Well, the "simplest possible" way when you only have 12v regulated 
available is a boost charger that can float the battery at 13.8v
You can current limit it to 100mA, which would then take several hours 
to charge (you don't say the battery capacity, but I'm guessing around 7AH)
You'd need a MOSFET, and inductor, a diode (1N5819), a comparator to 
measure the output voltage, and a comparator to limit the current. You 
may have to measure the current below the source of the mosfet to get a 
ground-referenced signal and extrapolate what the load is doing. Your 
incoming 12v regulated should be protected against short circuit in case 
someone does something they shouldn't (but WILL do).
--
Martin K

YAP wrote:
> Hi friends,
>
> I need to use a 12V SLAB for backup purposes in a design. Normally this
> design is powered but on very rare cases power can be out for 1 hour max
> during which the slab should supply power. It can take months between these
> power outages. I need s simple (hopefully) circuit to keep the slab fully
> charged. With the long time between outages fast charging is not  needed its
> OK if its take a week to charge the battery again after a power outage.
>
> How would the ***simplest possible*** working charger for a scenario like
> this look like?  I have 12V regulated. I have a PIC so I can measure current
> and voltage and temperature and do PWM. I also have some space left for a
> charger....  Ideas?
>
> Cheers
> /Ake
>   
-- 
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