Other Parts Discussed in Thread: BQ25798,
Hi Mike/Jeff,
Thanks for your response. Unfortunately, I made the initial posting as a “related question” to Christof Dunkels post, yet my problem has nothing to do with MPPT. For this discussion I am testing the BQ25798 with a 10watt 6 volt panel with Vmp~5.5volts, charging 2S Lipo batteries.
The datasheet states:
“Page 3:
Besides the I2C host controlled charging mode, this charger also supports autonomous charging mode. After power up, the charging is enabled with default register settings. The device can complete a charging cycle without any software engagements. It detects battery voltage and charges the battery in different phases: trickle charging, pre-charging, constant current (CC) charging and constant voltage (CV) charging. At the end of the charging cycle, the charger automatically terminates when the charge current is below a pre-set limit (termination current) in the constant voltage phase. When the full battery falls below the recharge threshold, the charger will automatically start another charging cycle.
And again on Page 22:
In addition to the I2C host controlled charging mode, BQ25798 also supports autonomous charging mode. After power up, the charging is defaulted enabled with all the registers default settings. The device can complete a charging cycle without any software engagements. It detects battery voltage and charges the battery in different phases: trickle charging, pre-charging, constant current (CC) charging and constant voltage (CV) charging. At the end of the charging cycle, the charger automatically terminates when the charge current is below a pre-set limit (termination current) in the constant voltage phase. When the full battery falls below the recharge threshold, the charger will automatically start another charging cycle.”
In our testing of the chip, we have observed the occasional but unfortunate situation where in standalone mode, without any MCU control whatsoever (I am however using the 2400 to monitor the registers), the chip enters Hi-Z mode and never comes out of it, unless the solar panel input is completely disconnected and reconnected. For us, this is an untenable situation and would potentially waste a day of solar generation at best. I have made a video of such, if you are interested in viewing, please provide contact details.
The problem occurs when the device is in PowerGood status and Vbus falls to approximately 3.42volts and Hi-Z is triggered. (Let’s say a cloud momentarily passes over.) When full sunshine returns the Vbus rises back to ~6v, and even after 7 minutes after which the poor source qualification should have re-enabled/cleared the Hi-Z bit it doesn’t happen. The only way to res-establish charge is by myself de-ticking the EN-HiZ (which obviously entails MCU interaction), or cycling the panel.
I have easily replicated the situation on a lab power supply.
Christof Dunkel’s post states:
“Another issue I found is that after some time where no input voltage was applied, it then raise slowly, it fails to start charging. After a quick Vin power cycle however it comes back normally.”
which may be a related phenomenon.
At present we are trying to circumvent the issue by implementing a solar panel input hysteresis circuit. Are you aware of this problem, and is there an easy solution?
Regards
Robert.
