Cheapest way to go is lipo alarms like Rusty and Roelof suggested. But with the prices of brushed escs like the Hobbywing WP1060 its probably best just to upgrade the escs to ones with lipo cutoff. You can get generic escs with LVC for less than $15 and the Hobbywing less than $20. The LVC's on the escs may be set too low for some so Lipo alarms can add extra insurance the pack does not get too low.