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Discussion Starter · #1 · (Edited)
I plan on using an AC fridge and AC water heater with a Victron 3000 multiplus. I was wondering if anyone has used a DC powered PID/temp controller with an AC relay in order to prevent the invertor from being on 24/7? I have a three ideas

  • Temp controller for both on separate relays. This seems the "easy" way but the water heater might cycle for 20 min then the fridge might cycle after.
  • Temp controller for both, single relay that turns both on when needed.
  • Timed controller that allows X (e.g. 15 min) on every hour. I'd have to really monitor this to make sure the fridge can maintain temperature.
 

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Seems needlessly complicated. Are those the only things you are using your inverter for? Probably don't need a big inverter to run the fridge so a second small one that doesn't have much idle consumption could simplify things considerably.
 

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If the inverter can handle both loads simultaneously, then each load could have its own relay (12vDC coil, 120vAC contacts) connected to the output of the inverter.

Does the inverter have some kind of contact that asks it to come on? You might be able to use some diodes so that either (or both) would turn on the inverter.

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Keep in mind that the inverter is probably ~85% efficient. So if your water heater used 1500 watts, the inverter will be pulling like 1765 watts (~147 amps at 12v) from the battery. In my opinion, 265 watts is a lot of energy to throw away in the van.

I have heard of people converting water heaters to 12v elements, which would avoid that loss.

Have you done the math on how much battery capacity it would take to heat the water? How will you recharge the battery? @GaryBIS can help with that math.

I truly love our 12v marine fridge, but it was spendy. Have you looked at 12v TruckFridge?
 
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Discussion Starter · #4 · (Edited)
Seems needlessly complicated. Are those the only things you are using your inverter for? Probably don't need a big inverter to run the fridge so a second small one that doesn't have much idle consumption could simplify things considerably.
I just edited my post. Its the Victron multiplus 3000 not just the inverter. I already have the components as well so there's no turning back :)

If the inverter can handle both loads simultaneously, then each load could have its own relay (12vDC coil, 120vAC contacts) connected to the output of the inverter.

Does the inverter have some kind of contact that asks it to come on? You might be able to use some diodes so that either (or both) would turn on the inverter.

========================

Keep in mind that the inverter is probably ~85% efficient. So if your water heater used 1500 watts, the inverter will be pulling like 1765 watts (~147 amps at 12v) from the battery. In my opinion, 265 watts is a lot of energy to throw away in the van.

I have heard of people converting water heaters to 12v elements, which would avoid that loss.

Have you done the math on how much battery capacity it would take to heat the water? How will you recharge the battery? @GaryBIS can help with that math.

I truly love our 12v marine fridge, but it was spendy. Have you looked at 12v TruckFridge?
I'll look into those DC elements for the hot water heater, it was just so easy to buy the 2.5 gal ones for $150. I have a 300 AH battery, 60 amp B2B and unknown but hopefully 600+ W solar. The plan was to have the engine on if the water is completely cold.

Down the road we'll definitely upgrade to a 12v fridge, they are just so much money and hard to get today. I'm currently wiring everything now so AC fridge and water heater it is! This is just going to be a weekend van so I think the math checks out. A small'ish negative net battery capacity per day isn't the end of the world for us.
 

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Another option would be to use something like an Arduino to read the temperatures, control the relays and control the inverter.

Even if you've already bought the water heater you should still be able to convert it to 12 volts.
 

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I purchased a Victron 12 | 3000 | 120 multi in late 2018 for my build & never installed it. IIRC they are 93% efficient.

I went the route of converter vs inverter 12v fridge & 120v water heater (honda generator for 20mins if off shore power & I really wanted hot water & did not want to use propane).

I think you could achieve what your intended post asks, but I don’t see the point of the complication.

What is your plan for batteries & more importantly how are you gonna charge them back up? In 2018 When I purchased my Victron my intent was a fully electrical van (no propane no diesel or gas heat) 🙄.

It can be done 💸💸💸💸, but after floundering around & bouncing about I choose 12v & propane.
 

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Discussion Starter · #7 ·
Another option would be to use something like an Arduino to read the temperatures, control the relays and control the inverter.

Even if you've already bought the water heater you should still be able to convert it to 12 volts.
Arduino is the plan! Luckily that part is easy for me :)

I purchased a Victron 12 | 3000 | 120 multi in late 2018 for my build & never installed it. IIRC they are 93% efficient.

I went the route of converter vs inverter 12v fridge & 120v water heater (honda generator for 20mins if off shore power & I really wanted hot water & did not want to use propane).

