LB90 is an interface circuit board to connect two batteries to a ZL90 and ZL92 control panels. The LB90 will keep the batteries charged while mains power is applied. It will switch over to the batteries when the mains power fails.
LB90 takes two 12V batteries. The battery size depends on the anticipated use while running on battery, and the overall time the power is off. In UK, we can expect power outages to be about 10 minutes, but recent storms have seen several days outage.