question

markus avatar image
markus asked

BMV Resolution and measuring small currents

Hello,

I have a remote off grid site, running a Raspberry Pi for monitoring and it is supplied by a 48Volt Battery Bank. Raspberry Pi + 4G Modem consuming together about 4W continuous power. The current measurement resolution shown in Venus is 100mA leading to a 5W resolution on a 48V battery bank. If no load is active except the RPi, I have a flipping DC consumption monitored and shown in Venus. So the VRM charts look like:

With a fully charged battery it gets logically worse, leading to zero consumption for longer periods:

Is there a way to improve this?

Thanks,

Markus

VRMVenus OSdc systembmv48v battery
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3 Answers
markus avatar image
markus answered ·

Hanging my head in shame, this is actually a very well documented setting, I had just overlooked.

07. Current threshold
When the current measured falls below this value it will be considered zero.
The current threshold is used to cancel out very small currents that can negatively affect the long term state-of-charge readout in noisy environments. For example if the actual long term current is 0,0A and due to injected noise or small offsets the battery monitor measures -0,05A, and in the long term the BMV can incorrectly indicate that the battery needs recharging. When the current threshold in this example is set to 0,1A, the BMV calculates with 0,0A so that errors are eliminated.
A value of 0,0A disables this function.

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wkirby answered ·

So, the BMV it's self has a resolution on 10mA (480mW), which is good. The BMV in a Venus environment seems to report a resolution of 100mA (4.8W), which is not a problem.
Now, as I'm sure you are aware, the measurements are taken as snapshots in time. The RaspberryPi is a computer, so its power consumption is contantly changing.
I wonder if the RaspberryPi and its associated components are spiking above the 4.8W threshold just as a reading is being taken?
It's awesome how such low power measurements (0.48W) are even possible over a shunt that is designed to provide readings up to 500A or 24KW!

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markus avatar image markus ♦♦ commented ·

Hi WKirby,

thanks for your answer. OK, so the SOC is calculated with 10mA resolution, this is good.

But I did some measurements with a good Fluke, and there were no such spikes.

DC Amp readings were rock solid when RPi was idle

So the reported resolution to Venus is a problem for me, a small one, indeed ;o)

Maybe there is a possibility to fine tune this somewhere in Venus itself.

Or to make a setting for this, in future firmwares...

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mattintheusa avatar image
mattintheusa answered ·

You could also use a different current shunt with the BMV that would give you more accurate current measurements. The BMV allows you to set the mV and amps for a non standard current shunt. I am using a current shunt rated at 200A at 100 mV, which I setup in the BMV as 150A at 75 mV. This gives me a resolution of 2A/mV versus the resolution of 10A/mV of the shunt included with the BMV. My system never exceeds 150A so this works fines. The shunt I use is part number FN-200-100, purchased from Newark Electronics.


Hope this is useful.

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