Victron BMV-700 faulty?

[Justifier]

Hi,

I am new to these forums so my apologies if my terminology is not quite right.

I purchased the BMV-700 battery monitor and I am having problems getting the monitor to show the correct amp usage and time remaining.  I have been very careful to wire the shunt correctly such that the battery side of the shunt goes straight to the 12v battery negative and the loan side of the shunt receives everything from my negative bus I have mounted.  I have an electric Watersnake 12v Boat Motor which is connected to the negative bus and therefore load side of the shunt and and the positive from the motor is going to a positive bus which is connected to the positive side of the 12v battery (with a 50amp circuit breaker).  The small red positive cable of the BMV-700 goes to this positive bus as well.

The problem is that the monitor is showing and amp draw of about minus
1.8-1.9 amps and minus 23w and is showing about 40 hours left from my
105 amp hour AGM battery.  I know from the specifications that this motor draws between 7.5amps and 35amps depending on the motor speed setting (1-5).  So the draw and time remaining are way off.  At full speed (5) on the motor it should be showing about 35 amp draw and time remaining of about 1.5hrs (based on 50% discharge setting and 105ah battery).

What am I doing wrong or do I have a defective unit?  I only purchased the unit a few months ago and I have only just installed it.

Someone's help would be greatly appreciated.

Thanks

[Justifier]

Hi jdp, thanks for your reply.  Yes I followed the set up instructions very carefully and have re-synced a few times at 0 load to re-test.  I am sure the wiring is correct too.  It seems very strange that the monitor is reporting -1.9amps (or -23w) regardless of the speed setting of the motor.  The specifications for the motor say about 7.5amps at speed 1 up to 35 amps at speed 5.

[superdiy]

46 minutes ago, Justifier said:

The specifications for the motor say about 7.5amps at speed 1 up to 35 amps at speed 5.

Under load or under no load?

[Justifier]

Yes, I am now guessing that it is not giving me similar readings to the specifications because it is not in the water.  I should have thought of that but I thought that the current draw would still vary out of the water with different speed settings (from 1 - 5) but it does not.....

[Mark]

I think that's spot on... no resistance on the propeller so therefore no need to work hard...

[___]

1 hour ago, jdp said:

Have you tried using a clamp-on ammeter to see if you get the same readings ?

It would have to be one of the better (read more expensive) hall-sensor type meters for that to work, because the cheaper clamp meters use a current transformer and can only do AC.

You can of course buy a kit with a Allegro hall-effect current sensor, eg:

http://www.bidorbuy.co.za/item/233496465/30A_Range_ACS712T_ELC_30A_Current_Sensor_Module_for_Arduino_PIC_or_ARM.html

http://www.ebay.com/itm/New-30A-range-Current-Sensor-Module-ACS712-Module-Arduino-module-ACS712ELCTR-05B-/200939884518?hash=item2ec8f34fe6

(I've used both sellers).

Google says it's a 500 amp 50 millivolt shunt, so 0.1mV or 100µV for every amp going through there. That's so low that your average DMM has no chance of measuring that, even a good oscilloscope might only go down to a 50mV range, so you'll have to put 10A-20A though there before you see anything.

Back to the original problem: I bought my son an educational electronics kit (known as snap circuits), and lately he's been fascinated by the effect you get when you put a lamp in series with an electric motor. The lamp only glows dimly while the motor spins freely, but if you take your finger and slow the motor down, the lamp's intensity increases as you load the motor down. Perhaps you have the same issue here, on a much larger scale of course :-)

Also, I believe with the BMV702, the shunt voltage can be adjusted so a different shunt can be used. If the adjustment is out, then of course the current measurement will be out too.

[superdiy]

A stock standard multi-meter with a 10A range should also do the trick if the max current is 7.5 amps on lowest speed setting - no need for a clamp meter etc. etc. - you only want to determine if the BMV's reading is fairly accurate, don't you?

If the meter is fused you'll probably blow the fuse on motor startup. To overcome that issue simply connect the multi-meter in series with the motor while also shorting the test leads at the connection points (tips) and once the motor runs at full speed move the shorted test leads apart in order for the current to flow through the multi-meter and then compare the reading with that of the BMV.

[Justifier]

Thanks for your replies.  I will try a multimeter and let you know how I go.

Cheers