I have copied this from another thread as it may help the original poster understand some of the reasoning.
If you do have any issues with stop start not working because of the battery (Your display will show which of the 3 elements are stopping it from working.) try disconnecting the earth from the battery for a few minutes.
This is for the ND1 so not stop start. https://www.mx5manual.com/page.html?docid=SM352783 The note about opening or closing the doors with the battery disconnected is ‘interesting’.
Roadster_RobbieMX-5 OC Technical Adviser
If you have an MX-5 with iStop, I’d connect the negative side of any ‘smart charger’ to the chassis for earth rather than the battery terminal. There is a current sensor on the negative battery terminal so connecting it straight to the terminal means the current sensor won’t ‘see’ any current going into the battery while charging. This may upset the PCMs calculated state of charge of the battery and may affect iStop operation.
