Originally Posted by
WillS
Please elaborate on this? Working avionics this goes against everything I was taught and understand about batteries/electronics.
Originally Posted by
WillS
This is how the internals of a Lithium Polymer battery looks
With that being said, the battery manufacturer determines the tolerances. The plates and how they transfer the power through the tabs is all part of this process. So how they manufacture the batteries determines the internal (inside the battery) resistance. Now the mah or milliamp hour of the battery is determined through the size of those plates. So an 8000mah battery can have the same internal resistance of a 200mah battery and vice versa. The size of that plate creating the larger milliamp hour does not increase voltage. They merely act as a larger fuel tank in the simplistic nature. Danny of SMC discusses this a lot on his facebook. He honestly is one of the most beneficial people to the industry. For stock a lower resistance pack will allow for more power to be transferred into the esc to be converted to the power needed for the motor.
Higher resistance = smoother (makes electronics work harder)
Lower resistance = punchier (makes electronics work easier, creating more power)
Higher mah = more run time
lower mah = less run time but possibly lighter (LRP sells these as lcg packs)
What I run in my cars:
Stock buggy 4250
Stock stadium truck 5400
Stock SCT 5400
I have ran the 4250 in all my vehicles until the 5400 have come out. I can make run time with the 4250 in stock sct (probably the most load pulling class raced at big races) but I can feel the discharge curve hitting its limit at eight minutes after two minutes of warm-up laps.
Yes, the quality and actual chemistry of the cells determines the internal resistance while the size of the plates inside the pack changes the mah of the pack and not the nominal voltage. However, the chemistry and quality determines the internal resistance per volume or surface area in the cell. Not per cell, period. The positive and negative plates are part of the chemical reaction and having a larger surface area to pull current from ends up with a lower cell resistance.
Let me know if this helps:
What if we compare them to plain resistors. Let us take two resistors that have the exact same resistance. If you put them in series, the overall resistance doubles. If you put them in parallel, the overall resistance is cut in half. If you pass a current through the set in parallel you will have a lower voltage drop.
Now, instead of resistors you have lipo cells (3.7 volt nominal) rated at 2000 mah, with an IR of 4 milliohms. Take those two cells and put them in series. You'll have a 7.4 volt pack with 2000 mah and 8 milli-ohms of resistance. Now put two of these cells in parallel. You will have a 3.7 volt pack with 4000 mah and 2 milliohms of resistance. Just like the resistance of resistors is cut in half if you put two identical ones in parallel, the same holds true for battery packs. If you add another "1s pack" to it in series, you will end up with a 2s pack with 4000 mah and an internal resistance of 4 milliohms.
Going a step further, instead of having two identical cells and putting them in parallel you simply make a cell that is twice as large. Assuming the tabs and other wiring isn't an issue, the internal resistance of that larger cell is smaller than that of a smaller capacity cell of the exact same chemistry and quality (or roughly the same as two cells half its capacity in parallel).
Comparing it to your fuel tank analogy, you need to add an additional fuel pump to make the comparison roughly similar. We can equate the surface area inside the cell to the fuel pump in a gas tank, so if you add another gas tank and fuel pump to your truck, that is more similar to adding an additional cell in parallel than just adding a larger fuel tank. Now that you have an additional fuel pump, you can draw more fuel without starving the engine. Same holds true for cells in parallel or simply larger cells.
Sorry if I rambled.