Real-time battery history visualization with:
- Subscribes to /saltybot/battery_state for continuous battery data
- Rolling 30-minute history window with automatic data culling
- Dual-axis line chart: voltage (left, cyan) and percentage (right, amber)
- Canvas-based rendering for performance
- Charge/discharge rate calculation (last 5-minute average):
* Voltage rate in mV/min with up/down/stable indicator
* Percentage rate in %/min with up/down/stable indicator
- Grid overlay, axis labels, time labels, and legend
- Current stats display: voltage, percentage, rates
- Responsive canvas sizing
Integrated into TELEMETRY tab group as 'Battery History' tab.
Follows established canvas rendering and data subscription patterns.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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