Hi Out there! I have a 2004 Toyota Tundra that I installed an extra auxiliary Deep cycle 12 Volt battery in so that I run lights and computers when camping. Under the hood I installed a Battery Relay Isolator and Relay so that the main battery would lose charge. This set up worked for a couple years but now I can get the Auxiliary battery to charge from my Alternator any longer. I can charge the auxiliary battery with a battery charger.
My alternator is charging my main starter battery with no problem. I the “Stinger” batter relay isolator only engages the Auxilary battery when the ignition is on. When The car is running I read with my volt meter 13,9 volts on all positive posts of the stinger. The Auxiliary battery only reads 12 volts at the battery. When the ignition is turned off I am reading getting only 12 volts on the Battery Relay at the Auxiliary battery line. The other posts that is connected to my starter battery reads around 12.9. It’s strange that the Auxiliary battery is getting 14 volts from the alternator but it won’t charge. and I can charge the battery fine with a battery charger. Does anyone out there have any ideas on what could solve this? thanks very much for taking the time to help. cheers