I've been driving an EV1 for about 4 months now, plenty long enough to have been exposed to the Great Inductive (GM) Vs Conductive (Honda/Ford) Charging debate.
Everyone agrees that the success of electric vehicles will depend in large part on the widespread deployment of a standard charging infrastructure. It goes without saying that this infrastructure should be based on the best technology available, and that a costly and confusing VHS-vs-Beta standards battle should be avoided.
This essay is my attempt to analyze these relative strengths and weaknesses of the various approaches to EV charging in the hope of informing the debate. I think I can maintain a fairly neutral position, as I personally feel that the jury is still out on the issue.
I am specifically excluding non-technical issues such as the current popularity of each type in public installations and the amount of marketing muscle being exerted by each system's champions, as this discussion is focused on the underlying technical merits of each approach.
The most obvious way to charge a big battery from the AC power line is with a switching power supply controlled by battery voltage, current, temperature and time. Switching power supplies are far lighter, cheaper and more efficient than conventional "linear" power supplies. A switching power supply can also operate at full power to rapid charge a battery until it is nearly full and then taper off to prevent excessive heating and gassing.
The offboard approach has the definite advantage of offloading much of the weight and volume that would otherwise have to be carried around in the car. It also gives the charger designer a somewhat more relaxed weight and volume budget. Among other things, this lets him do a better job in suppressing radio frequency interference, as good high power RFI line filters tend to be physically large. (As evidence of this, the 6.6 kW Magnecharger seems rather quiet, radio-wise, while the portable 1.2 kW charger is quite noisy on the lower HF bands up to about 5.5 MHz.)
Both the GM EV1 and the Honda EV+ appear to use switching power supplies in their chargers. The GM approach splits the switching supply right down the middle of the high frequency transformer. Everything up to and including its primary is in the charger, and the secondary and beyond is in the car. The Honda EV+ (and presumably the Ford Ranger) have onboard chargers, so the external conductive interface simply carries high current 240V AC.
An offboard charger that produces high frequency AC is fundamental to the inductive approach; an inductive coupler rated for, say, 6.6 kW at 60 Hz would be impractically massive. But an offboard charger could also be built with a conductive interface, with the same reductions in onboard size and weight.
For example, you replace the existing paddle and charge coupler in the EV1 with a conventional high frequency transformer in the car whose primary is hardwired to the charger. You'd also replace the existing 915 MHz RF car/charger control link with an optoisolator or signal transformer. Then you'd "cut the cord" on the existing Magnecharger and install conductive connectors. Instead of carrying the control link across auxiliary conductive contacts, you could make this an optical link.
This approach would still allow a common conductive charger to be used with a variety of EV battery voltages. You'd establish a standard voltage on the conductive connector, and the onboard transformer would have the appropriate windings to step this up or down to the desired pack voltage.
In short, both the inductive and conductive approaches lend themselves equally well to a stationary charger that can be left behind while driving.
But this too is not a fundamental difference between inductive and conductive charging. The Honda's onboard charger is only a 4.4 kW unit, so it doesn't have quite as much heat to dissipate; one would expect it to be at least a little quieter. I don't think there's any fundamental reason why an offboard inductive charger couldn't be made as quiet as an offboard conductive charger of the same power rating.
It's possible (and this is speculation) that it's inherently easier to quietly cool an onboard charger like that in the Honda EV+ by using the vehicle's existing liquid cooling system. But this has more to do with the onboard/offboard issue than with the choice of inductive or conductive coupling, and we've already established that an offboard charger is preferable for reasons of weight and volume.
I've seen several public charging stations with nearby disconnect and breaker panels that can be easily opened to expose bare 240V and even 480V conductors. Obviously these are far more of a potential hazard to, say, a young child than a well-designed conductive charging connector.
Again, the seemingly greater complexity of the inductive charger is really due to it containing circuitry that would otherwise be carried on the vehicle even if it had a conductive connector. And if this circuitry is to fail, it's better to have it happen in a stationary charger than inside your car. If a charger fails, you can always go to another one. If circuitry inside your car fails, you're stuck.
Conductive interfaces have also been criticized as unreliable. And perhaps they are inherently somewhat more so than an inductive coupler with no exposed metal surfaces to corrode. But an inductive coupler is not immune either to mechanical damage, e.g., to sensor microswitches, control antennas, door covers, etc.
In sum, I suspect that the learning curves will eventually produce both inductive and conductive couplers that are quite reliable.
While we engineers tend to dismiss such issues, the fact is that ease-of-use is very important to the mainstream adoption of any new technology. And it seems unlikely that any high-power, high voltage conductive connector can ever be quite as easy to use as the inductive paddle.
The efficiency issue also creates a cooling problem. The EV1 already runs coolant to the coupler, though it's not clear if this is really needed for 6.6 kW charging or is there in anticipation of future Level 3 (high power - 50 kW) charging. It is already known that the Level 3 paddles will have to be liquid cooled through lines down their cables.
In contrast, it seems doubtful that any well-designed conductive connector would dissipate so much power as to require liquid cooling, even at the 50 kW power level.
This approach, called reductive charging by Alan Cocconi (one of the original EV1 designers) has the advantage of requiring neither an offboard charger like the Magnecharger or the nearly equivalent onboard charger. The inverter would double as the charger, and with its existing liquid cooling system it could probably handle some fairly high charging power levels -- at least as high as those encountered in highway cruising.
This approach would dictate a conductive interface.
For safety's sake, this approach would require the complete isolation of the propulsion battery and all related circuitry from the chassis of the car. Both the negative and positive lines from the propulsion battery would be "hot" with AC with respect to ground while charging. The EV1 is already designed this way. But Cocconi reports that even with this isolation, small leakage currents can exist that will trip a GFCI in the supply circuit and careful design of the drive motor is required to eliminate them.
As an alternative, a stationary AC isolation transformer could be provided on the outlet used for charging. But because this would have to be a 60 Hz transformer rated for full charging power, it would be physically large and heavy. It could easily be heavier and possibly even more expensive than the existing Magnecharger.
The existing switch-mode chargers, for all their faults, do provide AC ground isolation "for free" through their high-frequency transformers -- either the one built into the car (conductive approach) or the one formed by the paddle and coupler in the inductive approach. And they do adapt readily to changes in AC line voltages. Both problems would have to be solved in any direct AC charging scheme.
The reductive charging scheme seems attractive because it requires no special charging hardware, either on the car or at the charging site. But it turns out that the charger is only a fraction of the cost of many charging sites. Simply bringing power to the parking spot turns out to be much more costly in most cases. Even 120V convenience outlets, sufficient only for very slow EV charging, are rare in parking lots. Higher power feeds have to be installed specifically for EV charging. This often requires trenching under asphalt in a parking lot or running hundreds of feet of conduit in a parking garage. Sometimes the runs are long enough to require a 480V feed and a step-down transformer near the charger, further increasing the cost.
It's also reasonable to expect that with volume production, Magnechargers will come down in price faster than the cost of installing them.
Comments on this essay are invited.