At least 1 is required. We not connect the negative output of the 3S without (including 3S 4P) XT60 plug in the design, it needs to use the front copper socket and ribbon cable to connect 3S for output. can provide charge and discharge equalization function for the entire system (multiple battery boxes) at the same time. 3S charge needs 12.6V charger. It is recommended that the charging current of each layer should not exceed 2A, otherwise the battery may become hot (for example, choose 2A charger for only one layer, and 4A charger for two layers). Do not put on the metal table top to use, pay attention to insulation do not short circuit. The battery up the circuit board, all the batteries face the same cannot interleave.The direction of battery installation is subject to the mark on the cirucit board,not the mark on the plastic battery box. Do not connect the wrong power supply, the circuit board is marked, pay attention to the observation. If you use a solar panel to charge the battery, you need to bring your own voltage control module. If the output voltage of the solar panel is too low, you need to increase the boost module. If the output voltage of the solar panel is too high, you need to increase the buck module. Type: 3S 2P 40A Model Number: Dog-2 colour:Black Material:ABS size:150 x 85 x 25mm Package Contents: 1 x Battery Box Only the above package content, other products are not included. Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.