BUT the battery, even if at 25.4V, in general at a lower voltage (25.4V against 28,8V) is not charged.

Every minute I suspend and enable the LTC4015 (setting and resetting the bit “suspend_charger” into the register 0x14 “CONFIG_BITS”). After every reset the LTC4015 enters in “absorbe_charge” (register 0x34), but no current flows towards the battery.

I checked all the voltages and they are OK. intVcc =4.96V, DRVcc is 5.02V, 2P5Vcc is 2.48V, DVcc is 5.02V.

I read continuously the register 0x39 SYSTEM_STATUS and I report at the display of my MCU. The content is 22C7 meaning that everything is as expected.

If I check by oscilloscope TG and BG they are fixed, no switching. TG is fixed at 25.4V a,d BG is fixed at 0V.

PDF 

I attach the schematic.

What do I have to investigate?

Any suggestion is appreciated.

Mario