Torch slow charges when fully depleted

[nseidle]

When the pack is fully depleted (~6.0V) the device charges so slowly that the charging system will shut off after an hour. Reconnecting, the device will continue to charge at ~2% per hour (~1W) but will shut off again if left alone. This is because the MP27692A has a ‘trickle charge and pre-charge timer’ length of 1 hour.

The Torch uses a 7.2V 6.8Ahr 49Whr battery. The Torch uses two ICs related to the batteries:

• BQ40Z50 Battery Pack Manager / Fuel gauge
• MP27692A Power Management / Charger

If the battery voltage is below VBATT_PRE+ (6.8V default), the total safety timer for both trickle charge and pre-charge is 1 hour. The complete charge safety timer includes a trickle charge and pre-charge timer.

The MP27692A trickle charges at 188mA until the pack gets to 6.8V (default) when it starts pre-charge at 180mA (default). These are so conservative/low that the device hits the 1 hour limit before getting to high rates of charging.

Possible solutions:

• We can lower the fast-charge voltage threshold from 6.8V (default) to 6.6V.
• We can regularly reset the timer while the battery is <6.8V - but only while the device is powered on.
• We can disable the timer (the length cannot be modified).
• We can increase the precharge current from 180mA to up to 840mA.

Because most users will turn off the device to charge it, we cannot rely on the timer to be regularly reset.

Doing some tests, lowering the pre-charge voltage and increasing the current to 840mA allowed the device to get to the fast charge state within the hour time limit. Let's do that, and implement a timer reset as well. I'm hesitant to modify the OTP settings of the MP27692A for now, so this approach will work with the following limitations:

• If a unit is completely depleted, the MP27692A's previous settings are lost, so a slow trickle charge will occur until the user powers on the unit once. I think a fully depleted device will be a rare enough occurrence. If enough users get into this state, and/or request OTP changes, then we can implement them.

[nseidle]

This is in v1.0.