Hy britcon, what are you seeing on your ammeter or voltage gauge? Any load large enough to deaden a battery while driving should provide other symptoms, something not working [lights, air conditoning] or smoke leaking out of a wire somewhere. Batteries can develop an internal short, battery cables can rub through on something hot or sharp and cause a pretty good short circuit. As Wrenchbender Ret. says alternator diodes can short out and give you little or no output. Charge the battery up and then load test it, if it passes the load test, then re-install it and as Wrenchbender Ret. says turn on some loads and see what voltage she charges at, at around a 1000 r.p.m. More batteries die in the heat of summer than in the cold of winter, hope that helps.
