Battery architecture
Settled the pack architecture around what the onboard charger expects rather than around maximum capacity. A 3S lithium-ion pack built from 18650 cells sits inside the charger's input window, which keeps charging on the platform and keeps the onboard current monitoring meaningful. A larger pack with an external charger was the obvious alternative and was rejected: it would mean the rover leaves the bench to charge, and the integrated telemetry stops describing the thing that is actually powering the robot.
What we learned
- Choosing the pack to fit the charger, rather than the charger to fit the pack, removed an entire subsystem from the plan.
- Individually replaceable cells matter more than pack density on a platform that will be opened dozens of times.
- We still have no runtime figure, and will not publish one until a pack has been drawn down under a real drive-and-compute load.
Next steps
- Measure capacity and runtime under a representative load once the drivetrain is back together.
- Confirm cell balance at the top of charge across several cycles.
Hardware referenced
- 3S lithium-ion packAssembled in-house from 18650 cells3S (11.1 V nominal), 18650 cells
- Waveshare onboard power systemWaveshareIntegrated 3S charging, regulated 5 V rail, INA219 monitoring
Open challenges touched
Charging verification
The onboard charger appears to bring the 3S pack up correctly, but termination voltage, termination behaviour, and cell balance at the top of charge have not been confirmed with an external meter. Until they are, charging stays supervised and the platform does not sit on the charger unattended.
Runtime under load is unknown
No figure is published for endurance because none has been measured. The pack has never been drawn down under a representative drive-and-compute load, and estimating it from cell capacity would be a guess presented as a specification.
Decisions referenced
3S lithium-ion pack matched to the onboard charger
Battery architecture was chosen to match the input window of the onboard charge circuit rather than to maximise capacity.
- Matching the charger keeps charging on the platform, which is the difference between a rover that gets used and one that gets disassembled between sessions.
- 18650 cells are replaceable individually, which suits a platform that will be rebuilt many times.
- Session length matters more than endurance at this stage; the platform is never far from a bench.
- Higher-voltage pack with an external charger — Rejected — more capacity, but charging would leave the platform and the onboard monitoring would stop being meaningful.
- Lithium-polymer — Rejected — better energy density, worse cycle life, and higher handling risk for no benefit we need.
Outcome Pack is fitted and charging. Capacity under load remains unmeasured and is deliberately unpublished.
Images
Cell arrangement and lead routing before the pack went into the bay.