Hi,
Quote:
Originally Posted by weazel
u should drop using any battery there all rubish id go for supper caps . . .
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I had looked at "supper caps" (ultracaps?) before but found there were problems:
- 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.
- 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.
- series ultracaps voltage balancing - the cheap solutions, accurate resistors, work by losing energy. All other solutions are terribly expensive.
- 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