The way I understand it the Volts * Amps should equal watts
but as you can see it doesn't is something wrong.\
I did do a zero calibration with batteries disconnected from camper load. so the only draw is the BMV itself
asked
BMV-712 Math Volt*Amps should equal Watts not (Watts*2)
I received and email from Jim Victron.... I am not sure if he is a "Victron" or representative from the company; either way I was impressed. He basically outlined all the things I have to check and submit to rule out operator error. First I read the exact procedure for zero current calibration. I did it exactly disconnecting load from shunt. (previously I disconnected the positive cable which I didn't think that would make a difference) Upon doing that I was getting the same result almost exactly 1 watt high reading on really low loads 3 watts versus 2 watts is a pretty big error. I did also decided to do the current calibration on the monitor itself rather than on the Bluetooth app to make sure all my ducks are in a row. I later decided to double check the monitor versus the Bluetooth again to make sure I have covered all my bases.
Kaboom...………………..! I was astounded the Monitor shows current in two decimal digits!
The Bluetooth shows only one digit of decimal digits. I never thought to look at the monitor itself.....? why would I when I have the Bluetooth app! Needless to say The monitor is correct.
12.98v * .1a=1.29W round up to 2 watts (no decimal in watt reading) Bluetooth was showing 3W
From Battery Monitor
12.98v * .19a=2.46W round up to 3 watts
There you have it a decimal place in the monitor that is not in the app.
It's great now that I know that..... I ask myself was I the idiot or was there some expectation of the app being the same. I would have expected it; at least if you get someone else with this kind of experience maybe they can read this post.
I am even more impressed with the BMV now that I know I can measure current in 2 decimal digits I just have to use the monitor and not the Bluetooth display. I think Victron should consider adding that digit to the Bluetooth display.
Thank you for everyone's help and insight.... appreciate it.
Jeff
Did you get the same "false" readings with bigger loads?
Sorry I posted above the question.... they seem to have changed the question answer format it wasn't clear to me earlier. The picture above is for a larger load but charging instead of discharging. I looked at it more today and it seems to be consistently high by 1 watt on low loads. I even changed the fuse on the temperature sensor to a 1amp fast fuse(only one I could find from 100ma) from looking at other threads regarding BMV issues. That didn't help. Is there a calibration issue with this unit or a simple software math error? I am debating whether to try and return the unit as it's kind of annoying because I am likely to be trying to measure low loads most of the time. Is this likely normal for this unit or would a replaced one fix the problem?
Thank you
No..... I attached picture it’s not perfect but close... Acceptable
You're measuring way down at the bottom end of the scale there. You promised have 3 bits of ADC resolution to measure with. You are measuring about 10 microvolts over that shunt with that small amount of current. You are displaying the smallest least significant digit, the next digit is 0.0
If you intend to measure really low current most of the time, then you'd be better off with a more sensitive shunt. A 100A / 50mV shunt would give you five time more sensitivity. Remember to amend the shunt setting's in the BMV if you decide to go this route. It think you get another least significant digit IIRC.
The 500A shunt which is included is selected for the most probable use cases where a few Amps is the expected minimum and a few hundred Amps at the top end.
Thank Kirby for the insight....the 3 bits of ADC I dont understand but i do understand what the big picture of what you are saying.
Does Victron have a shunt that I can purchase that meets your criteria and better serve my application
(100A / 50mv) that I can connect to the BMV?
Thank You
I'll take it as a given that the BMV-712 can be configured for a 100A shunt (I don't know either way). If so, there are DC current shunts that you can purchase from companies like Deltec or through distributors, etc. I didn't see a 100A option on the datasheet.
Ideally you can find one where the mounting screws on the shunt lines up with the BMV-712 PCB so you can do a drop in replacement... you might have to do some measurements and internet searches etc.
