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Battery Management - Chargers Forum
BQ27500-v130 BAT_GD disables BQ24075 due to temperature in discharge mode
we are seeing our system (which is very similar to SLUA490A BQ27500-v130/BQ24075 by TI)shutdown due to temperature in both charge and discharge modes.
we originally believed that the OT Chg and OT Dsg thresholds were what trip the BAT_GD line if temperature is out of range,
as these are under safety. I now think they just set the flags OTC and OTD. We have the default PFC=01. in the opconfig register.
I now suspect that temperature can disable a charger via BAT_GD through CHG Inhibit. I upped the value to 75C on the eval board
and showed that BAT_GD will de-assert in both Charge and Discharge modes. My question is why does it also do this in Discharge
mode while the documentation only states it does this in charge mode?
I see there are other registers for Discharge Current, Charge Current and Quit Current. there is some discussion of how the device
may sometimes be unsure if it is actually charging or discharging. could this be what is going on here?
I also watch the CHG and DSG bits in the flags register. when the BQ is in SLEEP with no charger, the CHG bit is set.
when we wake it up by adding a load, both bits are set. why does the CHG bit stay set when it is cleary in discharge mode?
Sometimes we have seen our own design shutdown at 45C when it is sleeping, but never when it is running as well.
We would appreciate any help we can get on this!!
1) The gauge will toggle BAT_GD when the temp goes outside the Charge Inhibit range. This happens regardless of the current state, because the gauge does not want to wait until the cell starts charging in order to disable charging. This results in an unsafe condition.
2) The Charge, Discharge, and Quit current govern the state transitions for the gauge. The gauge can be in CHARGE, DISCHARGE or RELAX modes. In each of these modes, the gauge is measuring and reporting current, but the ancillary functions are different. In CHARGE, the gauge basically just counts coulombs and looks for an end-of-charge condition. In DISCHARGE, the gauge counts coulombs, runs IT simulations, and provides updated estimates for runtime and remaining capacity. In RELAX, the gauge is looking to take OCV measurements of the cell, and update QMax based on these measurements.
The Charge, Discharge, and Quit currents determine the transitions between these states. Generally the gauge does not need to count coulombs if the charge/discharge current is on the order of C/10 or less, because this effectively looks like an open circuit for the cell, and the gauge can get accurate OCV measurements. Therefore, these parameters are usually set in this vicinity.
3) I admit that our naming convention for the CHG/DSG bits is weird and confusing. DSG means "Cell is currently discharging". CHG means "Cell is OK to charge"
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