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Old 12-30-2017, 03:19 AM   #1
Wingsy
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Default A Close Look At Battery Current

In testing a current sensor I plan to use in my speedometer & battery gauge project, I found some interesting things going on with charging current. This post ain't for everybody but some of you guys may find this at least entertaining.

First, this is an oscilloscope image of charge current when the charger is first plugged in, around 15A. The scope trace at center horizontal line is zero amps; above centerline is charge current and below centerline is discharge current. The scale is roughly 20A per major division at a time scale of 2.5ms per division. We have 120 pulses per second (from the 60Hz full wave rectifier in the charger) of around 30A peak, averaging near 15A like the charger meter indicates. So far so good. (Yes, it's full of electrical noise. I'll hit that later.)
Charge15A.png

Later when the charge current dropped to around 5A those 30A pulses dropped off to around 10A or so, averaging about 5A.
Charge - Variable5A.png

But what I noticed is that the peak currents were not the same. There was a variation in the amplitude over time. So I changed the time scale to 1 second per division to get a look at it over several seconds.
ChargeVariation(8S).png
Huh? Not what I expected AT ALL. (Those diagonal lines are what the scope displays in place of a saturated area, and shows just the peak-to-peak voltages.) I'm almost convinced that there was something wrong with my measurements because it makes no sense for the charge current to fluctuate like this, plus the meter on the charger was holding fairly steady at 5A. I will repeat these tests again tomorrow.

Something else I noticed. Occasionally there was a burst of high frequency current when charging at 5A. I eventually managed to capture it.
ChargeVariation - Burst.png
It lasts around 3/4 second and consists of pulses of charge AND discharge current. Had no idea the OBC did this. It's a reconditioning pulse used to desulfate a battery. I don't know if the OBC does this all the time or because it doesn't know my battery history and is assuming it needs it (had to remove a battery cable to install the sensor so the OBC got reset in the process).

I had the rear wheels off the ground for testing discharge current and captured this image. What you see is where I hit the throttle a couple of times, and you can see a little bit of regenerative braking when I got off the throttle the 2nd time.
Accel2.png
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Old 12-30-2017, 09:47 AM   #2
scottyb
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Default Re: A Close Look At Battery Current

Great post - thanks for the info
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Old 12-30-2017, 09:00 PM   #3
yawood
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Default Re: A Close Look At Battery Current

Very interesting, especially the desulfating bursts.
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Old 12-30-2017, 11:32 PM   #4
Htownblue
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Default Re: A Close Look At Battery Current

Well the fluctuations would seem to be due to the resistance from the batteries as current is forced back into the system. Just my thought.
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Old 12-31-2017, 03:55 PM   #5
cgtech
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Default Re: A Close Look At Battery Current

Quote:
Originally Posted by Htownblue View Post
Well the fluctuations would seem to be due to the resistance from the batteries as current is forced back into the system. Just my thought.
I don't think that explaination is possible. The current always flows from the point of higher voltage (charging system) to the point of lower voltage (the batteries). So, unless the batteries suddenly became the point of higher voltage, like a brief charging system shutdown, wouldn't be possible. I have not seen the "guts" of any of the newer smaller OBCs, so I don't know what's in them. I will be changing out a newer style one next week, sounds like it will be taking a bath in paint thinner to break up the potting.
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