The UC3906 implements the behaviour you describes. But in order to make it
work the input voltage required exceeds what a regular car battery under
load but without alternator charging can supply. I think I will be placing
a simple switcher of some sort (LM2588) in front of the lead acid charge
circuit to bring the input voltage to an acceptable level.

I would have liked to use the simple switcher in place of the pass
transistor required by the UC3906. But I think this will be rather
difficult considering the way the UC3906 does its current limiting.

Adi

> No it isn't. Lead Acid requires a minimum of 2.3V/cell to charge. That means
> you need 13.8V. Now usually this isn't a problem because the car alternator
> will produce somewhere in the ballpark of 14V. But if it's a true 12V it won't
> properly charge.
>
> This is a continual problem on my to do list and I still haven't taken the time
> to build a good charger. But here are the elements:
>
> 1. (Optional) current limited bulk charge until battery terminal voltage is
>    14.4V. You can use any voltage and it can be unregulated. But the current
>    has to be limited to the inrush current on the battery. This is usually
>    something like C/3 where C is the Ahr charge of the battery. For example
>    on my 33 Ahr gel cell the max inrush is 10A. My charger is going to be
>    powered by a 18V 4A transformer, so in this phase I'll throw in a 5 to 8
>    ohm resistor to limit current to 3.5A or so.
>
> 2. Voltage controlled main charge at 2.4V/cell. This mainline charge will
>    top the battery off. However at 14.4V the battery will gas with continual
>    exposure. This is real bad for sealed gel cells. Common wisdom states to
>    continue this charge until the battery is drawing less than C/100 current
>    which for my battery is 330 mA. One task I need to work out is how to
>    measure the amount of current drawn. I figure I can amply the voltage
>    drop across the resistor in step 1.
>
> 3. Trickle charge can be applied indefinitly at 2.3V/cell.
>
> Note that point 1 isn't required but will get the bulk part of the charge
> done faster.

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