I have four SOK 206ah batteries in series.With four batteries in series, the batteries are rated for 10.5kwh. However, when loading the batteries with a 2600W load, they only last for 75 minutes, which equates to 3.3kwh. This is only 31% of the rating. What could be the cause of this?

i have six solar panels feeding to two Rich inverters then to (4) 12V SOK 206 AH batteries in series. The kick-out setpoint in the inverters is 58V; however, the batteries only charge to 54V. Can anyone provide a suggestion to correct this?