kell said:
I won't use unlimited current.
In the original post I asked how to implement the transition from the
absorption stage to float stage. I read your response in that context.
It didn't occur to me you'd be talking about bulk charging!
Regarding the original question, I know I saw a fairly simple circuit
somewhere that used an op-amp.
I think we're in heated agreement.
Take a close look at your circuit.
Ignoring the issues when you hook a dead battery up to it. Let's assume
a subset of all possible initial conditions that don't go up in smoke.
U2 is a comparator providing digital latching function.
It's a two state state-machine controlled by S1 and the battery voltage.
During the first part of the charge cycle,
U2 does absolutely nothing. Vout gets pulled down with prejudice to
almost the battery voltage and tracks up as the battery charges and it's
voltage increases. So, you have initial charge current limited only by
the 0.2 ohm resistor and the current limit in the LM350. Go look up the
GUARANTEED specs on the current limit for this part.
We have some random current limit that's dependent on the LM350 and the
ambient temperature and the capabilities of the heat sink it's mounted
on. Once the system comes out of current limit at some battery voltage,
the
current starts to taper as a function of the vout-vbattery across the
resistor R7. When the current drops to the threshold of u2, it's output
goes low and reduces Vout. But guess what...R7 is a resistor.
Vout is known. So measuring the battery voltage is IDENTICAL in
function (mathematically equivalent) to measuring the current (voltage
across the resistor). You have three variables, knowing any two
determines the third.
So, you get a constant current (of indeterminate value) charge that
terminates when the
battery voltage reaches the threshold. At that point you have a state
transition to lower Vout
and remember that state with a digital storage element, u2 in this design.
We're in heated agreement about the process.
IF only the circuit shown
had predictable characteristics. Just put a splash cover over the
battery so when the LM350
finally gives up and melts itself shorted and the 18V causes the battery
to emit hydrogen that's ignited when R7 catches fire....
But heck, it probably works most of
the time.
Are we having fun yet???
mike
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