I asked how I easily could detect when a charger has reached 4V. This caused quite a discussion including some offensive postings here.
The item is a solar powered camping lamp. The indicator for fully charged can be discussed, but we have to see beyond ourselves. For people not into electronics, this is useful for them. They may ask this. They do not understand how a battery can be overcharged etc., and this LED will help those. From a productive point of view, this will extend lifespan and by that the quality of the product. As firstname.lastname@example.org correctly understood, I needed a one-bit-voltmeter.
John Fields suggested the LT6703, which is one option, I tried it, and it works. Thanks. Michael Karas suggested the ADM803, which is very much the same. There is another similar device, TC54 series from MicroChip, which meets my requirements slightly better, so I ended up using that one. Using a transistor it will only work when it is charging.
Systems using TL(c)431, and zeners below ~6V does not really work. Zeners below ~6V will conduct, and therefore always be on. And as posted, what happens above the Zener voltage, as when the battery is not connected. As it is integrated, it should always be there. Other issues as accuracy was considered.
Finally there is Phil Allison, who really did not add with something useful. Please find someone you own age to play with.