Here's a battery pack I made for my GPX. I used 18650 rechargeable batteries which is probably the most common kind used for built in battery packs like for laptops and flash lights. I used the plug off some old computer speakers that plug into a 4.2 low latency bluetooth transmitter (the white round disk). The power leads were then soldered to a Fatshark Battery Box used for powering Fatshark Drone goggles. You can use any 18650 battery holder but I liked the fatshark battery box because it has the battery indicator on the side. It's important when building an alternative battery sources to use good quality batteries and charger. The batteries are Panasonic rated at 3400 mAh. You can find batteries with higher ratings but it will be junk as 3400 is about the current max output for good 18650s. The longest I've gotten to hunt is 2 hours and I showed half charge so I feel confident I can go 3 to 3.5 before putting two more in and I have 8. The setup is extremely light and I have several bluetooth options including Sony and Minelab earphones but prefer ear buds as the sound is crisper and it's cooler in summer.