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Alan Piggot avatar image
Alan Piggot asked

Incorrect voltage at solar controller/ Spike when cut-off solar MPPT 150/35

I have a 150/35 solar controller connected to 3 x 144 watt solar panels. There is a discrepancy between the voltage at the batteries and the voltage taken at the solar controller. When I use the switch to cut off solar, the voltage at the controller on the battery side spikes.

Here is the situation:

House bank (2 x 210 amph lifeline 4d) will be at 12.7volts measured at the batteries while solar is on in bulk charging. At the controller, the battery terminals will measure 12.9v with a multi-meter and on the app (bluetooth). When I turn off solar using a solar cut-off switch, there is a momentary spike in voltage at the solar controller battery terminals up to 20v, then is slowly comes down to 13.8 volts. The batteries will drop to about 12.5 volts. I am not sure why the solar controller is not reading the voltage at the shunt which matches the batteries.

What is odd, is I have a BVM-712 that is reading the same voltage as the house batteries and it is sharing the same shunt? But, the BVM-712 won't read amps that the solar is putting into the batteries when it is charging. It will only measure current when I draw from the batteries. But, the controller is seeing and measuring the amps going to the battery.


I checked all the connections, tried a new fuse (250mA) going to the shunt from the positive battery terminal.


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MPPT Controllers
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2 Answers
Alexandra avatar image
Alexandra answered ·

@Alan Piggot

Is the negative of the solar charger connected to the shunt of the bmv or to the battery?

https://www.google.com/url?q=https://www.victronenergy.com/upload/documents/BMV-712-quick-installation-guide.pdf&sa=U&ved=2ahUKEwiUpNCO_6DtAhVWTxUIHTu7BPIQFjAGegQIChAB&usg=AOvVaw1WK-VMYtK2LCE75-LxU2t7

The DC breaker for disconnecting the solar charger would be better placed on the pv cables not the battery side, as the correct procedure for disconnecting the battery is to disconnect the pv first

https://community.victronenergy.com/questions/42014/disconnect-solar-when-no-battery.html

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Alan Piggot avatar image Alan Piggot commented ·

Wow. Thank you @Alexandra for a response.


The negative of the solar is connected to the battery. Do you think that is the cause?


I will consider moving the switch to the pv side. thanks

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Alexandra avatar image Alexandra ♦ Alan Piggot commented ·

@Alan Piggot

The negative of the solar should be connected to the bmv shunt as well. If you see the diagram in the manual you will see that. Everything is measured by the shunt so if it is not connected there it does not count the energy.

screenshot-20201126-225527.jpg

A better procedure for stopping charging would be disconnecting the panels. It is not safe to disconnect the battery while the pv is giving out power.

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Alan Piggot avatar image Alan Piggot Alexandra ♦ commented ·

Thank you. Yes, that makes sense. The solar neg is on the battery side of the shut, it should be on the load side. The load side of the shut is connect to a terminal strip.


I will change the solar cut off to the pv side. Thank you so much.


Here is the set up, it was almost complete.

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Alan Piggot avatar image
Alan Piggot answered ·

@Alexandra

Good morning. I place the negative wire on the Bmv shunt side and the monitor is reading amps on from solar. However, there is still a .2v discrepancy at the control vs at the monitor/batteries. Also, there is an current discrepancy. There are no loads on, the solar controller is charge enabled, says 6 amps current with battery at 14.24 and the bmv says 14.1 with 3.83 amps. I turned everything off and zero current calibrated the bmv. No change.

When the solar is charging, there is a discrenecy. If I use the app to disable charging, the solar charger and bmv and battery bank are all in agreement.

I can bypass all of this by using the smart network, where the bmv tells the controller the true voltage. Is that the best workaround?


I have yet to change the solar cut off from the battery side to the pv side. I am no longer using that to disconnect the solar, I am doing it though the app. I will change the switch later today.

Thank you again for you help. Any thoughts on the current discrepancy?

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Alexandra avatar image Alexandra ♦ commented ·

@Alan Piggot

Glad to hear there is an improvement. The .2v can be caused by cable length and connection resistance, basically ohm's law.

If you remove your earth does the current discrepancy disappear. It's the only thing I see in your circuit diagram that may be causing the issue.

I have a smart network at home for something similar to your set up and am happy with it, it should work well for your setup.

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Alan Piggot avatar image Alan Piggot Alexandra ♦ commented ·

@Alexandra

I installed a cutoff switch on the pv positive wire. All voltages are in agreement! No drops anywhere.

The amp discrepancy has been improved to just 1.8-2 amps.

I removed the earth and lost power.


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Alexandra avatar image Alexandra ♦ Alan Piggot commented ·

@Alan Piggot

Neat setup

The other thing that may improve the amp drop would be to re arrange lugs with the higher amperage stuff under the rest of the lugs on the connection points, especially the one on the earth. And there is good contact area. (Although I do know sometimes best practice and what is practical not always possible.)

And removing the earth would definitely cut power. Knowing it's a mobile setup now I would not have made that suggestion

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Alan Piggot avatar image Alan Piggot Alexandra ♦ commented ·

@Alexandra


thank you so much. I’m going to keep playing around to see if I can’t get some agreement on the amps. Thanks again!

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