NiCad charger trickle current

[Chuj]

Do you have an idea as to why this charger is allegedly able to reduce the current (see description under the schematic)
to trickle charge rate once the battery is charged? I haven't been able to replicate that in an ltspice simulation though.

https://powersupply88.com/12-v-nicad-battery-charger-200mah.html?replytocom=617#respond

By the way an answer to Jerry Seerungum's questions (at the bottom of the page) would also be welcome.

 

[Audioguru]

You need to learn the basics of electronics to understand how that simple charger works:
1) A 12V Ni-Cad battery has 10 cells. Each cell is about 1.55V when fully charged according to the author so the battery will be 15.5V.
2) R1 turns on T1 but R2 and D1 cause it to produce a current that is limited to the voltage of the zener diode D1 (5.6V) minus the base-emitter voltage of T1 (0.6V) divided by the value of R2 (68 ohms) so the charging current will be about 74mA.
3) VR1 is adjusted so that D3 begins to turn on T2 when the output voltage reaches 15.5V.
When the battery is fully charged then its charging current automatically drops since the voltage is limited.

BUT it is an awful way to charge a Ni-Cad battery because the fully charged voltage is different with different cells and the fully charged voltage changes when the temperature changes.

A battery charger IC detects the output voltage dropping a little when the battery is beginning to heat and its internal pressure begins rising when it becomes fully charged.

Use Ohm's Law and simple arithmetic to calculate the value of R2 for a different amount of charging current.