Biggest batteries I've found to fit in the stock XRT battery tray are these HRB 9500mah 4s 120c packs. The Lectron pro 5200's the hobby sold me are way too small. These
NEW TRAXXAS T-MAXX 3.3 EZ Start 7.2v 1800mah NIMH Battery Pack +Charger RXB | eBay
i accidently left the battery plugged in my Traxxas for about 2 years, now the charger won't recognize the battery is even plugged in. : r/rccars
GOLDBAT 11.1V 5000mAh 3S 50C Lipo RC Battery Pack Hard Case with Deans T Connector for Traxxas E-maxx Axial RC Car Truck Buggy Truggy Racing (2 Packs), Battery Packs - Amazon Canada