Here's a nasty piece of work from ST that claims to do the job.
Simplicity the main attribute:

      http://www.emesystems.com/pdfs/parts/PB137.pdf


Ah
This is it
UC2909 UC3909
Unitrode :-)
Now TI

    http://focus.ti.com/lit/ds/symlink/uc2909.pdf *************************=
*****

Whoops. No. I don't think the original was SMPS/.
Maybe
BUT this does seem to be what you want to look at the workings of

R

  DESCRIPTION

The UC3909 family of Switchmode Lead-Acid Battery Chargers accurately
controls lead acid battery charging with a highly efficient average current
mode control loop. This chip combines charge state logic with average curre=
nt
PWM control circuitry. Charge state logic commands current or voltage
control depending on the charge state. The chip includes undervoltage
lockout circuitry to insure sufficient supply voltage is present before out=
put
switching starts. Additional circuit blocks include a differential current =
sense
amplifier, a 1.5% voltage reference, a =963.9mV/=B0C thermistor linearizati=
on
circuit, voltage and current error amplifiers, a PWM oscillator, a PWM
comparator,
a PWM latch, charge state decode bits, and a 100mA open collector
output driver.

-- =

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