[BrianG]

This post is for anyone using an Eagletree device to gauge their amp draw in their setup.

I was playing around with my XT8 truggy (Medusa 36-80-2200, MMM, 5s 5Ah RCM Lipos) and decided to get an ET graph just for giggles. Once I downloaded the results, I was overall pleased, but a little disappointed at one area of the results. At 161A, the voltage fell to 15.74 (3.148v/cell). The puzzling part was that the rest of the graph didn't support this reading.

So, I plotted a number of different voltage and current points at various places and the total pack resistance varies from 0.0131 to 0.0133, at currents spanning from 53A up to 121.5A. That's a pretty consistent resistance number.

So, something is amiss here. I figure one of two possibities:

1) The pack resistance increased dramatically somewhere between 121A and 161A. Well, I don't believe this to be true. I highly doubt the pack resistance was pretty constant all the way up to 121A, and then rose dramatically at just 40A more. Possible I suppose, but unlikely.

2) Eagletree errors. If I assume the pack voltage was correct (15.74v) and use my extrapolated resistance number of 0.135 (slightly higher to compensate for the higher current), I get a calculated actual current closer to 200A (40C).

Option number 2 makes more sense. And to support this reasoning, I took a real close look at the graph at that 161A spike and noticed the spike flattens abruptly at around 155A, but the pack voltage spike was pretty peaked. This tells me the ET device drastically loses accuracy much beyond its 150A rating. A look at the Hall-Effect datasheet for the sensor the ET uses, I see that it is rated up to 150A, and that's it. It doesn't even give a little extra margin.

So, long story short; if you are using an Eagletree device to gauge your setup, be aware that the currents you are reading may be a lot less than they actually are beyond the rating. I'm thinking a 300A version would be better to use for larger vehicles.