Battery Voltage difference between BMV and Victron controller

Sharon_is_SAM

Can someone using both the Victron controller and shunted battery monitor confirm the real time battery voltage difference between the two devices?  How big of a voltage difference is there between the 2 devices with the controller wired to the load side of the battery monitor shunt vs the battery side of the shunt?

This is a good question for someone who does not have a shunted battery monitor, but does have a controller and wants to know if they can monitor their battery off the controller voltage.

Denny16

The controller voltage reading only shows the actual batrery voltage at night when the solar panels are asleep.  The Battery shunt monitor reads the actual voltage of the battery when the battery switch is off.  Both will read the total voltage at the battery, plus any charge voltage (from the solar panels or converter when plurged into shore power),  The battery monitor also tracks the battery load, vs the charge rate, and averages the two to determines how long the battery will be able to maintain its minimal voltage.
cheers

Sharon_is_SAM

Thank you @Denny16.  I am curious about the actual voltage number.  

I know that battery voltage is a constantly moving target, but, at any given time, how does the controller voltage differ from the BMV voltage?  Are they identical with the battery switch turned on?  Or will the controller voltage during the day reflect the solar input vs the BMV reflect the load?

Also, for just reading voltage, does it matter if the controller negative is wired before or after the BMV shunt?  I know the wire placement impacts amp hr measurement via the BMV, just not sure about the voltage reading.  

I wish I had the TaB in my yard so I could reinforce this info first hand🙄

Dutch061

I can't say exactly how much difference or lag time there may be between the two, but I can say that if you have both that you can create a network via Bluetooth that allows the two devices to see each other. I can also say that it uses the voltage and current both (according to the screen) to allow direct communication of this information between the two devices. My understanding is this is supposed to allow more accurate charging and battery state of charge indications.

Brad

rh5555

The difficulty in answering the @Sharon_is_SAM question is that those of us who have both BMV and MPPT boxes also will have set up the network referenced by @Denny16, above.  When such a network is established, the MPPT controller reports the BMV's battery voltage (because it is more accurate), so the two boxes appear to report identical battery voltage readings.

All is not lost, however.  If you want to use your MPPT controller to get an accurate battery voltage reading, this can be done in 2 ways:

1. If you turn your MPPT controller off temporarily, then the "Battery Voltage" reported will be your true battery voltage.  This is done on the Battery Settings page
   
2. This involves some math and use of Ohm's Law.  When the MPPT controller is active (and not networked to a BMV), the "Battery Voltage" reported is the voltage at the MPPT's output terminals.  To get the true battery voltage, you need to subtract from this the voltage drop caused by your solar charging current as is passes down the wires between your MPPT and the battery.  By Ohm's Law, this voltage drop is equal to the current flowing * the wiring resistance.  So what is this wiring resistance?  If you have 10AWG wiring (like I do), then it is 0.001 Ohms/ft.  In my TAB there is about 10 feet total (count both wires (+ and -) for this), so my resistance is 0.01 Ohms.  The voltage drop is therefore 0.01 x the charging current.  If the charging current is, say, 15 Amps, then the reported battery voltage is 0.15 volts too high.  You can also calculate your wiring resistance by direct measurement (if you have a multimeter):  While the solar charger is producing at least 50% of its rated output make a simultaneous recording of the MPPT's "Battery Voltage" (BV), "Battery Current" (BI) and the actual voltage on your battery terminals (V) with your multimeter.  The wiring resistance is equal to (BV - V) / BI.
   

Denny16

@rh5555, nice trick turning off/on the MPPT controller to get the battery voltage.  Yes, if both the MPPT and BMV are networked, the readings should be the same.  When you add the temp probe accessory cable on the Victron BMV, the battery monitor sends the MPPT controller the actual battery temperature, which is the main info shared by the BMV, and the MPPT sends the battery monitor the solar charge current/voltage info, so the battery monitor can determine time to full charge, and battery charge vs load info.  

This is one of the main reasons for adding the Victron BMV to the MPPT, to improve the solar charging and get accurate battery use monitoring.  Otherwise you are guessing about your battery chargemstste, and how long it can sustain its current average load.  It is just not tracking the current battery voltage, any voltage meter can do that, it is getting the whole electrical system info.
cheers

Sharon_is_SAM

@rh5555 - I like the first way to check voltage😂

Now, I have the BMV, but I have a Renogy controller that does not display voltage.  If I disable the controller, I should see a change in voltage on the BMV?  So, I would note the voltage on the BMV with and without solar?

rh5555

Generally you should be noting the battery voltage in the absence of charge or discharge.  It may take a while for this voltage to stabilize, but when it does I got these values for the Harris AGM batteries in my TAB400:

m_lewis

It is dark now and my BVM reads 13.19V, while the MPPT reads 13.20V. Even though they are networked together, I never see the two devices read the same. I sort of recall seeing a .05 - .10 volt difference before.  I wouldn’t be surprised if a one hundredth of a volt difference is well within the spec accuracy of the devices. 

