I know of at least one inverter/charger that does have a pass thru to monitor current. I believe that is a feature on Samlex, and it has an input for the solar charger. It is not a standard feature on most inverter/chargers that I have seen. If running a full Victron Setup with a Victron Cerbo you could invoke DVCC "Distributed Voltage and Current Control" and it would take care of that for you. Most of the charging devices are unaware of any other inputs other than voltage as they are all attached to the same battery. They don't know the current is coming out or what particular stage a charger is in.
So could it matter, does it matter, and should you be worried comes down to the equipment you have, the settings of that equipment, the size and chemistry of the battery bank.
The size and chemistry of the battery will determine the preferred charge rate of the battery. Wet deep cycle batteries will prefer a smaller charge , such as .1C to .2C or C/8 . AGM batteries can typically go higher such as "C/4" or .2C to .3C. In all of the above C is capacity of the battery. Take into account that any consumable current needs to taken into account.
So here lies the possible issue with combining chargers, if your inverter/charger is already set to maximize the charge rate of your particular setup, then adding solar could be an issue. Again that all depends on the capacity of you solar system. So if you were aggressive and charged at a the higher rate the additional solar could put you over your target range and cause issues.
Again chemistry comes in here also, I would be more concerned with a Wet deep cycle battery than a AGM. The other big factor is what stage are the charger ins. The only stage where the full charge rate output will be used is in Bulk, so if your batteries were discharged and you got home, plugging in at night would be the preferable thing as you would have most like exited the Bulk phase by morning. Again, if you were to plug in early in the morning before solar was at full capacity, you would be better than turning them both on at full capacity.
Now we move to the absorption phase , where as the Bulk phase was just a constant current, the absorption phase will be a constant voltage with a controlled current. So the current supplied will start to decrease as we move thru this stage. The bigger issue, will the stages may be out of sink. Some chargers may going into absorption around 80% SOC some closer to 90%, many chargers will have the ability to set the time of absorption, and some don't. Some even give you the capability of lengthening the absorption phase based on depth of discharge. In reality this probably won't be an issue, it will be even less if the two chargers have common set points.
Float will be the final stage, again if both chargers float voltages are the same not so bad. Probably the bigger issue will be when Solar wakes up in the morning. Most solar controllers will start the whole process from the beginning. They will quickly transition in absorption, might stay in that phase longer than one would like, that could be determined by the controller settings. While not ideal, this shouldn't destroy batteries. Even some shore chargers may have storage phases, and setpoint where they will come out of. All theses special features that a charger has implemented may be negated by the sharing charge controller.
So it would be best to make sure the combined currents aren't to high.
For Lithium I always just let one charger run at a time.
greg
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