The batteries are only half of the circuit. The cables between them carry every amp the motor asks for. Acid vapor eats the lug and then the strands inside the insulation, where a quick glance at the post will not show it.
What a bad cable looks like
Green copper at the cut end, a lug that is thin or pitted, insulation that is swollen or stiff, or a cable that is warm after a short ride. Any one of those is a replacement, not a wire-brush job. Cleaning is for light crust on a lug that is still solid.
Replace cables with the pack
If you are buying a new lead-acid set or a lithium kit and the cables are already green, replace the cables in the same job. The new battery will otherwise dump current through a high-resistance joint and the post will pit again. Measure the length and the gauge before you take the old cable off, and label both ends.
We stock parts for the small hardware and can confirm cable gauge if you send a photo of the tray. Do not assume a lithium kit includes every cable your cart already ruined.
