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LTC4015 will not start charging

Category: Hardware
Product Number: LTC4015

We have a battery charger circuit based on the LTC4015.  It is strapped for 6 cell Lead Acid, programmed mode.  I connect a battery, which is showing 12.5 V open circuit.  Then I connect input power, a benchtop supply set to 15 V, and current limit 10A.

There is only about 20 mA load on the SYS side.  The charger is not charging, the battery current is 4 or 5 mA drawn from the battery, that is, - 5 mA charge current, and the SW is not switching. 

If I power up the circuit with the same 15 V DC at Vin with the battery disconnected, I see about 7 V at the battery terminals. 

I am looking at the telemetry over the I2C bus.  If I read back CHARGER_STATE I see 0x0200 (yes, I know its little-endian), so absorb_charge is set.  CHARGE_STATUS is 0x0000, so its not in input undervoltage, nor input overcurrent, nor constant current limit, nor constant voltage limit. 

I read back NTC_RATIO, which is around 0x21E4, which seems a bit off from the actual room temp, but not too warm as to cause temperature compensation foldback.

The measurement system can read back the battery voltage correctly, VBAT is 0x3F92, which scales to 12.51 V.

If I set suspend_charger, then clear it, the charger still does not start.

Since with no battery, I see 7 V, I assume its in limit for some other reason?

Anyone have ideas on how can I find out why the charger is shut down?