Many vehicle designs have a single bus that carries the current to the three systems. Using mosfets, or in the simple case - diodes, you can switch current onto the bus, or shunt it off into the system you're dealing with. Typically in a dumb system the bus is kept at voltage within a certain range, but that voltage is allowed to move up and down within that range, and each system acts differently based on the bus voltage.
Taking each system one at a time:
Alternators
The alternators attempt to keep the voltage at, say, 52V. As long as it's at or near 52V, they regulate their current output so the bus stays between, say, 50 and 52. When the voltage is below 50V, they run full tilt at 250A to get it back up there. That's their only job, since they never take current from the bus.
Motors
The motors don't care about the bus voltage. The throttle shunts current from the bus to the motors, and the bus voltage drops if the other systems aren't adding current quickly enough.
Batteries
The batteries are the more complicated system. If the bus voltage is 50V or higher, they take current from the bus, and charge the batteries. Just activate the 0.5A charger when the bus voltage is above 50V. 50V is the signal that the alternators have excess capacity, and the batteries can charge. They do not put any current onto the bus.
If the voltage is 48V to 50V, the batteries disconnect themselves from the bus if they aren't already supplying current. If they are supplying current, they continue to do so, trying to keep the bus voltage above 48V, but below 50V.
If the voltage is under 48V, the batteries start trying to force current onto the bus, keeping it between 48V and 50V.
The batteries only do this, of course, until they run to about 50% of their capacity, then they stop supplying current to extend the life of the batteries.
Result
When the motors draw no current, the alternators are running on a light load, charging the batteries.
When the motors are drawing 250A or less, the alternators are running at full load, and the batteries are disconnected.
When the motors are drawing over 250A, the alternators are running at full load, and the batteries are fully connected to the bus, pushing as much current as they have left onto the bus.
Reality
This system has its faults. If the battery-->bus or alternator-->bus regulators aren't happy with each other, you'll end up with system oscillation which might not be easily resolved.
Most modern systems use active elements and a single bus controller that tells each system what to do. This controller knows how much current the motor is commanding, and tells each subsystem how much current to supply, while monitoring other bus and system vitals to prevent overvoltage, overcurrent, and overtemperature violations.
But with careful design and tuning, the dumb system can work, and works well. An advantage is that the system can handle more devices either providing or using current without having to specifically communicate with a central controller. If you add solar panels, or a cabin cooling system, they hook directly to the bus with a little logic that turns them on or off depending on the bus voltage. On a central bus system you will likely have to reprogram the central controller and may even have to have these devices communicate with it so it doesn't accidentally discharge the batteries when it believes it is in a charge state, for instance.
Best Answer
The normally recommended maximum charge rate is C/4 to C/5, ie. 1/4 to 1/5 of the battery capacity in Ah.
If your battery capacity is 90Ah then 30A is C/3. The battery should handle this OK but the voltage will rise faster. Above ~13.8-14.4V (2.3-2.4V per cell) the battery will 'gas' as the water breaks down into hydrogen and oxygen. Apart from the explosion risk, this is bad because it wastes power and the battery will have to be topped up with deionised water.
At C/3 the battery will probably reach gassing voltage at around 50-70% of full charge. To get a full charge the current must then be gradually reduced to keep the voltage below gassing level. Good chargers do this automatically, but your 12V power supply won't.
A 12V power regulated supply will hardly charge a 12V lead-acid battery at all because it doesn't put out enough voltage. An unregulated supply will continue to charge the battery at gradually reducing current until it reaches its unloaded peak voltage, which could be 40% higher than its rating and is dependent on the mains voltage. Another problem is that without current limiting a discharged battery may initially draw too much current out of the power supply, causing it to either shut down or blow up!
For all these reasons and more, you should use a proper charger designed for lead-acid batteries. A regulated power supply can be used only if it can be adjusted to put out 13.8-14.4V and is designed to work in continuous current limiting mode.