BQ24074: Problems with BQ24074 never reaching 4.2 Volts

Kitty Last Name

Part Number: BQ24074

I am using the BQ24074 in a charger design and I am seeing a problem. I have set the device to a maximum of 1A charge rate and the test cell is a 2000mAh Lithium Polymer (from Farnell). Starting from a discharged state, when I look at a log of the charging cycle I can see that it initially charges at 1A but after about 30 minutes the current starts to drop off and - importantly - the output voltage does not rise to compensate for this and maintain the programmed charge rate - it never gets to 4.2 volts. Also, final charging voltage never reaches 4.2 volts - it only gets to approximately 4.15 volts.

Can anyone tell me what I might be doing wrong here?

over 5 years ago

0 Raheem over 5 years ago

TI__Mastermind 21590 points

Hello,

This looks like the charger is entering the CV phase early.

When you measure the battery voltage, is this on the battery side or on the BAT pin side?

Also what voltage do you measure on BAT just before the charge is terminated? This is on the IC side.

Is there any impedance between BAT pin and battery?

Does any ground shifting occur during charging?

0 Kitty Last Name over 5 years ago in reply to Raheem

Prodigy 20 points

Hi Raheem,

I am measuring the battery voltage on the battery side. There is no impedance between BAT pin and battery. The leads are very short and there is no ammeter in the way.

FYI, the graph below is a plot of minutes against volts and shows the voltage while charging a 2Ah cell with the max current set to 990mA.

I left it running for just over 7 hours and the charge never terminated. As you can see from the graph, the voltage increased more and more gradually over time but it never got to 4.2 volts. This is not as per the data sheet which says that it should maintain max current until it reaches 4.2 volts and this should be maintained until the current is less than I(TERM) at which point it should switch off. Note that we have tested this on several different devices with three different LiPo cells.

There is no ground shifting during charging.