question

Had a similar issue just lately, but with an mppt charger in my van. Its hooked up to the starting battery, then the secondary battery is charged trough a cyrix coupler and monitored by a bmv. There was a difference between what the mppt and the bmv said about the starting battery voltage, but the mppt was still outputting power when the sun was out. The issue was a loose ground connection from the mppt to the car body, still good enough to let some power flow, but not good enough for it to have no voltage drop.

Id check the cabling between alternator-starting battery-orion, my guess is theres a loose/bad connection or maybe a damaged wire. Contant movement can break the copper while the isolation might still be ok.

You can try the following to narrow down things:

With the orion turned off, engine running, measure the battery voltage. Turn on the head lights. This will add about 150W of load to the system (2x 55W head lights plus some tail lights and other marker lights) so thats about 11A at 14V. Compare the battery voltage to before, there will be a bit of a drop, but it should not drop down to 11.6V. If it does, try again with the engine at some higher RPMs, 1500rpm for example. The alternator might simply not be capable of outputting anything serious at idle speed. If the voltage still drops significantly, id guess the issue is with the alternator, battery or its cabling. If the voltage stays stable-ish, then the charging system of the car itself should be good

Thank you for your response. I will try your diagnostic technique and provide feedback soon.