The voltage decrease you saw is known as “bubble voltage” and is quite typical for Lithium Batteries. Other cell compositions react differently than LiFePO4 batteries. The relationship between voltage and load is not linear. If your voltage is between (13.3 and 14.6), (26.6 and 29.2), or (53.6 and 58.4), you’ll have much more than 99 percent capacity. This effectively indicates that “Bubble Voltage” represents less than 1% of total capacity, and thus the decrease when a load is put is instantaneous.