[bwilson4web]

Hi,

Originally Posted by weazel

u should drop using any battery there all rubish id go for supper caps . . .

I had looked at "supper caps" (ultracaps?) before but found there were problems:

1. voltage energy storage device - to extract energy, we have to convert the stored ultracap voltage into usable voltages and currents. In my case, a pretty massive DC-to-DC converter that generates 48VDC at 15A (720W max) in addition to the ultracaps. Furthermore, the DC-to-DC converter will have to deal with either small voltages and massive currents or large voltages that have a huge swing. Let me give an example for my 720W maximum load:
   • 2.5V (1) -> 288A
   • 5.0V (2) -> 144A
   • 12.5V (5) -> 57.6A
   • 25V (10) -> 28.8A
   • 50V (20) -> 14.4A
   • 125V (50) -> 5.76A
   • 250V (100) -> 2.88A
   • 500V (200) -> 1.44A
   • 1,250V (500) -> 0.58A
   The wide voltage swing means a DC-to-DC converter that already will be running ~90% efficiency ... Opps just lost 10% of the energy ... that has to have a very wide input and close to discharge, some massive currents. They can be built (look for inverting switching regulator) but in this energy and voltage range, pretty expensive.
2. energy density - most of the ultracaps I've seen are cylindrical. This means intra-cap space of cylindrical units is unusable. In contrast, these poly-cell batteries are flat, stackable plates so we'll get a minimum sized power pack which is important on a space limited, electric bicycle. Worse, we're looking at .7-1.8 MJ/kg versus .56-.1 for supercapacitor which means a heavier ultracap for the same amount of energy. My electric bike needs to be lighter, not heavier.
3. series ultracaps voltage balancing - the cheap solutions, accurate resistors, work by losing energy. All other solutions are terribly expensive.
4. surface versus volume energy storage - all caps require two surfaces with a dialectic between them and store the energy in the voltage field. In contrast, these lithium iron phosphate batteries store their energy inside the mass of the electrodes, not a surface layer.

To contrast, these lithium iron phosphate batteries have:

• 90-100 Wh/kg
• 220 Wh/L
• 3.3V constant discharge voltage

So for ultacapacitors, we are looking at:

• 0.5 to 30 Wh/kg
• 0.33 to 3.89 kg/L -> 117 Wh/L (best case)
• 0-2.5v per ultracap

There are applications where an ultracap makes sense. It has extremely high discharge rates but that is not needed in my eBike application. If I were making a 'rail gun,' an ultracap would be the way to go. They might also work with a battery to provide a high start up energy surge that these batteries don't do as well in providing.

Bob Wilson