Electric 818 | AC & LiFePO4 configuration

[KrissMotors]

Why not use a step-up transformer to get higher voltages? I understand the controllers create a variable frequency waveform, making a variable frequency transformer necessary -- and I've never heard of one -- but why couldn't we power an inverter from the battery with a step-up transformer and a rectifier to power the motor controller? Bug zappers and Tasers both create high voltages without extra batteries.

As I understand the question, it relates to using more powerful motors, not efficiency. When it comes to making big power - whether gas or electric - nothing can be called "efficient"! I think the answer to the question really is that there is nothing to be gained. Stepping up the voltage would either result in fewer amps at the stepped up voltage (as I believe would be the case in this particular configuration), or fewer miles out of the battery pack. The latter could be a worthwhile trade-off in some usage cases, but I don't think it applies to the battery configuration under discussion here.

This topic gets complicated quickly. Bottom line: there is no realistic way to transform the voltage/current profile of a battery pack. As RM1SepEx says, efficiency loss is one reason. Each conversion step, even a relatively efficient one, loses at least 10% of the total energy, and that translates into a 10% mileage range reduction.

But there is a more profound limitation imposed by physics, and I'll try to outline the technical issues below.

TRANSFORMERS
Transformers take advantage of changes in magnetic flux that induce current flow in a nearby conductor. So, if you run alternating current (AC) voltage through a primary conductor, the rise and fall of magnetic flux induces current in the secondary conductor. This is a typical experiment in high school physics class. The ratio of windings in the primary and secondary either steps up or steps down the voltage, and this is the basis of long distance power transmission that uses AC.

Here's the physics limitation: the transformer requires a change in magnetic flux and you get that automatically with alternating current. But a battery pack outputs direct current (DC) with a constant flux. Thus, you can't use a transformer with a DC battery pack.

CONVERTERS
This does not mean that you can't change DC voltage. You can, but first you need to convert it back to AC, then run the current into a transformer, then convert back to DC. The device that does this is called a DC-DC converter. In fact, electrics cars routinely use DC-DC converters to take a little bit of the main pack voltage (say, 120v) and step it down to 12v for automotive relays, lights, dash instruments, etc. This avoids the need for a separate 12v battery and alternator/generator setup. For this rather simple job - usually about 40 amps at 12v - you need a DC-DC converter about 6x6x6" and it gets hot.

But there are practical limits to DC-DC conversion. To run the DC motors we're talking about, you need about 360v at 300amps. Here's the math: 900 amps at 120vdc converts into 300 amps at 360vdc (actually, you would lose at least 10% in the conversion process, but I'm ignoring this inefficiency). First, the LiFePO4 pack I'm talking about can't generate 900 amps ... but assume for a moment that it could. You would then need electronics that can handle a 900 amp input. Probably two DMOC645s will do the job; they are extremely expensive. But you also need a transformer that can handle 900 amps. Typical residential power transmission lines have transformers that could do the job ... but look at them on the power poles! They're enormous, heavy (probably over 1000 lbs) objects unsuitable for automotive use. Laws of physics just won't let you get there unfortunately.

The engineering lesson is that the battery pack must be designed to match the operating characteristics of both the motor/controller and the transmission/use of the vehicle. You can't transform or convert your way to a better place.