Recently I was given a Feron EL15 rechargeable light fixture with bricked 4 V gel battery. This device cannot run directly on mains. It can only either charge battery from mains or run on that battery. So if battery is bricked, the whole unit becomes useless. I decided to repurpose it to run directly on mains only.
I charged the battery overnight, but it did not accept charge. Since there is no line isolation in this fixture, I first disconnected it from mains, and only then measured the battery voltage. There were only 260 millivolts per the whole battery, or 130 millivolts per cell. That's how I noticed that it was bricked. Nothing strange, it's from 2010! So I removed and properly recycled it, avoiding short circuiting during both procedures.
The board used in this fixture requires major changes to run LEDs directly from mains. Also, I want line isolation. So I decided to remove not only battery, but also power cord connector, and then attach a line isolated power supply at correct polarity instead of the removed battery. Modern micro-USB power supplies can deliver about 7 V at no load, which drops to slightly more than 5 V when loaded with 800 or 1000 mA, depending on type. A fully charged 4 V gel battery that is going to be replaced by this supply gave about 5 V at no load before it was bricked, so I had to use something else. Pre-micro-USB power supplies can have either higher or lower parameters. This particular one produces 6 V at no load. Its non-removable output cord was torn, and I attached a new one, avoiding touching leads of filter capacitor at primary side, that stores charge after disconnection from mains.
After all disconnections and connections were done, I made some measurements. There is a three-position high-off-low switch in this fixture. At high setting, it consumes 600 mA (20 mA per LED), and power supply voltage drops to 4.5 V. It exceeds 550 mA which this power supply can safely deliver, so I decided not to run the fixture in this mode. At low setting, current is 460 mA (15.(3) mA per LED), and power supply volatge drops to 5.4 V.
Here is the fixture in action. Its 30 LEDs connected in parallel have ~6400 K color temperature, which is close to what a vintage monochrome CRT phosphor has. But, despite its low power (less than 3 W), it is much brighter than a typical 1960s large tube-type B&W TV set consuming ~180 W! Therefore I also use it as a SAD lamp and as a diffused light source for macro photography.