fix battery drain when battery charge threshold is set

[hojjatabdollahi]

When the battery sustainer parks the pack at a charge limit it enters CHARGE_CONTROL_IDLE, which sets manual current and voltage to 0. charge_request() then takes the !voltage || !current path and, on NVDC chargers, requests VBAT + one voltage_step. Per the comment on that branch, the margin exists so that "the BGATE FET body diode" does not "conduct and discharge the battery".

However one voltage_step does not appear to be enough margin: if the rail sags below VBAT under load the diode conducts and the pack supplies part of the system current. Charge current is pinned at 0 in IDLE, so it is never replenished, SoC falls until it crosses the sustainer's lower threshold and triggers a full recharge.

This is what I observe: a shallow charge/discharge between min/max charge threshold. The EC automatically sets max-5 as the min. When I set the charge threshold to 60%, it charges to 60%, then goes to IDLE (observed in ec console) but then as soon as there is some load on the system, it starts drawing a constant 5-9W from the battery. Until it gets to 55% and charges back up to 60%.

Stock EC, Min=55%, Max=60%

• Note it stayed parked for some time, but then suddenly started draining at a constant rate

Stock EC, Min=Max=60%

• Ignore the first couple of charge/discharges, I was testing something.
• Notice how much more it stayed parked, going below 60% switches to Normal mode and replenishes the diode

Patched EC, Min=55%, Max=60%

This patch potentially applies to other boards too.

My framework forum post: https://community.frame.work/t/puzzling-battery-threshold-behavior/83941

EDIT: looks like this was found before: here.