Is there still a N needed for 3phase? The big selling point of triphased power is that you don't need a neutral line, which is obviously a big benefit for transport. Is there a benefit to add a neutral line for battery loading?
No on the distribution network (aka up to the transformer in the street usually, but for sure not on the xxx kV lines) because there the lines are balanced.
In your house, you do, because the voltage between 2 phases is 400V, but the voltage between 1 phase and N is 230V. So you have "low" voltage for "normal" appliances, but high-voltage (and thus high-power) available for high power applications. For example: EV charging, induction cooking, home heating / AC etc.
For an EV usually the N wouldn't be needed, if you always charged at a balanced power on all phases. But from my experience, the full 3 phase power is only used when the battery is empty. At some point the charger switches back to single phase to better modulate the current I guess.
Sidenote:
On a 3x230V net, you don't have an N, but that means you also don't have a non-power conducting wire either! Meaning: double pole switches and breakers are required to prevent shocks. This is why these are generally required in Belgium btw.
I think it's more, 3-phase+N can be converted to 1-phase whenever needed with simple wiring. So even in France, you might have 3+N coming to the breaker panel, and then 1-phase from there on.
The neutrals in either of these situations are used when you want to use some partial multiple of the power, instead of a full multiple for some or all of the load. For example, most US electric clothes dryers will use the full split phase 240V for the heating element, but 120V for the light and sometimes the motor. Same with US electric stoves. So they need a neutral to be able to do so.
If they don’t need that 120V (half the voltage), they can just use straight 240V, and no neutral.
Same with 3 phase - if connected in a delta configuration, you have three distinct loads, each connected phase to phase.
Same as in split phase, if you have two 120V phases, each connected to one half of the split.
When designing AC->DC rectifiers a key concern is ripple (aka how consistent the DC output voltage is per unit time). 120V half phase AC is particularly terrible for this, but a single phase of a 3 phase system will also be not great. You spend a lot of the cycle with no meaningful power available, and need to smooth out that very spikey output with capacitors or the like.
The ripple on three phase (if using all three phases) is going to be a lot lower, and power flow will be much more consistent, as you’ll have 3 waveform ‘peaks’ per cycle, unlike split phase which has 1, or tapping a single phase of 3 phase which has one. (Depending on your definition of peak - some would double the count as the negative voltage side of the waveform technically counts too!). That means with only minimal additional component count, you have a nearly perfect continuous flow of power.
At the type of power levels we’re talking about, the capacitors for smoothing out the ripple (assuming it’s needed when charging the batteries - I would assume so, but I’m no EE), will be enormous and expensive if using single phase.
I know for industrial motors, the smoother/more continuous waveforms are a huge help in making smoother and more powerful (for their size/weight) motors. Almost all industrial motors run on 3 phase AC, and it’s common for even small hobby machine shops to either get 3 phase pulled in, or use phase converters.
