NC courtesy (interior) light issue.

[Ston]

Hi, I've recently got an MC MX5 and its bit sitting around a bit prior to my purchase, so I know the car battery isn't in a great state. This isn't too much of a problem as I only use it on the weekend and when I need to, I trickle charge the battery to full.

The problem is that the interior/courtesy light doesn't always work. When it does work, it's consistently after the battery has been fully charged. If I leave it for a day or 2, it then won't.

To try and solve the issues, I've replaced the battery (with a bog standard one as its all I could get my hands on at the time) but now it hasn't worked since the switch over. Is anyone aware of some minimum volt/amp cold crank requirements for the interior light? Does it have a safety cut of just in case its been left on, not to drain the battery fully and both batteries I have, haven't been able to meet it, so the car thought the battery was on the way out and doesn't trigger the interior light?

Any thoughts appreciated.