I have a John Deere Z255 that is not charging the battery. The battery is practically new and holds a full charge when jumpers from another vehicle are used to charge it. When the mower is running, I only measure about 12V instead of the 13-14V I should be getting, so the battery runs down after an hour so of mowing.

Using my multi-meter, I get about 3.5A out of the battery side of the motor circuit and 15V out of the other side which are within the ranges they should be. The diode on the charging side also checks out ok and the fuses also check out good. Any ideas about what may be preventing the battery from charging?