Lets take a typical scenario. We have a lead acid battery bank hooked up to a shore power charger, an alternator and solar panels. The recommended bulk charge voltage for the battery bank is 14.4 volts.
At the dock the boat is typically plugged in to shore power and the battery soon gets fully charged. So the worry is that if solar panels are running at the same time will they overcharge the battery because it now has two sources charging it. Well assuming there is a charge controller fitted on the solar installation then both charge sources, the battery charger and the solar system, are regulated. They read the battery voltage and are programmed to not allow it go beyond a certain limit, they are also programmed to go into float mode after a certain time or when the charge current reduces by a certain amount. So the battery doesn't get overcharged.
The alternator isn't normally running when the boat is also connected to shore power but if it is it doesn't matter. The alternator is also a regulated power supply and the program will limit the maximum voltage as well as the time spent at that voltage.
But what about when the boat is away from the dock: Can having multiple charge sources mean that one sees the voltage produced by the other and none of them work properly?
Lets assume the battery is quite depleted. The solar panels haven't been working all night and at dawn they turn on the engine and the sun comes out at the same time. If the battery is big and empty then they might not get it to 14.4 volts right away. While this is happening they are all working at full power. Once the battery gets to 14.4 volts they are all limited and cannot let it get above that voltage. That is the maximum charge that the battery can accept so nothing is being wasted. As the battery gets fuller the charge voltage will remain at 14.4 but the current will reduce because the batteries internal resistance increases. At this point although all the charge sources are sharing the work it is quite likely that one is doing more work than the other. It doesn't matter which one is doing the work, the batteries are still be charged at their maximum rate. Eventually the charge sources will go into float mode and the voltage will reduce and we are back to the situation at the start of this article.
That is the broad outline of what happens. As usual the Devil is in the details. Of course the way the individual devices are programmed, or even how accurately they read voltage, will not be the same. During the course of a charge cycle one may do all the work and then the other, they may share it equally, or it may fluctuate. The overall effect may not be quite as efficient as if they were truly working in harmony. It works though and batteries hooked up like this where all the regulation devices are working correctly do not get undercharged or overcharged.