Is there a mathematical relationship between the battery AH rating, and that of the transformer in the charger
I assume that your 12V 150 Ah battery is lead acid (LA).

If that 150 AH figure is not a typo that is a large and valuable battery and should be treated well.

It is very very very important to get the battery into a reasonable state of charge NOW. "A few months" often turns out to have been 6 months +. While a battery may survive OK for that long it is liable to be in  alow state of charge and at risk of "sulphating".

I'd recommend that you 


Urgently have  alook at sites like Battery University  that give you a good idea of why large lead acid batteries have a fairly complex charging arrangement. 

Battery University on charging lead acid batteries
Check the following advice is good from at least one technical source that you trust and then DO IT ASAP. These voltages are somewhat uncertain as they are measured open circuit. Battery university will have better advice on loaded tests. 

Measure battery voltage. 

If over 12.5V panic less

If 12 to 12.5V get a suitable charger NOW.

If under 12V take urgent action.
Use anything available to bring the voltage up. A small car charger able to provide an amp or more will probably do.


Much better advice is needed on a proper ongoing charger. I do not have time at present (deadline looms) BUT if you are serious about keeping that battery in good order you need a smart charger to someextent.



Voltage per cell:

Open circuit fully charged 2.10 - 2.13 V / cell.
 Open circuit, fully discharged 1.95 V - 2.0 V / cell
 Loaded, fully discharged 1.75 V/cell
  Gassing threshold 2.35 V / cell