Recently I heard about an inexpensive and simple low voltage alarm for LiPO and LiFePO4 batteries so I got my hands on one to try out. They are very simple and plug into the balance charger connection of your battery. As this connector’s size is directly related to the number of cells in the battery the device knows how many cells to monitor. It simply scrolls through each cell and shows the voltage and then the overall voltage of the battery. You can set the alarm value to anywhere between 2.2v and 3.8v so as not to discharge the battery below a safe level. $1.94 from China via eBay.