Was flying my quad when suddenly I lost sight of it and apparently lost radio signal. I still had telemetry though. My copter went into RTL and for some reason climbed up to 432 meters above me.. My thoughts are that my battery came loose and fell…

I just flew my quad with a dual battery setup using 2x Zippy compact 5000mah 3s batteries. I had my dad machine a custom plate for mounting the batteries and it works great.my only concern going into trying this setup was the extra 350g of battery…