It sounds to me that your battery has lost most of its capacity, ive also had this on batts when they reach their end life. If the battery is old and appears to look fully charged on your charger with no warning lights on it after charging, but it still drops off in voltage fast, i would say your battery needs changing. Example: Its like say a 100ah batt is only fully charging say 20ah of it because the rest (80ah) is dead because its old and had many cycles already, thats why the battery drops off fast because it probably only has 20% good left in it that works. But it will still look fully charged (12.7 - 12.9v) on the meter, because you may affectively only have 20 amps left in it that charges properly . This is my understanding anyway. Good luck m