So, I was just updating the project details for this project, and I happened to see this blog post about reading the DC voltage input on the TS-7670v2.
I haven’t yet gotten around to finishing the power meters that I was building which would otherwise be reading these values directly, but they were basically going to connect via Modbus to the TS-7670v2 anyway. One of its roles, aside from routing between the physical management network (IPMI and switch console access), was to monitor the battery.
I will have to explore this. Collectd doesn’t have a general-purpose I²C module, but it does have one for barometer modules, so with a bit of work, I could make one to measure the voltage input which would tell me what the battery is doing.