I have been using a PB137 to Charge a 12Volt 7Ah battery from a 18 Volt source.

I wanted to take the source from another 12 Volt battery of higher capacity and the 7Ah Battery did not charge.

I looked at the datasheet but cant see if there is a minimum input voltage. I thought that it would work similar to a regulator just with internal current limiting. I want to find out if the source is not at a minimum will the PB137 not have an output.

 

It is all in the datasheet. The PB137 is a Linear regulator; not a boost converter.

The specification that you seem to have missed is "DropOut Voltage" Vd, which is the voltage drop that the regulator needs to do its job. To charge a 12V SLA, it needs a minimum of 2.3V per cell, or 13.8V at 25degC.

If Vd is 2V, then the min. input to the regulator would have to be 13.8v+2.0V = 15.8V. Obviously, starting from ~12.9V (the resting voltage of a full-charged SLA) ain't gonna charge another battery. Even paralleling a fully-charged SLA with an uncharged one is not going to do much...

Read the ST Datasheet again, looking for the word "DropOut". It is mentioned in several places. Pay attention to fig 5, 9, 10, and 11.