I am building a lithium battery system for a boat. I have two battery banks and two BMSs. The batteries are in parallel.
The positive from each battery is connected to a bus that feeds all loads.
The negative from each battery goes to its own BMS, the outputs of which are tied together at boat ground.
The BMSs are FET based.
The BMSs on the ground side of the batteries and disconnect the battery ground from boat ground on high voltage, low voltage, high temperature, and unacceptable cell imbalance.
Is there some simple way to detect when one of the BMSs has opened the connection to ground while the other has remained connected?
I want to detect this disconnect and operate a relay to sound a warning alarm if it happens.
What you want is a RELIABLE way to detect the fault condition, not the simple way.
This is the Arduino forum.
I suggest to look for professional advice on proper design, engineering and construction of dangerous, high power battery systems, especially on a boat, where there is no place to run from a lithium battery fire.
I have a very well engineered LiFePO4 system installed by an ABYC certified electrician using high end marine components in full compliance with applicable standards and even "gold standard" recommendations. The project I was considering would be an add-on method of tripping a buzzer, strictly for my convenience, when the BMS has safely disconnected one of the banks. An arduino would be plainly unsuitable for a life safety, or fire safety application, especially in a marine environment.
I take the point that I have asked for help in the wrong place and will see if I can close the thread to further comments.
You provided no useful information about the project, and asked for the wrong kind of help.
Have fun with your professionally designed power system, and if you have questions about it, the ABYC certified installer could be a valuable resource in the future.
Not really.
Many people come here with non Arduino but electronics related questions and many of the volunteers can provide valuable answers. If it give it a few hours you will be flooded with responses. and suggestions.
However you may receive several non-helpful responses.