Low charge to no charge issue pulling hair out
Disconnect the bulkhead connector. Then remove the terminal from each half and clean them.
Finishing at 12.2 Volts is not right. The charger should be at 12.8 Volts or even better at 13.+ for the battery to get fully charged.
When a good battery is fully charged, its surface charge will be a little over 13 Volts and no measureable current (electrons) will be seen going into the battery.
So at this point I don't know if its the charger is incapable or the battery is damaged.
The fact that the battery was drawing 20 amps from the alternator when the engine was running suggests to me this battery probably can be recharged. If its a flooded battery with removeable caps then you can check the condition further.
It works like this. The amount of power drawn for recharge depends on both the battery's charge state and the voltage of the recharge.
Have you ever noticed that lights run brighter when the alternator is running than when power is coming from the battery?
Same idea with the battery recharge. If a battery recharging at 12.5 Volts is drawing 20 amps, then if it was given recharge at 14 volts it would draw 30 or 40 amps. That much current will (a) damage connections and wires, and cause the battery acid to bubble and cook off (or worse).
You may be confusing current and voltage.
Current is electrons moving. Voltage is electrons looking to move.
An analogy can be made to pressurized air or water system.
Voltage is pressure created by the compressor or held in an air tank.
Current is the amount of air flowing in the system.
Check the ground terminal too.
Well this part is easy thanks to your additional measurements. Look at where the resistance occurs with current flowing through.
Disconnect the bulkhead connector. Then remove the terminal from each half and clean them.
I threw a lot of information at you to help you understand how the battery charges.
Finishing at 12.2 Volts is not right. The charger should be at 12.8 Volts or even better at 13.+ for the battery to get fully charged.
When a good battery is fully charged, its surface charge will be a little over 13 Volts and no measureable current (electrons) will be seen going into the battery.
So at this point I don't know if its the charger is incapable or the battery is damaged.
The fact that the battery was drawing 20 amps from the alternator when the engine was running suggests to me this battery probably can be recharged. If its a flooded battery with removeable caps then you can check the condition further.
It works like this. The amount of power drawn for recharge depends on both the battery's charge state and the voltage of the recharge.
Have you ever noticed that lights run brighter when the alternator is running than when power is coming from the battery?
Same idea with the battery recharge. If a battery recharging at 12.5 Volts is drawing 20 amps, then if it was given recharge at 14 volts it would draw 30 or 40 amps. That much current will (a) damage connections and wires, and cause the battery acid to bubble and cook off (or worse).
"reporting back current"
You may be confusing current and voltage.
Current is electrons moving. Voltage is electrons looking to move.
An analogy can be made to pressurized air or water system.
Voltage is pressure created by the compressor or held in an air tank.
Current is the amount of air flowing in the system.
The visible corrosion further reinforcing my suspicions.
Check the ground terminal too.