every few months i buy 1 or 2 of these https://www.amazon.com/gp/product/B07R5XKD36/ref=ox_sc_saved_title_9?smid=A2KUZVNQ9LP7N9&psc=1

$18-30 ( i have 7 now ) they come back in stock pretty often, or you can get them new for $27

https://www.amazon.com/RAVPower-20100mAh-Portable-Delivery-External/dp/B0816BGCJR/ref=sr_1_3?dchild=1&keywords=ravpower+20100mah&qid=1600243647&sr=8-3

charge my max 85-95% very happy with them