BQ25570: The voltage across the battery rises very slowly

Hello, I built an energy harvesting circuit with BQ25570 as the core component. When using a friction generator as input, it can charge a 1500uF or 5mAh battery normally, and can charge to the set charging cut-off voltage of 4.2V. But when I changed the battery capacity to 55mAh, the charging voltage at both ends of the battery rose very slowly, and there was almost no change after charging to about 3.4V. What's going on?