BQ24074: Will not charge dead battery when system load is connected

Peter Naylor

Part Number: BQ24074

Hi all,

I am using the BQ24074 with a solar panel at the input (6V, 370mA), a 3.7V Li-Ion battery, and a system load. When the battery goes dead, the 6V solar panel will not charge the battery unless I remove the system load. If I remove the load, the battery will charge as expected. Once the battery has a tiny charge, I can reconnect the load and away we go. This is a problem, because it requires our customers to turn OFF the device (system load) when battery dies to start charging again.

Can anyone help me by explaining why this is happening? What may be possible solutions?

Thank you!

over 7 years ago

0 Raheem over 7 years ago

TI__Mastermind 21590 points

Hello Peter,
How many mA is the load and what is the input current limit?
Regards,
Raheem

0 Peter Naylor over 7 years ago in reply to Raheem

Prodigy 60 points

Hi Raheem,

The load is dynamic, but draws about 500mA at startup. ILIM is set to the max of 1.5A (1.1kOhm resistor)

Thanks,
Peter

0 Raheem over 7 years ago in reply to Peter Naylor

TI__Mastermind 21590 points

Peter,
Can you please post your schematic?
Regards,
Raheem

0 Peter Naylor over 7 years ago in reply to Raheem

Prodigy 60 points

0 Raheem over 7 years ago in reply to Peter Naylor

TI__Mastermind 21590 points

Hello Peter,
Based on your schematic, you have SYSOFF floating. For proper operation you want this pin connected either high or low. Since you want to charge the battery, i'll suggest connecting SYSOFF to ground so the BATFET stays on.
Regards,
Raheem

0 Peter Naylor over 7 years ago in reply to Raheem

Prodigy 60 points

Hi Raheem, I tried connecting SYSOFF to ground, and it still does not charge. I think this is because the load is drawing all the current from the battery/solar so the charge controller never gets a chance to 'turn on'. It was suggested by another person that we should never let a Li-Ion battery run down to 0%, so I will be adding application code that shuts down the functionality of the load at maybe 3% battery charge, and then start back up the charge has risen above 5%.

0 Raheem over 7 years ago in reply to Peter Naylor

TI__Mastermind 21590 points

Hello Peter,
You stated that the load is dynamic but pulls about 500mA at startup. This shouldn't be a problem as the battery should be charged with the pre-charge current which is about 1/10th the fast charge current. What is the status of the /CHG pin? Leave TMR floating and see if /CHG starts to blink at a frequency of 2Hz after 30 mins. If that's the case, connect TMR to ground again and wait for a while to see if the charger enters fast charge. You generally don't want to deeply discharge the battery as this reduces the lifespan of the battery.
Regards,
Raheem

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BQ24074: Will not charge dead battery when system load is connected