I’ll try to get a reading tomorrow in full sunlight and post it here. 

Denny16

Looks like as close a reading as you are going to get.
cheers

Sharon_is_SAM

@m_lewis - assuming your devices are networked, did you switch off the controller like rh5555 did above before the reading?

m_lewis

I checked several times today, full sun, shade, bulk and acceptance charge rates. The two devices always read within 0.01 - 0.02 volts of each other. 

If they were not networked, I am sure they would read slightly different when the solar was charging at several amps due to voltage drop in the wiring between the battery and the MPPT charger. 

In any event, using only a voltage reading to attempt to estimate battery state of charge is unlikely to be useful. A true state of charge meter that monitors voltage and current across time is the only effective solution. 

m_lewis

Sharon_is_SAM said:

@m_lewis - assuming your devices are networked, did you switch off the controller like rh5555 did above before the reading?

No. The different light conditions (night vs day) effectively did that. 

Please tell me what you are trying to accomplish so I might be more useful to you. 

Sharon_is_SAM

TaB and TaG owners frequently ask if they really need a battery monitor.  We often tell Newbies to learn battery management by learning about amp hr usage and voltage readings.  We encourage owners to buy plug in meters and measure battery voltage with multimeters to monitor their battery.  Those new to solar ask if they can just use the Victron solar controller battery voltage to monitor their battery, so I am trying to see if there is any significant difference between the controller battery voltage vs the shunted BMV voltage.  I know the value of the shunted BMV in terms of how it calculates SOC and remaining time etc. - we have a BMV but I can’t compare the voltage to our own controller voltage because it has no display.

m_lewis

I see. 

Pretty sure you have your answer from the above comments. Salient points:
State of Charge can be determined by voltage only on a resting and rested battery. That means no load and no charge currents for several hours (24 is often recommended) and during the voltage reading. Any current into or out of the battery will skew this reading, with larger currents creating larger errors. This is effectively impossible to accomplish without disconnecting a battery cable. Too many phantom loads, including the solar MPPT controller even when the Battery Disconnect switch is open. 

A coarse approximation can be obtained by measuring a battery that has very minimal currents. This can be somewhat useful, but can easily result in errors greater than 25%. The tank monitor is worse, but not by much. 

A plug in meter that has resolution of one tenth of a volt, i.e. a display of 13.2, is not likely to have an accuracy of better than 1/10V +- one digit. Therefore it could easily read 12.9 when the actual voltage is 12.7. According to the chart above, that would be an error of about 20%! And that is on a rested battery. Not too many people are interested in disconnecting their battery for a few hours just to find out how much juice remains. 

Additional errors are introduced by wire resistance, again with higher currents producing larger measurement errors. The further from the battery, the larger the error. So the trick of turning off the MPPT charge function is useful. Combined with opening the battery disconnect switch, this would result result in the best result. 

As best I can tell, the Victron voltage readings are rather accurate. Still, I cannot recommend using any in circuit voltage reading for determining state of charge, except for the crudest of approximations. 

rh5555

My experiments of looking at battery recovery time after periods of significant discharge (~50 Amps) show that the battery stabilizes to within 0.05V of the final resting voltage in about 30 minutes.  I haven't done experiments for stabilizing after periods of charging, but would be surprised if they were significantly different.

So, @Sharon_is_SAM, you could suggest that TAB400 users without a BMV turn off their MPPT controller, turn off their main battery disconnect and wait 30 minutes before making a measurement and that should be good.

Be aware that this resting time and the table I published above are for the Harris AGM battery.  Other batteries will be different!

Sharon_is_SAM

@m_lewis and rh5555 - thank you for this!  I think to give an easy way to rest the battery to help owners understand that concept to better approximate the voltage is a good start.  The bottom line is you can’t get a very accurate voltage with active charging or loads.  Good to know that the shunted BMV voltage and the controller voltage are so similar.  

“So, @Sharon_is_SAM, you could suggest that TAB400 users without a BMV turn off their MPPT controller, turn off their main battery disconnect and wait 30 minutes before making a measurement and that should be good.”