Buy battery holder hard plastic case with 3.5mm pin pack
CentIoT - 3S x 18650 Three Series Lithium Battery Holder - for 12.6V li-ion Plastic case with Lead Wire Hard pin Spring Retention - 1PCS Black - Mifra Electronics
Battery holder: Li-Ion 18650 Battery Holder (2S2P) With 2.6" long 20AWG & PCB