Why is my battery not holding a charge after solar input?

That sluggish voltage behavior almost certainly points to massive internal resistance in the battery itself. When a battery has high IR, it can handle very little current, even if it has capacity left. The charge controller thinks it's pushing power, but because of the internal resistance, the battery can't pull the current effectively, and the voltage plate just flatlines. It's the battery's restriction, not the controller's.

I've run into this on a few projects where I used old sealed lead acid batteries that had been sitting for a while. Once thier plates start degrading, the resistance just skyrockets. This prevents them from taking a proper charge even if the voltage is technically still above the cut-off point. You won't see the typical charging curve because the battery is essentially limiting the flow.

If you want to verify this without swapping batteries, you need an IR meter. A quick check on a new, healthy cell versus your questionable battery will show a huge difference. A healthy deep cycle battery shold have very low milliohm readings. If yours is hitting tens or hundreds of milliohms, it explains why your solar array power is just dissipating across the battery's internals instead of going into the chemistry. You won't be able to fix high IR just by fiddling with the wiring or the controller settings. The cells themselves are compromised.

For a DIY system to actually work, you need batteries that can handle the current the charge controller is pushing. If you are using anything close to 8 years old, even if they seem okay, the chances of high internal resistance are very real. It might still have some usable capacity for low-draw things like an LED light, but for charging a main system, it's just not capable of accepting the load.

I've seen this happen with sealed batteries that look fine but are secretly fried. Once you confirm the internal resistance is crazy, the controller often limits the actual current flow before it even hits the battery terminals. It's self protection kicking in because it detects an impossible load, even if the voltage looks high. If you measure the output amperage from the solar setup right at the controller output, it should be dropping significantly when you connect the battery. That drop confirms the battery is the bottleneck, stopping the charge before it even tries to hit the chemistry. You are correct, the cells are the issue.

The stagnation of the terminal voltage without it crossing the upper threshold for absorption suggests a potential limitation beyond just resistance, because the chemistry requires a specific potential to accept the charge. It is not merely about the current flow, but the battery's ability to sustain that full charging voltage long enough for the chemical reaction to properly initiate. You need to verify that the controller is successfully ramping up to the maximum voltage specified for that particlar battery chemistry, because if the voltage cannot hold that required ceiling, the cell is simply incapable of reacting at that level, regardless of the current the array is generating.

Update: It turns out that sluggish voltage behavior was exactly what you guys said about the internal resistance being way too high. I ended up swapping the unit and saw the charging profile imediately correct itself. It looks like my original battery had completley given up its ability to accept a charge.