Application considerations for multiple battery disconnects

It is standard practice in telecommunication power systems to have multiple battery strings in parallel. Multiple strings allow power engineers to achieve higher ampere hour back up capacity as well as building in system redundancy by installing additional battery strings. In today´s applications, we are seeing battery plants with greater than a dozen strings in parallel. Ease of growth and the need to reduce floor loading are forcing engineers to look at larger quantities of smaller batteries. This larger number of battery strings brings about the need for additional maintenance and new methods for making maintenance easier. In many of these applications, power engineers are specifying individual battery disconnects for each of the battery strings. These disconnects come in several forms and serve many functions from isolation of single battery strings for maintenance to emergency disconnect. They are also specified for low battery voltage disconnect functions to prevent over discharging of the batteries. Regardless of how the disconnects are used, it is important to understand pitfalls and misapplications that can occur. The problems associated with multiple battery disconnects occur during the disconnect and reconnect conditions. Contactors, which are most commonly used, have characteristic pull up and drop out times that differ from device to device. This paper looks at the dynamics of multiple battery strings and battery disconnects. It focuses on the precautions that need to be taken when sizing cables and selecting suitable disconnect devices. It briefly describes tests that were performed to actually demonstrate multiple contactor disconnects and how the last contactor operating actually breaks the entire discharge current