48V Battery Voltage Regulator
Firstly, your use of a 7812 voltage regulator for you initial circuit may have seemed fine when you tested it but the 7812 needs at least 14V (or above) for it to provide a regulated output of 12V - if you check what the output is when the input drops significantly below 14V you will see that I'm correct. At 11.58V, it's likely that your "regulated" output is less than 11V.

I'm saying this because you have a misconception as to how linear voltage regulators work and you are in danger of believing you can apply this to higher voltage supplies like the 48V in your question.

You have a desired input voltage range of 43.2V to 67.2V and your required output is 48V. Without compromising your intended requirements, the only way this can be achieved is with a buck-boost switching regulator.

In addition to this, powering fans in series will likely destroy one fan - it will go open circuit (or burning) and then nothing will work. To simplify this, consider using a switching buck regulator that converts your 43.2-67.2V to +12V, then power your fans in parallel.

Maybe try one of these: -



I'm assuming you don't need more than 1A at 12V output as per your original design.

EDIT
A higher output version is this and now, you will have to use external transistors on the device: -



I would also seriously consider the LTC3810. Here's a picture and also see figure 19 in the data sheet: -