Why does short circuit current jump so far above the MSJ-400?

Short circuit current is set by internal resistance and cell construction, and it does not track capacity in a straight line. Internal resistance falls from 0.30 to 0.28 milliohms across that step, and CSB's measured result is 9,953 amps against 6,065. The practical consequence is that a protective device and its interrupting rating are sized off the fault figure, so an upgrade from a 400 bank to a 500 bank is an electrical review and not only a capacity swap.

MSJ-500 or MSJ-1000 for a 500 amp hour requirement?

The MSJ-500 meets it in one cell per position. Choosing the MSJ-1000 would give roughly double the reserve at double the weight and double the depth, which is the right move where the reserve target is going up later and the floor loading allows it. Where 500 Ah is the settled requirement, the MSJ-500 is the correct size and the lighter install. We will say so rather than sell the larger cell.

What torque goes on the M8 terminal bolt?

109 lbf-in, with a tolerance of plus or minus 22, which is 12.3 N-m plus or minus 2.5. Copper alloy inserts take a higher torque than the lead alloy inserts on CSB's 12 volt lines, and under-torquing is the more common error because installers carry over a figure from a smaller battery. Use a calibrated torque wrench on every connection and record the reading.

How long can these sit before commissioning?

Self discharge is under 10 percent after 90 days at 25°C. Storage runs to six months as shipped, and to ten months if an equalization charge is applied part way through. A full recharge is required before the bank goes into service either way, and a warmer stockroom shortens every one of those windows. Tell us the install date and the cells get dated to suit it.