question

joshgray avatar image
joshgray asked

BMV Instantaneous power fluctuates; how to tell power draw?

I’ve noticed that while charging via solar, the BMV-702’s value for watts being used fluctuates with the power coming from the charge controller, even when the load is steady. What might I be doing wrong?


I’m using a Raspberry Pi to read the data from the battery monitor. When I say `watts being used` this is the “P” value, which they document as `Instantaneous power`. Overnight when not charging this value is stable at about -19w which is a bunch of AC devices like cell modem, wifi, Rpi, etc. I understand this as it’s negative because it’s drawing power. As soon as the sun comes out, this value goes above zero, as expected because it’s now generating watts, but if I take the value that the charge controllers says is charging and subtract P-value, I still get a much higher number than roughly 19w and the graphs track with the intensity of the solar charge.


Obviously I’m not understanding something here?


300ah AGM (3x 100ah) batteries

Renogy Rover 60a Charge Controller

Renogy 2000v Inverter

Victron BMV-702 Battery Monitor


Wired:

Shunt wired between batteries and bus bar, everything else connected to the bus bar(s)


[1] http://www.tiarora.no/Victron/VE.Direct-Protocol-3.24.pdf

BMV Battery Monitor
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1 Answer
joshgray avatar image
joshgray answered ·

Here's what I'm doing. Two screenshots of graphs showing middle of the night, and middle of the day. I should also preface the BMV is not synchronized correctly yet but I thought that just affects SOC%?

Screen Shot 2020-08-09 at 4.16.08 PM.png
Screen Shot 2020-08-09 at 4.15.46 PM.png

Green - Value coming from the charge controller

Yellow - P-value coming from BMV

Red - Value I'm calculating based on the below:

if ValueP > 0:

inuse_Power_Calculated = chrgPower - ValueP

else:

inuse_Power_Calculated = ValueP * -1


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