The Hobbico meter throws a 1A load on the battery, so if it is on the tail end of a charge you will see a precipitous drop in voltage.

Under normal circumstances the servos do not put this much of a load on the battery, but at high deflection even a single servo can draw MORE than 1.5A... throw two or more servos into the mix and you'll see why you may need to cut back on the number of flights per 2000mAh pack let alone a 800mAh pack.

Consider making your batteries field changable so you can swap in a freshly charged pack, for long days of flying.