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BQ24040: termination current

Yehoram Bar
Prodigy 50 points
Part Number: BQ24040
Other Parts Discussed in Thread: BQ25100

Hello,

In our system we have a very low capacity of Li ceramic battery (45mAh).

we set the max charging current to be 0.75C=33.7mA.

according to the battery spec the termination current should be 0.05C=2.25mA.

according to the BQ24040 datasheets, we can define the termination current to be 5% to 50% of Iout (1.68mA to 16.85mA).

we found in TI forum that the accuracy of the termination current is guarantee down to 6mA (but it is not written in the datasheets).

what does it mean? do you have some accuracy information?

if I will still define the termination current to be 6.6% (2.25mA) what might be the problem?

is there any kind of risk involved with that? might the battery explode in case of over charge (although it is CV in that phase and it make no sense)?

or the actual termination current might varied a lot (let say between 1mA to 5mA for example)?

Thanks a lot!

Yehoram

   

  • 0
    TI__Genius 12485 points
    Yehoram,

    The consequence of setting a larger termination current is that you might be losing on capacity.

    After the CC phase, when we switch to the CV phase, the battery current will slowly taper. As soon as it reaches termination current, the charging process is done.

    Now setting it higher will mean that you might not be charging it all the way to a 100% but maybe lower. Please check with the battery vendor for explode or heating issues.

    Some folks try to change the resistor on the fly to adjust preterm and term currents based on the voltage of the battery. (which I wouldn't recommend unless you trust the MCU :) )

    You are correct at CV, the voltage accuracy plays a more significant role.

    Reagards,
    Gautham
  • 0
    TI__Guru* 75936 points
    For small battery application the bq25100 is a better device it is designed to support low charge and termination currents with very good accuracy.

    The bq24040 is designed to support up to 1A and can be scaled down to support lower currents but accuracy of the current decreases for both charge and termination.

    Area of risk:
    1.) Fast charge "0.75C=33.7mA"-- the K-ISET will vary from 480 to 600 about 10%. At lower charge current the accuracy is lower.Charge current could be lower or higher than target.
    2.) Termination current "6.6% (2.25mA)"-- The K-TERM will vary from 174 to 224 about 12%. Again lower or higher termination currents.

    The higher or lower charge current could lead to the longer charge timer at lower currents. Higher currents increase risk of exceeding safe charge current.
    Termination current that are higher will reduce charge on battery, while lower termination points may lead to no termination. A no termination condition will set the fault timer and stop charge.