Tool/software:
The setup on our board is this:
BQ25 <> BQ28 <> secondary battery protection.
We're trying to do "9.3.4 Device Power Up from Input Source".
Expected behavior:
- Connect battery
- Connect charging source
- Disconnect charging source
- Enter backup mode (rest of the circuit continues working)
Current behavior:
- Connect battery
- Connect charging source
- Disconnect charging source
- Did not enter backup mode
What we know and tested:
- The secondary protection disables battery discharge if battery is first thing connected. External voltage source resets it to normal. This is also according to the manufacturer.
- Applying a sufficient voltage on V_PACK (Fuel Gauge input) resets the secondary protection, and starts the BQ25798 as if it was "9.3.3. Device Power Up from Battery without Input Source".
- Checked both sides of Q3 (Ship FET) and they're at the same voltage, and measured SDRV at ~6V, so we're sure that the Ship FET is being driven.
- On the original setup, we checked the BQ25798 register CHRG_STAT_1 field CHG_STAT and it states "Not Charging".
- On the original setup, the voltage measured at V_PACK is 0.1V. This leads us to think that something is stopping the BQ25 from charging the battery pack to, in turn, reset the secondary protection back to normal.
Here is a picture of the schematic connection of the battery part of the BQ25:
