[e*clipse]

I guess I'll chime in here.

It's generally known as not a good idea to mix & match cells. Even to the extent of mixing old/new batteries of the same type. Why? they will have different discharge curves and the pack will be limited by the worst performer of your batteries. Even different ages of the same cell will have different discharge curves. Expecting similar performance from cells with different characteristics will lead to many grey hairs. If you are entertaining the idea of mixing Lithium with Nickle-Metal Hydride or Lead Acid, you will be mixing different discharge curves and different start/end charging voltages. How you would (or some BMS) keep track of that - on the fly - is a mystery to me.

For example, I have a lithium powered LED flashlight, using two of the same cells the Tesla pack is made of (the 18650 laptop cell) in series. I purchased a large amount of these cells to build a test battery management system. So, being cheap, I grabbed two of the cells that had already been cycled many times and sat for several years. One was pretty worn out, the other pretty new. Long story short, the total run time of the flashlight is limited to the run time of the worn-out cell. The flashlight's LED system limits power output depending on incoming voltage. When the worn-out cell hits the "fall-off" point of its discharge curve, it shuts down the whole system, even though the other cell is more than half full. At one point, the run time of the flashlight was pathetic, worse than your average "c" cell powered incandescent bulb flashlight. Solution? replace the worn-out cell with one comparable to the stronger cell. Way better performance/run time.

Yes, I'm describing a simple flashlight, but the same thing will happen when you mix cells for something big like a car pack.

To make things worse, cycleing the worn out cell heavily like that will cause it to fail prematurely, unless you have a BMS that individually monitors cells, and takes care of the weak cell.

*Edit*
Also, regarding voltage doublers: They put out the same POWER ( minus the efficiency loss ) as the input power. Say you have a 250V pack. The motor you have is 50kW and wants to be supplied with 500V. The efficiency of the voltage doubler is 95%. (I'm just pulling #'s out of my ***. ) So, using the doubler will result in a 5% loss: 50kW * 5% = 2.5kW.... Therefore, the power being supplied by the pack must be 52.5kW. Of course, there are other losses for the inverter, wiring, etc - but we're focusing only on the doubler. Power = I*V . Therefore the current being supplied to the inverter by the doubler is 50kW/500V = I = 100Amps . The trick is that the doubler is not a constant current device, it is a constant power device. Therefore, the current that must be supplied by the batteries is 52.5kw (taking doubler losses into account). 52.5kW/250V = 210Amps. So, in drag racing the bug, theoretically you can run just as fast as with a high voltage pack, you will just be demanding 2X (or whatever) the current from the lower voltage battery pack.

A question, why are you interested in, say a prius pack (or battery pack) vs individual cells? Certain cells, like the headway are set up for being easy to inter-connect. Others can be a royal PIA, requiring welding the cells together.

- E*clipse