Hello there,
Some nice links in this thread.
Charging Lead Acid is relatively simple. More simple than charging NiMH for example. But even though it's simple, you should use a circuit that can do it right. You can ignore the 'float' charge phase of this because that is only necessary when you want to keep a battery fresh over time when it's not being used. If you really need that, only then would you need to incorporate that too.
The basic idea is to use a voltage limited power supply that is also current limited. The current limit is set to less than the max the battery can take according to the manufacturer's specs for that battery. The voltage limit is set by the number of cells. To get a circuit like this requires a few components that are not expensive.
For example, using an LM317 set to around 14.2 volts (for a 12v battery, half that for a 6v battery) and using a simple current limiting resistor on the input, you can charge a LA battery (under 1.5 amps charge current). But a zener diode shunt regulator (which looks like what you have shown) is not the way to do it. A series regulator is the way to do it, and it's not a great idea to use a zener when there are many better methods to get that right voltage.
So i guess the question then is, have you ever used a LM317 and if not would you want to try this? Also, what is the ampere hour rating of your battery? (this is important too)
Current limiting is simple enough when you use a small series resistor to measure current, or even just a simple series resistor sometimes will work when the power supply output voltage is relatively constant. With the LM317 you can use a series resistor if the required max charge current isnt too high, like maybe around an amp. Adding active current limiting isnt hard either though, a small transistor and a couple resistors and you have your current limit.
One thing to remember is that charging LA is like charging Li-ion, you need a voltage and current limited supply, but the voltage output tolerance has to be more accurate than the current limit. The current limit can be done with a small transistor and resistors and doesnt have to be accurate at all. For a 1 amp current limit, if it was actually 0.8 amps to 1.2 amps it would still be ok for example, but the voltage needs to be fairly accurate, within a few percent. And also if you charge at other than room temperature you may have to adjust the voltage a little to compensate for the temperature of the battery.
If you'd like to see a LM317 battery charging circuit i could post one here.