It’s sounds like your solar engineer didn’t advise and install a system that programs the charge controller to stop the battery from discharging when it gets to at particular voltage, for example 11.8v then a transfer switch kicks in either a generator or ECG to take over supply the load and also charge the battery at the same time. That system would have cost more for the transfer switch but at least your batteries would still be functioning. Sorry you’re going through this. That’s a negligent install in my opinion. Glad you’re going lithium ion phosphate. Best decision, but you still must get an inverter that has an internal auto transfer switch or buy a transfer switch $600 and have the system re-wired to accommodate same. Blessings my sister. Love you, you know that. Stay strong and keep up the good work🙏🏾❤️
Don't mix the old gel battery with a new gel battery. The batteries need to be around the same age. You also need a LVD(Low Voltage Disconnect) btw your batteries and inverter. Most inverters have it inbuilt. Lithium batteries can be deeply discharged but they too don't look like to be completely discharged to 0%. LVD is important
You may want to switch to lithium batteries. They are more durable than gel batteries. Besides, you can run them down to zero, but they can be recharged to 100 %
The inverters they selling to you guys are not good inverters, now there are inverters that can run all appliances in your house, in this case you will not need ecg, you go on full solar.