Two-phase charging

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huntiedk

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Feb 11, 2016
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Although a very happy i3 REX owner for a few months now, it's always been bugging me that the i3 can only charge using one phase. 32A outlets are not so common here in the EU, although I'm currently able to pull 20A from a 32A circuit at my work without blowing too many fuses :D

It would be perfect to do multiphase charging, allowing for faster charge times on a 3x16A outlet. So... here's the nerdy and quite theoretical part :ugeek:

I'm an electrical engineer, and as such of course interested in the inner workings of the i3. Looking into the tech docs available in the global i3 facebook group, it seems that 32A / 7.4kW charging is actually handled by two chargers in the i3. One 3.7kW charger in the EME (electrical machine electronics, present in all models), and one 3.7kW charger in the KLE (convenience charger). The KLE seems to have had some problems as from what I can read in this forum, but hopefully they're fixed by now. As I understand the document, not all i3s are fitted with a KLE, but it will be there if the DC option is chosen.

The interesting part is that the (single phase) AC connection is run from the charging socket through the KLE and on to the EME, each pulling up to 16A. So my geeky mind would believe that, given special cabling, these two units could be run from a phase each. So connect each charger to its own phase, let the EVSE tell the car to use 32A, and presto, it will use 2x16A. That would be really nice!

One thing I could be concerned about would be the fact that the two chargers would be run on AC that is out of phase. If they shared control electronics, the active rectifying of the AC could go wrong (if the timing is only based on one of the phases). However they do not seem to be connected through other connections than the CAN busses.

Too bad BMW did not give us this option - it could actually be implemented using a few contactors controlled by the LIM (charge control unit). However 2-phase charging probably isn't a standard, even if would probably work perfectly fine with all 3-phase EVSEs.

It could be fun to do a wiring hack, but given warranty issues etc., I don't think I've got the guts for it right now :?
 
The i3 can accept 32A anyway, so there's no point in changing it. All that would be necessary would be an EVSE that could accept 2 phases at 16A per phase and combine them into a single 32A supply for input to the i3.
 
PhilH said:
All that would be necessary would be an EVSE that could accept 2 phases at 16A per phase and combine them into a single 32A supply for input to the i3.
Combining two phases is not an easy task from an engineering point of view, as the phases are, well, out of phase ;) That's the whole point about a 3-phase system - you can't just put two phases in parallel to get double current - that will be a short and blow a fuse.

It could be done by rectifying each phase into DC, then make AC from this. This would be overly complicated, as it would be simpler to stop at the DC stage and use that for CCS charging instead.

Still, we already have the i3 charging split into two separate units that should be able to work from separate phases.
 
Hooking up an EVSE with less than design wiring is asking for problems. The car only tries to draw up to the amount it wants or the amount indicated available by the pilot signal from the EVSE. If you wired up an EVSE capable (and reporting) it had 32A available to the mains when less was available, it would likely blow a fuse or trip a breaker on its supply...nothing radical or out of the ordinary there.

The EV standard allows for multiple phase charging, and it might be incorporated in those cars sold in Belgium, as that power seems more common there in the EU, but the generic plug and wiring in the car is not setup for it for most (all?) i3's sold. Multiple phase charging is much more common on large commercial vehicles, where sometimes huge batteries need to be charged as fast as possible and DC charging is not feasible.

One of the goals on the i3 design was to watch every gram of weight. Extra wires, or longer ones were all considered, and I recently read, one reason why the i3's charging port is where it is, is so that the cable didn't have to be any longer and thus weigh more.
 
jadnashuanh said:
If you wired up an EVSE capable (and reporting) it had 32A available to the mains when less was available, it would likely blow a fuse or trip a breaker on its supply...nothing radical or out of the ordinary there.
Well, this thread is meant as a theoretical technical discussion anyway. I probably wouldn't want to mess with the car's wiring even if I was a trained i3 technician. Rather, the point is that all the infrastructure for two-phase charging is actually right there in the car. When charging with 32A, each charger in the car pulls only 16A and could do so from two phases, given BMW had designed the wiring this way. So no, the fuse wouldn't blow due to each charger pulling only 16A in my example. But yes, it would a nasty hack ;-)

jadnashuanh said:
The EV standard allows for multiple phase charging, and it might be incorporated in those cars sold in Belgium, as that power seems more common there in the EU
Now that would be nice, but I haven't found anything online about multiphase charging in EU models. Anyone here in the EU having a multiphase charging i3 version?

