randyp avatar image
randyp asked

Use a Victron smart battery protect BP-65 to disconnect solar panel controller from solar panel ?

I would like to use the Victron smart BP-65 to disconnect the solar panels from the solar controller at the battery high voltage set point as sensed by a BMV 721 smart battery management device.
I hope to use the BMV 712 high voltage relay as the remote relay for the BP 65, with the BP 65 in the [ , Lithium Ion Mode.
The BP 65 would see the solar cell voltage (Voc 24.06, V mppt 19.67),
the BMV 712 would see the battery voltage.
LiFePO4 Battery.
Should this work or is it problematic.

BMV Battery MonitorBattery Protect
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2 Answers
Justin Cook avatar image
Justin Cook answered ·

@RandyP, I don't see any reason why this wouldn't work; obviously the SBP's "in" post would be connected to the PV panels and the "out" post would be connected to the controller, then the remote "H" terminal would be connected to the NC (normally closed) relay-out of the BMV-712, with the BMV being configured to open the relay at your selected high voltage setting.

Obviously a couple of keys here will be to A. make sure that in the future you don't add panels in series, which would raise your voltage over the capacity of the SBP; and B. make sure that, however many panels you're putting in parallel, you don't exceed the SBP's maximum current switching capability.

Other than that, although this is a proposed usage of the SBP that I haven't seen before, I can't find any documented reason why it wouldn't work perfectly well.

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ttbb avatar image
ttbb answered ·

@RandyP What exactly is the intended purpose of doing this?

Can i assume you are wanting to bulk charge then cut off charge preventing it going into absorption? Or possibly the voltage curve of LiFePO4 chemistry making you want to hand off control to the BMV-712 ? Unless i am missing something else?

I have found the settings in the victron just fine for Lithium but i have not got LiFePO4 chemistry.

In battery expert mode in LiFePO4 preset i set the absorption duration to fixed 0. I also set the equalization to fixed 0. It is greyed out to 'disabled automatic' under LiFePO4 anyway but just for good measure, it makes me feel better ). Then i set the re-bulk offset to 0.01 and use the float voltage as the trigger for when bulk cuts in again. That works really well but the voltage tracks down nicely on the battery i have to trigger re-bulk.

Are you aware you can also shut off charging with the RX pin on the charge controller? If the VE.Direct port on the charge controller is not being used this is survey a much better option.

"The Rx pin on the VE.Direct connector can be used for other functions. This functionality was introduced in firmware version 1.17. Available options:

  1. Remote on/off (This is the default setting on all larger models). This setting allows remote on/off control: (0V = switch charger off, +5V (or a signal sent via VE.Direct communication) = switch charger on)."

@Justin Cook i will just add a few things to what you said.

The battery protect is a solid state relay so it must have a voltage drop. I have not specifically tested the drop on a battery protect. mine has 24v being stepped down to 12v the other side anyway. On other solid state relays i have used it's somewhere between 1v- 2v so if that is true of the battery protect PV voltage will take a hit.

Every time i have disconnected the PV form my controllers in daylight, then connected them again a few hours later it has made it think it was night then a new day. It totally messes up logging and to sort it out i have had to disconnect the PV / battery then reconnect it again.

There is no time / date setting anywhere in the controller. Who knows how the internal clock works but my controllers always start a new day well before midnight when it gets dark early. It does not appear they know the time that accurately. cutting the pv voltage could cause some issues in that respect because low voltage makes it think that it is night.

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Justin Cook avatar image Justin Cook ♦♦ commented ·

@TTbb, given that the SBP is a FET-based relay device, voltage drop through the device is considered near-enough to be 0 that it's not generally worth consideration.

In re to the internal clock of a Victron MPPT, though, you're definitely correct; the Victron does not have an internal clock but rather measures time as a function of PV input; ie, removal of PV input will make it think it's night... if that's a consideration for @RandyP, it's certainly something to look at. I have the impression, however, that he won't be using a Victron MPPT, so that may not be a consideration.

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