Should be ok, as long as complies with the following rules:
1. Batteries cranking capacity (starting or short term pulse, SAE) to be bigger than than your 130% short-time load requirement for transient time. In this case, 12kW at 48V, makes 250A, 130% will be 325A. Lead or gel car batteries have something like 550 to 800 A cranking current (55-75Ah). Did not have experience with Ni-Cd this size, but any producer will provide data for them.
2. Voltage of these batteries must be same as output voltage, means 48Vdc.
That makes 4x12V, meaning 4 pcs. batteries. For other voltage, do the math yourself.
Lower capacity of batteries will be useless, higher will be expensive.
Of course, must be done maintenance of them on a regular basis.