Although I've been tinkering with bits, this was another home fix exploit. The lights didn't work at all (outdoor LEDs).

At first I thought it was corrosion on the battery contacts (which is a good first port of call), but when I injected bench supply power I could see it was drawing amps - which meant that either the board was not putting power to the LEDs or the LEDs were bad.

I checked the contacts for the LEDs with a multimeter, and it showed a voltage (bouncing - which would make sense if the lights pulsate).

At it happened, the string of LEDs were a simple positive and negative wire running all the way through (other systems, which allow for individual failures, will have a third wire - I forget exactly how it works, but it means a bulb or LED can go out, btu the line continues to work).

In this case, I kept cutting the wire in half until I found the LEDs working - which happened only when I had a single LED (bad luck me), but then I started splicing large runs of the LEDs back together.

I'm not 100% sure I'd put them outside again (since I used shrink wrap rubber tubing to protect the wires, I doubt if it's fully waterproof), but they do work again, so it's one more thing saved from landfill.