If you are not monitoring the battery voltage while working with your battery-powered VOXL, it is possible that you are over-discharging the batteries to the point of failure. Especially with LiPo batteries, it is unsafe to over-discharge them, so please be careful. (It seems that you are also using LiPo batteries, in addition to ModalAI Li-Ion batteries also shown (green)). There is nothing wrong with using LiPo batteries, but you have to be more careful with those. Over-discharged batteries lose capacity and Lipo Batteries may become dangerous.
It is a good idea to use an appropriate DC power supply when working with VOXL2 while it is powered on continuously. 12V 3-5A Power supply, connected to the APM (which will provide 5V for VOXL) should be sufficient. You should have no problem finding one, but in case you need, here is a link to the AC adapter that we sell : https://www.modalai.com/collections/accessories/products/ps-xt60
Alex