The only way to confirm your batteries are good is to determine the internal resistance of each battery. Most quality battery chargers provide that capability. Low internal resistance indicates that the battery chemistry is degraded. Some significantly degraded batteries will still charge to almost full volts. If batteries are confirmed good then....do the following: If the voltage does not drop at the batteries when connected to the loads but one or more of the loads is showing a significantly reduced voltage level across it... you have a high resistance connection somewhere between the battery and one of the loads. It's basic ohms law.... you have inadvertently created a voltage division circuit. http://www.solved-problems.com/circuits/circuits-articles/482/voltage-divider-voltage-division-rule/ ...... Also all loads must be wired in parallel to avoid voltage division. On the otherhand if the voltage read across the battery terminals suddenly drops while one or more loads is connected, then you one or more of the loads has become very low resistance and therefore is acting much like a short circuit.