Solar Power Manager (for 9V/12V/18V solar panel) refuse to charge

Hi, i recently bought few pieces of Solar Power Manager (for 9V/12V/18V solar panel), when i connect the 18V solar panel to the solar manager without 18650 battery, and adjust the MPPT to 9V, 12V and 18V respectively under strong sun light, the solar charge led is on, and i check the input from the green connector it show 9V, 12V and 18V perfectly. The problems is, when i connect the solar manager with 18650 battery, the solar charge led switch off and i check the input of solar charge is show 4.5 V to 5.5V, no matter i on the MPPT at any voltage. So choose to off the MPPT, the solar charge light switch on weakly, and the input is just around 5V. When i reconnect the battery from solar manager, the solar charge led on brightly and the input also show according to MPPT. After test all 3 pieces of this solar manager i have, same thing happen. Does anyone have this problem?
2021-04-16 15:16:26 It is possible that the power required by the load is higher than the power output by the battery panel, which will result in a voltage drop. If there is not enough sunlight, the power of the battery panel will drop and it will not reach the power required by the load. There is also a situation where the circuit is short-circuited. You can check the circuit connection.
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