Hey everyone, I’ve finally finished my first build after browsing these forums regularly for the past 4 months. I don’t have any pictures, but I made a video instead. Let me know what you think:
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Do you get any voltage sag?
What pulley set do you use?
15T/36T, 255mm 5M HTD belt, it can be found here:
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Nice video, good to see it’s possible to solder those batteries. Do watch you joints, some looked to me a bit brittle and ‘cold’. Bit scary with the metal box tho.
But it’s nice to see you made almost everything yourself.
Very nice first build!
I actually insulated the bottom of where the vesc was sitting to make sure nothing would short, I was worried about this too.
Voltage sag is relatively small, but still significant with my specific pack because of the internal resistance of the cells.
Every cell has ~20milliohms IR which will be the cause of the power dissipation and voltage sag. Total power at the terminals of the pack is
P = IV - I^2*R
where I^2*R is the loss dissipated as heat due to internal resistance. Since we’re only drawing 60A maximum,
I^2*R = (60)^2*(.020*8 cells in series) = 576 watts
and then at the terminals, ideal power is:
3.7V*8 cells in series * 60A = 1.776 kW
so we can set up a ratio to find voltage sag and actual power:
1776W/29.6V = (1776W-576W)/x volts -> x = 20V
so we’re getting a voltage sag of approx
29.6V - 20V = 9.6V
at maximum current draw, and voltage sag will decrease exponentially from P = I^2*R as the ESC draws less current
All calculations aside, it accelerates quick enough, and although voltage sag is an unfortunate characteristic of non-ideal voltage sources, we can still draw more current to account for power loss as long as it’s not above 60A
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