Thanks so much to Rene for putting this guide together.
I have two Segways that we sadly haven't used this past year, and they weren't starting up, and one charging light on each was solid red.
This guide led me to buy a power supply,
https://www.amazon.com/gp/product/B09WD85XCX/ but I already had a voltmeter. I gritted my teeth when I got to the part in the guide about taping up nails to stick in the batteries, but everything worked like a charm, and both Segways are up and running now!