For a cyclic charge, you can just leave the 15V connected for however many hours it needs for a full charge, then switch off.
The15V supply needs to be current limited to below the battery maximum charge current - often quite low with small sealed batteries.
Simply reaching full voltage does not mean the battery is full; you need to monitor current as well.
First the voltage goes up, then the current starts to drop.
The battery is only somewhere around half to 2/3 charged at that point.
The battery is considered full when the current drops to a small percentage of its AH capacity - so a good charger will allow you to select the battery size as well.
For standby charge (permanently on charge) you can use 13.8V - so add a couple of diodes in series with the charger.
More complex chargers use both, to give faster charge and less battery degradation; 14.8 until the current drops to a low level, then switch to 13.8 continuously as a float / maintenance charge
You could do that with a second relay that shorts out the diodes.
You could easily add a low value current sense resistor in the battery negative connection, giving a small positive voltage you could read with another PIC ADC input.
Use a series resistor from the shunt to the PIC pin and a small capacitor between ADC and 0V, to filter noise and protect the pin from spikes if the battery leads are shorted! Also a diode back to the PIC power to discharge the filter cap when power is lost.
Don't forget to also subtract that value from the overall voltage measurement, as well as doing the current calculation.
More info on lead-acid charging here:
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