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Old 06-19-2009, 03:55 PM   #6 (permalink)
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Thanks for the comments, jyanof. You nailed it: the second pack is like a big capacitor bank.

Regarding the inductor: this topic actually started on another thread before I moved it here, and that's where I stated that I'd have to add an inductor in series with the batteries, as well as probably adding another diode in series on the positive lead, too, just to guard against the possibility of overvoltage on the input side.

The LCP cells are a nominal 3.6V. They charge up to 4.25V, which is 144V in my case. The LFP cells have a nominal voltage of 3.2V, and charge up to 3.65V per cell, which is 146V. So the two strings are about equal at full charge. The LCP cells settle down to about 4V/cell after charging, or 140V. The LFP cells settle at about 3.35V/cell after charging, or about 136V. Under load, I expect the LFP cells to sag about 8V at 50A. Based on the internal resistances of the two strings, I expect to need between a 15V and 20V difference in the two packs to get the full 70A. So when the current draw on the LFP cells goes to 2C sometimes, they should sag maybe 16V, and the full 70A will be applied. At lighter current draws, I may not be able to get the full 70A. I might need to reduce the number of LFP cells to 39 or 38.

Only real-world testing will tell.


Last edited by wjdennis; 06-19-2009 at 04:20 PM..
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