Li-ion battery with charge indicator for mobile medical devices

The task

Mobile medical devices are still often powered by primary cells. For the user that means changing batteries; for the manufacturer it means that a rechargeable replacement has to fit the same devices, with the same contact surfaces, the same label and no change to the device.

The client wanted this battery as a product in its own right: several sizes for its device family, charged through a USB-C socket directly on the battery, with a charge indicator visible from outside and a standby time long enough for months of storage. Development followed the requirements of IEC 60601-1 and of IEC 62133-2 for cells and batteries, within the scope of the MDR and the Battery Regulation. And inside the device, the battery must not do anything the device does not expect.

The solution

The electronics sit between the cell and the contact surfaces and decide what the device gets to see. While the battery is charging, the output is disconnected, so the device cannot be operated from the charger. Protection functions shut down in the event of a fault: an impermissible voltage source at the USB socket, a short circuit at the contacts or overcharging of the cell. A fuel gauge with a profile we measured specifically for this cell provides the state of charge, which an RGB LED shows on the outside through a light guide.

All sizes use the same PCB and the same firmware; the variant is a matter of configuration. The firmware is built on semf, our software architecture, with a state machine for the battery state. In standby, the controller goes into deep sleep and wakes only to check the cell.

We validated the system design on functional samples before finalising the hardware. The complete device was tested against the client's acceptance tests. For the transfer to manufacturing, the assembly sequence and the documentation are set up so that all sizes are built from the same parts.

The project delivered the system requirements, the electronics with manufacturing documentation, the firmware and the documentation for manufacturing.