I miss interpreted your first post. If you mean that only when you put the ESV with a 250 milliamp load on the system, the meter reads below acceptable limits yet the battery (on a separate tester) checks out with more than ample milliamperehours left; then I suspect that your planes wiring has a high impedance somewhere, most probably in the switch or one of the connectors to the battery or switch. Try bypassing the old switch assembly and wiring with a known good one and see if you get the same response. There seems to be a definite voltage drop somewhere between your ESV and the battery. If so, this could also cause serious problems in flight.