I have a spotlight lamp that runs off a small 6V lead acid battery which no longer holds charge. I replaced it with some used laptop battery cells. The charger power adapter is just a simple transformer with full-bridge rectification; its output voltage depends on the load and can be as high as 11V . The lead acid battery is tolerant of overcharging, but not the lithium ion battery. So I decide to add circuitry to limit charging voltage to 8.4V. There is a blocking diode before the battery, so the output of the power adapter should be 9V or below. I put a 5.6V Zener off a 3.3V LDO to generate 8.9V. The circuit is first drawn using KiCAD before assembled on a prototype board that is small enough to be placed inside the power adapter.
