My question is about all the COMMUNICATION (edited) between chargers, inverters and batteries. What are they saying back and forth that would make me want to spend tons of money on, say, eg4 batteries over just buying cells and a BMS and building a homemade battery?
The communication can be a nice feature, but it is not required in many cases.
The data typically consists of the calculated State of Charge (SoC), the total BMS current and voltage, the minimum and maximum single cell voltages, temperatures, and the maximum charge and discharge current the BMS can accept.
From this data, the inverter, charger, chargeverter, etc. can determine the charge rate and discharge current limits to keep the battery safe. On LiFePO4 batteries, having the BMS calculate SoC can be helpful, but the charge limit should still use the battery voltage since the SoC is only a rough calculation. One major advantage of the BMS to Charger communication is the ability to slow the charge rate down as a single cell begins to top out in the upper knee. For example, you are charging at say 25 amps. This is slow on a large battery bank and should never cause a problem with a voltage limited charge. But the cells have gone out of balance, and a single cells is starting to hit the upper knee. Without BMS/Battery communication, the charging power source has no idea this is happening and just keeps charging at 25 amps. The balancer, even at 2 amps, is not able to slow the run away cell and the BMS just hits it's cell over voltage protection and turns off all charge current to that battery. If you have a bank of multiple batteries, it is not a huge deal, but a single battery going into disconnect like that can be a big issue for some charge controllers and inverters. With the battery communication, the BMS can command the inverter to reduce the charge current as a single cell becomes near full into the top knee. The battery is near full, so this reduced charge rate is not a big deal. And if it drops from 25 amps to 4 amps, the 4 amp balancer can cut the high cell's charge rate in half, giving a lot more time for the other cells to catch up. If the cells are well matched and closely top balanced, then this should not be a problem, but it can happen. A similar thing can happen on the discharge side with an inverter. A single cells starts to reach the bottom knee. The overall pack voltage is fine, so the inverter stays running at full power. The BMS then just shuts off the discharge current, oops. With the BMS communication, the inverter will see the available current dropping. If it is running local loads, this can't help much, but the inverter can do a safe shut down and keep the status monitoring working since the battery is not shut off yet. If it was exporting power from battery to grid, it can reduce or even stop that current flow as the battery commands less maximum current. Again, this won't be an issue with well matched and balanced cells, or if you are not trying to pull all the energy from the system. If you have well matched and balanced cells, just leave a bit of SoC room at both ends, and stay well below the max C-Rates of the cells, and the communication won't change how the system operates. But if you want to push the batteries for the maximum cycle capacity, the communication can make it safer and help protect the batteries for a longer life.
