Auxillary battery malfunction

More then likely you have a sensor on the main battery that measures the amount of amperage and voltage output. However, this is for the overall state of charge of both batteries. The display says the auxiliary battery is the issue but I suspect the main battery is drained at this point. What happens is the main battery gets drained down and as it drops it uses the smaller battery as a crutch. Then this light comes on and you replace the aux battery but the main battery is still drained. A lot of times this will make the light to go out for a short time then it will come back on. Basically this could happen with either battery but in essence one gets drained, pulls down the other, and then you replace one of them and the cycle continues.

I would go to the parts store and have them test the main battery and see what the state of charge is.

https://www.2carpros.com/articles/car-battery-load-test

You are welcome again :). I thought your name was looking very familiar.

Let us know what else you need. We are here to help. Thanks

Unfortunately they don't have a circuit diagram for this but if all that checks out then the body controller thinks the current is too low so it displays that message. The battery has what is called an intelligent battery sensor that basically just tells the computer the amount of current that is flowing from the battery system. Not just the aux battery but the entire system. Do you have a scan tool that can monitor the overall state of charge? I am not sure what they will call it on the tool but you are looking for something like battery state of charge and there will most likely be a percentage. Let us know what that is and we can go from there.