I think you could achieve what your intended post asks, but I don’t see the point of the complication.

What is your plan for batteries & more importantly how are you gonna charge them back up? In 2018 When I purchased my Victron my intent was a fully electrical van (no propane no diesel or gas heat) 🙄.

It can be done 💸💸💸💸, but after floundering around & bouncing about I choose 12v & propane.
We have the Victron 3000 multi, 300 AH lithium with room to add. I think we'll be good capacity wise.
 

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Arduino is the plan! Luckily that part is easy for me :)



We have the Victron 3000 multi, 300 AH lithium with room to add. I think we'll be good capacity wise.
I just looked it up, If I am reading this correctly;

93% efficiency

&

8 to 20 watts for “standby”. The PID controllers I assume use some energy for constant temperature detection.

Also the Victron has 6000W peak (not saying you will use that). What is the load if your fridge & HWT draw at the same time & can your 300 AH Lithiums provide that? 3000W divided by 12volts = 250 amps & although it doesn’t work exactly like that math (actual battery voltage, resistance, inverter efficiency, blaa blaa blaa) & in my case back in 2018 the equation wasn’t could I do it but why & how am I gonna charge up the batteries.

Maybe @jracca or others who have gone this path can shed some light, but if standby uses 8 to 20 watts & you are concerned about that “energy spend” on 3000W or 6000W potential use I assume you are interested in 12v energy conservation. I am not sure I am reading the standby energy correctly, but here is the chart (zero load power).

Font Rectangle Parallel Screenshot Number
 

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Discussion Starter · #9 ·
I guess the question would be if the multiplus would ever hit standby mode with an AC fridge plugged in? The point of the DC powered controller would be to completely eliminate AC power.


Sent from my iPhone using Tapatalk
 

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I guess the question would be if the multiplus would ever hit standby mode with an AC fridge plugged in? The point of the DC powered controller would be to completely eliminate AC power.


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That makes sense. I was blinded to the parasitic 120v appliance draw for “temp control”.

How much energy will the PIDs consume to stay alert monitoring the temps?

Victron comes up with bluetooth controllers & add ons all the time it seems. Maybe they have a controller that could be added? @el Jefe seems to be big into Victron & maybe he will chime in.
 

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That makes sense. I was blinded to the parasitic 120v appliance draw for “temp control”.

How much energy will the PIDs consume to stay alert monitoring the temps?

Victron comes up with bluetooth controllers & add ons all the time it seems. Maybe they have a controller that could be added? @el Jefe seems to be big into Victron & maybe he will chime in.
Yes to 12vdc elements. You could use the Cerbo or Multiplus relay or the relay on the BMV712.
Also, you can use Smart Battery Protect with a simple temperature controller.

If you have lithiums, best to control using SOC as opposed to voltage.

And, if not using a Smart Battery Protect, you will need a relay as most of the relay (switch) contacts are only rated for 10A. Most 12vdc elements start @ 12-13 amps @ 150 watts.
If using the load output on a charge controller, you will still need a SSR due to the amperage draw.

Anyone want to add to the myriad of methods to heat copious amounts of hot water using excess solar or B2B chargers?
I am keeping 40 gallons @ 130-120⁰f during the winter. Seems to adequately heat our bed area too due to the loss thru the adjustable insulated tops on our standard rv water tanks.
 

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You can use an assistant with an aux input on the multi to turn the inverter on or off with the two wire BMS assistant. You can get a simple 12v thermostat (https://www.amazon.com/bayite-Fahre...1637873915&sprefix=12v+thermo,aps,220&sr=8-18) and wire the output the aux input and tell the inverter to shut off when the signal is open (or closed) depending on your thermostat options. However you'd obviously want to provide an override switch in you want to use your inverter at other times. This will tell the inverter to shut down as the multi will receive a signal that the battery is empty and turn off the inverter.

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I know you didn't ask, but this is a very odd way of doing this though; not at all how I would design a system.
 
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Arduino is the plan! Luckily that part is easy for me :)



We have the Victron 3000 multi, 300 AH lithium with room to add. I think we'll be good capacity wise.
Amazon lists super cheap temperature controlled relays. Use one for the refer and use the output to "enable" the inverter. Let the hot water float ( or parallel a second relay) since the refer will need power many times during the night. I think I got two for $9. Same deal for small DC to DC converters. 12v in 5-35v out. Super cheap interface between 12v and anything using a wall wart (LCD screen, car vacuum, laptop charger).

Takes all the fun out of rolling your own...
 
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