BQ27Z561: BQ25185DLHR LOW CURRENT CHARGING

Hello Craig, 

Can you please clarify what you are referring to with the battery pack cutoff and the charger cutoff? It sounds like the charger cutoff is the current that the charge tapers down to before cutting off, is this correct? 

Regards, 

Jonny. 

sure. I'm looking at the inr18650-35E data sheet. It specifies a standard charge (constant current/constant voltage), of 1700mA (constnat current mode), 4.2V (constant voltage mode), that dwindles to 68mA cutoff current to achieve rated capacity of 3400mAh. We have a 4p pack so 4*68mA = 272mA cutoff for the pack.

Currently our charger cutoff (when the charger stops providing current) is 30mA. Solar currents are expected around 50mA. 

30mA is well below the battery cutoff current...Thus this charger isn't really operating like one would expect where it's usually expeceted that charger current cutoff is at or above the pack cutoff. 

Basically 'current' is not how we'll detect a full charge with the charger-Is that how the gauge alogrithm works? It determines charge purely on current?

We might/probably have to do some manual control of the charger to preclude overcharging. 

What should I use for the term charge taper current? Is it possible to run the learning calibration assuming a standard charger, and then use that image in the solar charge setup (would it still gauge accuratetly?).

Will this gauge work with our current setup?

Thanks, Jonny.

Here I was asking a similar question here:

BQ27Z561: bq27z561 learning cycle question - Power management forum - Power management - TI E2E support forums

I guess my question is the charge term taper current, will not really indicate end of charge in our setup. Our charge will always be supply less than battery cutoff so it's not a good marker for FULL charge. Is the taper current used to indicated FULL charge?

We'll have to manually verify end of charge by other means (shutdown the charge process in software monitoring voltage, or setting a timer....TBD).

Is the gauge smart of enough to track charge for the little current that is injected to it OR will it see a low current and say FULL charge when its actually just in the middle of of charge? - THAT IS MY CONCERN.

This is the IT algorithm doc:
Evaluation 3: Theory/Implementation Impedance Track Battery Fuel-Gauging Algo (R ev. B

From there:

DODcharge is assigned equal to DOD0 aft first DOD0 update after CHARGE TERMINATION BY TAPER CURRENT

where 

Qstart = Qmax (DOD0 - DODcharge)

Thus Qstart would equal ZERO and thus be equivalent to a full charge? 


I'm concerned that the taper current is being used to determine FULL CHARGE


Additionally. Our application has an active state and sleep state with current ranges from 1.8A during message transmission, averaging  maybe 500mA and hopefully sub-milliamp (microamp) current levels during sleep. The sub=milliamp is hopeful/optimized levels we have NOT reached yet. Are realistic current discharge is probably a couple milliamps

Will the gauge work accurately given the low current draw?

What DSG Current Threshold should be used given our application-Should it be based off the wake state?

I'm not an expert in the alogrithm. Most of my time spent on the driver was implementing the calibration procedures on a microcontroller and working through errors in the documentation. 

From a battery perspective (not an expert), the end of charge is determined by 4.2V being reached (Constant voltage mode entered) and the current dwindling decreasing from a current higher than cutoff current (say 1700mA), down to the cutoff current. 

We can't track this decrease as our solar charge current is at best 50ma (always below BATTERY cutoff) and thus cannot use this for determining charge. To me, the IT alogrithm indicates taper current (perecursor to cutoff current) is utilized in indicating full charge

IF the gauge is using current level as the determining factor to indicate FULL CHARGE then i fear it could indicate full charge when it's actually not. OR the gauge doesn't work for our setup.

I feel the gauge would work fine with a standard charger (High charge current in CC phase that dwindle to cutoff current in CV phase)...but our current setup is suspect to me.

So three 'cutoff currents at play'

CHARGER cutoff current -    30mA

GAUGE Assigned taper current (related to the cutoff current of charger), set by us but deduced from charger...say 35 mA

BATTERY cutoff current  at standard charge 1700mA CC mode, 4.2V CV mode, 68mA for a cell--->272mA for the 4p pack

I'm not certain these are aligned-hence I'm concerned about hw (charger->gauge->battery) compatibility

Thanks!

Hello,

Today's a national holiday. We will get back to you tomorrow.

Regards,

TI Apps Team