TRV604 18650 adapter

The TRV604 wifi valves run natively on 3 AA cells, which in the case of NiMh rechargeable cells provide ca 7 Wh (3-4.5V). This energy allows the TRV604 to run between 1-6 months, depending on temperature changes, heat source temperature variation, user interaction etc. This AA to 18650 (1s2p) adapter and cover allows to run the valve with one or two 18650 Lithium cells with a TP4056 board as BMS and USB-C charger. Two 18650 cells provide 18-25 Wh which allows to more than triple the TRV604 run time. The cells need to be welded with two soldered wires to connect to the TP4056. Two more soldered wires connect the TP4056 and two cylinder head M3 screws that make contact to the TRV battery compartement. The welded cells are strapped to the adapter by means of two cable ties. The USB-C port is accessible without removing the cover. I would strongly recommend to change R3 on TP4056 to 3.3kOhm or more, as the standard 1A limit may lead to thermal damage of the TP4056 when charging batteries with less than 3.5V. All parts can be printed with PLA in parallel. The snap-in back cover may be omitted if the opening is horizontal or facing downward.