Salt Meter Battery Adaption

My salt meter uses four these AG13 batteries. It's my only device that uses this battery size, so I don't keep spares on-hand. When it came time to replacing these batteries, I was left wanting. Instead of buying some more AG13 batteries, I decided to modify the salt meter to work with the common 18650 (rechargeable LiPo) battery.

To accommodate the larger 18650 battery, I made a modification to the salt meter battery lid. I extended it to enclose the 18650 battery assembly via a 3D printed part. It adds about 80mm to the overall salt meter length, but not a problem for my use.

The 18650 battery provides a nominal 3.7V. A bit low to reliably operate the salt meter. The salt meter operates nominally at 6V (4 x AG13 batteries) and displays a low voltage indicator when the voltage drops below 4V. I use a MT3608 voltage step up inverter module to lift the voltage to 6V and all is good. The MT3608 does not have reverse battery protection and will fry itself if the 18650 is in