Here is the only way to finally solve ES2/ES4 external battery connector issues once and for all. : r/ninebot
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Amazon.com : PAOWANG 36V 6600mAh External Rechargeable Battery Replacement for Segway Ninebot ES1,ES2,ES4,E22 Electric Kick Scooters : Sports & Outdoors
Ninebot Kickscooter external battery – Ludi Electric
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Segway External Battery for Ninebot KickScooter ES1
Ninebot Kickscooter external battery – Ludi Electric