Add automated cell balancing for Tesla batteries (bleed+charge cycle)

[hpetersen]

Summary

Adds an automated balancing procedure for Tesla batteries that solves the problem of high cell deviation causing premature charge termination.

The problem: When one cell reaches e.g. 3650 mV while others are at 3300 mV, the BMS stops charging and reports 100% SOC — but real capacity may only be ~75%. Manual intervention is tedious and time consuming.

The solution: An automated state machine that exploits Tesla's built-in passive bleed procedure (active when contactors are open) and cycles between bleed and charge phases until cells are balanced, automatically handling the isolation clear + BMS reset sequence required between cycles.

How it works

1. Open contactors — Tesla BMS passively bleeds high cells down via internal resistors
2. Wait until cell deviation drops below the configured threshold (default 20 mV)
3. Automatically run isolation clear + BMS reset (up to 5 retries if contactor close fails)
4. Close contactors and charge at a limited power (default 500 W, ~1.4 A at 350 V)
5. Re-bleed if deviation exceeds the re-bleed threshold (default 50 mV) during charging
6. Stop when any cell reaches the configured max voltage ceiling (default 3650 mV LFP)

Configurable settings (via webserver)

Setting	Default	Description
Max cell voltage ceiling	3650 mV	Hard stop — never exceeded
Charge power limit	500 W	Gentle charge to protect imbalanced cells
Open contactors above deviation	50 mV	Re-bleed trigger during charging
Close contactors below deviation	20 mV	Exit bleed, start charging

Events & MQTT

Six new events (all automatically published to MQTT via the existing event system):

Event	Level	Trigger
AUTO_BALANCE_START	INFO	Feature activated
AUTO_BALANCE_CONTACTORS_OPEN	INFO	Bleed phase started (data = cycle number)
AUTO_BALANCE_CONTACTORS_CLOSED	INFO	Charge phase started (data = cycle number)
AUTO_BALANCE_STOP	INFO	Ceiling hit, normal operation restored
AUTO_BALANCE_RETRY	WARNING	Contactor close failed, retrying (data = attempt number)
AUTO_BALANCE_ERROR	ERROR	Hard abort (data: 1=open timeout, 2=close failed after 5 attempts)

Files changed

• TESLA-BATTERY.cpp — 12-state machine in update_values(), charge power cap, contactor hold flag in transmit_can()
• TESLA-BATTERY.h — 5 new member variables
• datalayer.h — 5 new settings fields
• events.h / events.cpp — 6 new event types with messages
• webserver.cpp — 5 new HTTP routes
• settings_html.cpp — template vars, JS functions, HTML section (Tesla-only, hidden for other chemistries)

Notes

• Tesla Model 3/Y only (chassis type > 1), same guard as existing manual balancing
• Charge power limit is independent from the manual balancing float power
• Contactors are held open by sending ACCESSORY/GOING_DOWN/OFF instead of DRIVE on CAN 0x221 — no hardware changes required
• State machine runs on the existing 1-second update_values() tick — negligible CPU/memory impact