Detecting "excess power" to run hot tub pump and other devices

rpvietzke · July 2016

Hi folks,

I am looking for a way to detect when I am generating solar energy in excess of what is required to recharge the batteries. I am assuming the best way to do this would be to detect when I have been through charging stages and was in float. (I use my MPPT's controller's relay to run the fan on my battery box.)

Are there any voltage-controlled switches that track charging stages that would be an inexpensive solution? Some sort of way to read the status from the combos? Ideally what I want to do is run my hot tub pump every day when there is excess sun, but NOT when we have a string of cloudy days... I can wire a traditional relay or SS-relay for the pump, I'm just trying to figure out what can trigger it.

I have a 672AH 24V battery bank, Schneider SW4024, Combox, MPPT60 and 6x250W panels.

Thanks!

Rob

BB. · July 2016

Xantrex makes a Battery monitor with programmable "alarm" output (set "on=90% SOC, set "off=75% SOC") (digital interface too?):

http://www.xantrex.com/power-products/power-accessories/linkpro-battery-monitor.aspx

Victron makes battery monitors with remote "alarm" contact (digital interface too?):

https://www.victronenergy.com/battery-monitors/bmv-700

There is the Pentametric Battery Monitor System - Input Unit. (plus various i/o units). Don't know any details.

In a past life, we used to use a "tape/glued on" photo cell to LEDs to monitor equipment warning lights... You could do that with this Midnite voltage based monitor: Green LED on, turn on your optional loads.

http://www.solar-electric.com/mnbcm.html

-Bill

Dave Angelini · July 2016

If your are using the aux from the mppt for your fan, you could also use the aux from the inverter to control a load. This is the old fashioned way as it involves controlling the mains on your load.
If your loads are wifi enabled you can control them without interrupting their mains with wifi.
I have not seen a wifi hot-tub yet but who knows? I mainly do this for mini-split heat pumps.

rpvietzke · July 2016

Thanks BB, Dave,

I'll see if one of these suggestions can work. Unfortunately the Conext SW40240 inverter does not have another relay output. The one on the charge controller is the only one I have for the battery fan. There is a relay on the combox, but it doesn't appear to be programmable.

It drives me a bit nuts to think I would spend $150+ on a basic switch, but the battery SOC trigger does seem like a good approach for a stock solution.

I've also been thinking maybe I could rig up a raspberry pi with a relay shield to poll the serial port for SOC and take action based on that... If I could get it working, that would be a much nicer (and cheaper) solution.

Rob

BB. · July 2016

There are voltage controlled switches too:

http://www.solar-electric.com/volconswit.html

My concern is--The more complex the system, the more chance that something can/and will go wrong.

Perhaps combining your Raspberry Pi for program-ability--And a second pure voltage controlled switch (turns off at 12.7/etc.) volts as a backup... that way, a crashed R-Pi will not literally kill your battery bank with a "stuck on" load.

-Bill

rpvietzke · July 2016

Bill - Good suggestion. The "load" is on the inverter, so I always have LVD for the whole system as a fail-safe for the batteries. That said, it wouldn't be hard to add some backup to this scheme, which is a great suggestion.

BB. · July 2016

LVD's -- Make sure you know what the settings are for your inverter... Many are set at 10.5 volts (12 volt system0 or equivalent. That is a "dead" battery--And one that will probably not come back to any meaningful life when recharged.

Some systems/inverters will let you program the LVD (simple voltage settings, others can also have time limits)... You could use 12.0 for resting/near zero current loads... And 11.5 volts for > 30 minutes, and >10.5 for 10 seconds as other set points (lets you have surge/heavy loads without trips).

If you are specifically managing your optional loads--Then you can pick float (13.x volts) or even 12.7 volts minimum (battery does not discharge until below ~12.7 volts). And set the hysteresis to 13.8-14.2 (or whatever you like) to reset back to "on" (at least battery is receiving charging voltage)--Assuming this is the backup LVD device.

-Bill

South Africa · July 2016

There are a bunch of guys here in SA that are all writing software to calculate production to see when there is spare power to use for say geysers etc.

Some use Raspberry Pi's, some use Windows tablets to read the data for up to 6 devices simultaneously. All the data read is stored in databases with calculations etc done.

Victron BMV monitor is core to most of the systems. Some even use the BMV's relay to switch AC loads on/off or inverters on/off based on a set SOC of the battery bank, to save the bank. All started because of power failures here, using the installed arrays to generate optimal benefit.

As Bill said, it can get very complex very fast, or not. Depends on what you want to do.

Detecting "excess power" to run hot tub pump and other devices

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