Battery Tender Failed
I've kept a 0.8 Amp Battery Tender on the front battery forever. I moved it from my previous truck, a C3500 with a 6.5 diesel. Normally the battery tender LED tells the status of the charge. Solid green means the battery is fully charged and the charger is in maintenance or float mode. Flashing green means the battery is >80% charged. The charger also has a non-green LED which might be amber or red, depending on the vintage of your charger. When this amber or red LED is solid, it's charging. If it's flashing, that indicates an error state. The charger thinks the wiring is wrong, the battery is too sulfated to charge, or the battery is too flat (<3V), and it's not charging your battery.
My ~5-year-old Battery Tender 800 started flashing yellow. I poked around with a meter and couldn't find a problem. I started to suspect the Battery Tender was defective, so I ordered 2 more, one for each battery. I installed them, using the old wiring that was already in place, and they work fine. They both show solid green after a day of charging.
So if your Battery Tender starts flashing yellow, it may be time to replace it. I like having two now, to make fault isolation easier. Next time, it will be much quicker to determine if I have a battery problem, a wiring problem, a fuse problem, or a failed Battery Tender, since I can switch between 2 different Battery Tenders.
The photo below shows them in operation. I haven't decided where to locate them permanently, yet. The plug hangs out the corner of the cab, making it easy to plug in the truck. I use an orange extension cord that runs across the windshield en route to the plug, so I can't forget to unplug.
I know 0.8 Amps isn't much for a 1000 CCA battery, but I only ask the Battery Tender to keep the batteries topped off. They do this very well for me. Even if I don't start the truck for a few weeks in the dead of winter, the batteries are always fresh and 100%, and I get a quick start every time.
I also sometimes use the truck for short trips around town, where the engine doesn't run long enough to fully recharge the batteries from the load used in starting the truck. The Battery Tender may take all day to do it, but it does top them off when that happens.
Good batteries aren't cheap. Battery abuse leads to battery failure, which can happen at extremely inconvenient times. The Battery Tender keeps my batteries topped off, pampering them and maximizing their life expectancy. I never have to wonder about the state of my batteries, increasing my confidence in the truck. For me, the Battery Tender is worth the trouble and expense.