Hey all,

I'm trying to charge (let's say) a single Li ion battery (float charge) to
4.0V.

The maximum charge current will be controlled with a BJT (tell me if there'=
s
a better way) and so I've been doing some simulations to see if I can
actually get 4.0V across the battery this way.

Screenshot of simulation: http://solarwind.byethost7.com/pic4.png

I'm pretending that R2 is the battery, gradually increasing its impedance a=
s
it's state of charge increases. The voltage across the battery never seems
to hit 4.0V. I'm doing another simulation with R2 from 1-100k and that seem=
s
to get closer and closer to 4.0V as the current goes to 0 and the BJT's
impedance rises rapidly.

I can understand that at the beginning, the BJT needs to decrease impedance
as R2 increases in order to maintain the overall impedance so that the set
amount of current goes through. I don't understand though, why just before
40 ohms on the x axis, the BJT impedance increases again. Shouldn't the BJT
try and lower its impedance as much as possible so that the most current ca=
n
go through (even though the maximum amount allowed by the base current can
no longer go through)?

What is this minimum impedance and why is it there?
--=20
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