jadnashuanh said:
One of the goals on the i3 design was to watch every gram of weight. Extra wires, or longer ones were all considered, and I recently read, one reason why the i3's charging port is where it is, is so that the cable didn't have to be any longer and thus weigh more.
It would be a minor weight difference, but of course you're right about the design goals :)
 
huntiedk said:
Although a very happy i3 REX owner for a few months now, it's always been bugging me that the i3 can only charge using one phase.
I'm not an electrical engineer, so my knowledge of electricity is far inferior to yours. However, the J1772 socket has AC1 (phase 1?), AC2 (phase 2?), and ground pins. My L2 EVSE is powered by 2 120V AC power lines 120° out of phase, or 2 phases of 3-phase power. L2 EVSE's of many EV owners are powered by 2 120V AC power lines 180° out of phase. Aren't i3's being charged by such L2 EVSE's being charged by 2-phase power?
 
alohart said:
huntiedk said:
Although a very happy i3 REX owner for a few months now, it's always been bugging me that the i3 can only charge using one phase.
I'm not an electrical engineer, so my knowledge of electricity is far inferior to yours. However, the J1772 socket has AC1 (phase 1?), AC2 (phase 2?), and ground pins. My L2 EVSE is powered by 2 120V AC power lines 120° out of phase, or 2 phases of 3-phase power. L2 EVSE's of many EV owners are powered by 2 120V AC power lines 180° out of phase. Aren't i3's being charged by such L2 EVSE's being charged by 2-phase power?
No...

Typical home in the USA has 240vac coming in...one phase. To get 120vac, the secondary of the transformer has a center tap, which, if you reference it to either end, gives you the 120vac to the 'neutral' or halfway point across the secondary of the transformer. In the USA, we also tie that neutral to a system safety ground.

In Europe, they still supply 240vac coming in, but they do not use a transformer with a center-tap on the transformer. IOW, both are getting single phase power. It's just that if you looked at one end of the transformer's secondary winding, when one end was high, the other end would be low, then they switch. This is not two-phase power.

True two or three phase power uses a different kind of transformer, and 3-phase is more common. IN that situation, each phase is 120-degrees delayed from the others, one forward of it, the other backwards, so each one is 120-degrees from each other. The plug specification for both the USA and the EU DO include contacts for that, but because they are not used, neither our cars or the typical plugs include them. If you happened to find one with those pins, they wouldn't plug into our cars because the sockets aren't in there. They could have made empty holes for them, just like when you plug in a grounded or ungrounded plug into a typical USA 3-prong receptacle, but they didn't. Try to plug a 3-prong plug into a receptacle with only two slots...it doesn't work, so hopefully this example clarifies things...they are both standards, and often interchangeable, but not on our cars. 3-phase power is used on some larger, often commercial, things like busses and trucks where they need more power faster to recharge them. I do not know of any EV that supports it.

The advantage of multiple phases is that to end up with the same power, you can use multiple sources, each with less. On the other side, this also means more wires, more expensive plugs and sockets, more complexity inside if you need to be able to switch from one type to another, and also more weight. I can see why it is not done for our EV's. It's much simpler on the i3 with its two power supplies to just feed them in parallel, rather than having to have a serial-parallel switch in there and detect when it needs to separate them. IN our case, feeding them in parallel, they each can share the load equally. They each would need similar cooling. The control logic to throttle the current as the batteries get closer to full charge could be the same. It could get very messy if say one phase had 20A available, and the other 15A...there's only one pilot signal to tell the car how much power it can draw...not one from each supply. Then, the interlock circuits would need to be able to support two supplies and the resistor network that detects what's attached would need some work to be compatible.

It's not as simple as you might think!
 
jadnashuanh said:
True two or three phase power uses a different kind of transformer, and 3-phase is more common. IN that situation, each phase is 120-degrees delayed from the others, one forward of it, the other backwards, so each one is 120-degrees from each other.
The power in our apartment is 120Y/208V. Isn't that 2 phases of 3-phase power with each 120V line 120º out of phase and 208V between each phase? Our EVSE is powered by both 120V lines, or 208V across these lines. Seems like our EVSE is using 2-phase power that is passed through to our i3's charger.

jadnashuanh said:
3-phase power is used on some larger, often commercial, things like busses and trucks where they need more power faster to recharge them. I do not know of any EV that supports it.
Renault Zoe.
 
Multi-phase power is more common in commercial or larger facilities. The reason why you only get 208vac when you use two legs is because they are 120-degrees out of phase rather than them adding together. If the car also used a neutral in this situation, you'd have more power available; it does not...the EVSE is only connected to P1 and P2 (or P2 and P3, or P1 and P3) without the neutral. This is also why a 30/32A EVSE connected to this type of power won't max out the i3's power needs since power=volts*amps, and the amps are limited. Many of the public EVSE's use that as their power input. Functionally, though, without the neutral, it is single phase.
 
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