Any approach to specify a DC load to switch off based on state of charge

[jameshowison]

Could someone point me in the right direction?

I'd like to find a way to tell a DC load to shut down based on state of charge of the batteries. Use case is to ensure that the fridge is always powered (until the very last smidge of DC power in the batteries). We will have a DC HVAC system that may use plenty of power but should always be turned off if things run low.

System is victron, including a Cerbo GX and a smart shunt. I figure this is something about relays, with something watching either voltage or shunt state of charge and triggering a relay that turns off a switch on the relevant circuit? Hmmm, that seems like a pain as that essentially requires a networked disconnect.

Perhaps this is something I can do with a Victron battery protect on the HVAC circuit, shutting that down if voltage drops to a particular level?

Any suggestions for other disconnects that can be triggered by relays? Amperage is low (like 40amps), 48v nominal system.

 

[HRTKD]

I have a Battery Protect 65 in my system. I have it programmed to shut off if the battery bank gets down to 12.0 volts. So far, that's the highest low voltage threshold that I could see in the Victron Connect app. There may be further configuration possible, but I haven't dug deep enough to find it.

The Cerbo GX has a relay that is programmable. There are two, but I've read that only one is useful. The GX should be able to get the remaining Ah from the shunt and from that the relay can be tripped.

 

[rmaddy]

I have it programmed to shut off if the battery bank gets down to 12.0 volts. So far, that's the highest low voltage threshold that I could see in the Victron Connect app. There may be further configuration possible, but I haven't dug deep enough to find it.

12.0V is the highest amongst the presets. Select User Defined and you can set any voltage you want.

Perhaps this is something I can do with a Victron battery protect on the HVAC circuit, shutting that down if voltage drops to a particular level?

A Battery Protect's main job is to disconnect DC power at a user-define voltage.

It also has a relay mode. The manual states:

Relay mode (relay connected to the alarm output):

• In case of under voltage, the relay will engage after 12 seconds. The SBP will disconnect the load after 90 seconds and the relay will disengage.
• In case of over voltage, the load will be disconnected immediately and the alarm output will remain inactive. Overvoltage trip levels: 16V respectively 32V

 

[Johncfii]

I'd like to find a way to tell a DC load to shut down based on state of charge of the batteries.

System is victron, including a Cerbo GX and a smart shunt. I figure this is something about relays, with something watching either voltage or shunt state of charge and triggering a relay that turns off a switch on the relevant circuit? Hmmm, that seems like a pain as that essentially requires a networked disconnect.

Your objective is worthwhile and do-able. This is also a fairly common desire that solar equipment manufacturers are not recognizing. At least not yet. But using battery voltage as a surrogate for state-of-charge is far from ideal. This is because battery voltage can vary widely relative to SOC depending upon size of load at any given instant. This is a manifestation of battery internal resistance.

Your intuition to use SOC as the parameter to use to automate load shedding is correct. Your CerboGX has one internal relay that is programmable to trigger, and then release, at preset SOC values. And the CerboGX can track SOC very accurately if properly set up, and with periodic recalibration.

40 amps is not a trivial load for relay switching, but there are a wide variety of high current relays, or solenoids, that can do the job. And the relay logic is pretty simple. It really shouldn’t require a control ”network”.

BTW, you can gain programmable functionality of the other relay built in to the CerboGX by installing an open-source version of CerboGX firmware that incorporates an add-on app called “NodeRed”. It is a bit technical, but there is a lot of good information about it on the Victron forum.

 

[jwelter99]

Move to the Victron large image and use node-red to control one of the relays to turn off that load based on the SOC (available in Node-Red)