What does the 300 in MSJ-300 refer to?

Capacity at the 8 hour rate, rounded down to the nearest hundred amp hours. The measured figure is 304.00 Ah to 1.75 volts per cell at 25°C. The same rounding holds through the range: the MSJ-400 measures 405.60 Ah, the MSJ-500 measures 506.40 Ah and the MSJ-1000 measures 1016.00 Ah. Sizing off the badge leaves between 1.3 and 1.6 percent of the bank unclaimed, which is a small and safe margin in the direction you want it.

How many cells make a -48 volt string?

Twenty-four, at 2 volts nominal each. Substation control banks usually run 60 cells for 120 volts or 120 cells for 240 volts. Because each cell is an independent jar, you order the exact count the bus needs rather than rounding to a block size, which is the main reason 2 volt plant is specified at this scale in the first place.

Will MSJ-300 cells drop into a rack laid out for MSJ-400s?

They will sit in it, but they will not fill it. The MSJ-300 is 5.91 inches wide against 6.69 inches for the MSJ-400, so a 24 cell tier gains about 18.7 inches of slack that has to be taken up with spacers or the cells will move. Going the other direction does not work at all. Tell a specialist which rack you have before ordering and the fit gets checked against the drawing.

Why are two different amp hour figures published?

Discharge rate changes usable capacity. Drawn gently over 20 hours the cell gives 314.00 Ah; drawn over 8 hours it gives 304.00 Ah; taken to a 1.80 volt endpoint over 8 hours it gives 296.00 Ah. Standby plant is conventionally specified at the 8 hour rate to 1.75 volts, which is why that figure leads here. Match the rate in your specification to the rate in the table before comparing this cell against another brand.