Battery problem: car won't start with a click-click-click, lights are dim or out, jump works and car drives normally afterward.
Alternator problem: car ran fine, then started losing electrical (dim lights, dying radio, dash gauges going wonky), then died. A jump might start it but it dies again within a few miles because the alternator isn't recharging.
Our roadside driver brings a real alternator load tester. If your alternator is bad, we tell you and tow flatbed to a shop instead of leaving you with a jump that will fail in 10 miles. Same $249 / hour rate.