Dear Stanley,

if you are asking, why your board hangs:

Can you have a look with oscilloscope at battery voltage that is reaching your micro board? I guess, you will see nasty dips causing some brown-out condition.

Remedy: Using a MAX1232 reset circuit with brown-out detection.

There's one other thing you can do: Put a big storage capacitor (electrolytic, e.g. 220µF/10V) at input of 5V regulator and connect input pin of regulator to battery via a ferrite choke of about 10µH. This should filter out the power supply dips. If current consumption of micro board is not to high, you can also add a small resistor in series to ferrite choke, something between 1...4.7Ohm. This will eliminate any possible ringing of the LC filter according to the criteria:

R >= SQRT(2 x L / C)

If R is greater than this value ringing is impossible.

Finally: Unacceptable voltage dips of battery under load can also be caused by old battery performance...

Good luck,
Kai