One MSJ-400 string or two MSJ-300 strings in parallel?

A single string of the larger cell is the better engineering in almost every case. Paralleled strings share current by their internal resistance, so any manufacturing spread or aging difference sends more of the load to one string than the other, and the imbalance widens as the bank gets older. Two MSJ-300 strings also double the intercell connections and the monitoring points. Where the reserve figure can be met by one string, use one string.

What physically changes going up from the MSJ-300?

Depth goes from 6.69 to 7.76 inches, width from 5.91 to 6.69 inches, and weight from 48.05 to 63.03 lbs. Height stays at 13.15 inches, which is true across the whole MSJ range. The width change is the one that catches people out, because across a 24 cell tier it adds 18.7 inches of run that an existing rack may not have.

What charge current does a 24 cell string need?

The per cell ceiling is 120.00 amps, and cells in series all see the same current, so a 24 cell string is charged at up to 120 amps total rather than 120 amps per cell. Most rectifier plant is sized well below the ceiling and recharges over several hours. The number matters when a site has to be back at full float on a short schedule between events.

Is the flame retardant case an option or standard?

Standard. The MSJ container is flame retardant ABS to UL 94-V0 as built, with no FR suffix to specify and no cost adder to negotiate. On several other CSB lines the flame retardant case is an option priced separately, so it is a line item that disappears from a comparison when the bank is specified in MSJ cells.