I sometimes use old notebook LiPo accus from our institure battery recycling box. Because they are for free.
Usually in the package only a few cells are completely damaged, but the rest can still be charged.
Disassemble enclosure. Check every cell. Good cells have voltage is 2 V. And solder them all in paralell.
Biggest assmbly I have made is with 30 cells. Charging takes ca 1 day with a RC model LiPo charger.
It went into a wooden bird's house put on the tree with glass on entrance hole and some silicon for rain protection.
Autonomous running time ca 3 months.
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