I have a 25KV stand-by generator made by generac. It is powered by a 4 cylinder mitsubishi engine. All works well for about 24 hours then the generator will shut down. If I install a fresh battery, it starts right up and will run for another day, then repeat it's little process. Any assistance would be appreaciated.

You didn't say if this is gas or diesel. My guess is gas (propane or natural gas) which means it has spark plugs. You have a charging system problem. The engine gets enough spark from the battery to run for 24 hours, and the charging system isn't able to keep up so that drains the battery until there isn't enough juice to get the plugs to spark and it quits running. It could be your alternator, or your voltage regulator (which might be inside the alternator). Remove the alternator and take it to a service shop. They can test and fix or exchange it. A temporary solution would be to connect a battery charger to the battery and leave it in place while it runs. That should provide enough juice to keep it running indefinitely, or at least quite a bit longer. If you hook up the battery charger so that it is powered by the output from your generator, this could work as a longer-term work-around - since it will only provide juice to the battery while it's running. I hope this was helpful. Al K