[jracca]

This is not something I can answer without some research, but purely from memory

I am not sure if you can trigger on 500W of AC output, but you can setup the inverter to trigger a relay if overloaded. This is typically used to start a generator, but nothing says you cannot use it to turn off a relay that would shut off the water heater.

I am pretty sure you can trigger off inverter output with a GX series device, but I don't see one in your diagram. I think setting up a generator start assistant if the inverter is overloaded will work fine, the inverter can decide when it is overloaded rather than setting a fixed 500W.

I might switch the water heater element to a 1500W anyway as it won't make much difference in usability but can save some stress on your system

 

[dog_house]

This is not something I can answer without some research, but purely from memory

I am not sure if you can trigger on 500W of AC output, but you can setup the inverter to trigger a relay if overloaded. This is typically used to start a generator, but nothing says you cannot use it to turn off a relay that would shut off the water heater.

I am pretty sure you can trigger off inverter output with a GX series device, but I don't see one in your diagram. I think setting up a generator start assistant if the inverter is overloaded will work fine, the inverter can decide when it is overloaded rather than setting a fixed 500W.

I might switch the water heater element to a 1500W anyway as it won't make much difference in usability but can save some stress on your system

I have a Cerbo and I didn't look in the generator start assistant for a way to do it. That's exactly what I was looking for. If something triggers an overload, turn off the relay.

I think the system will be okay, it's spreading the load across 4 batteries so each one is discharging at .15C and less than 1/2 the BMS's rating. I would rather hit it hard while we are driving and heat the water while we have alternator power and save the batteries for running the AC. If for some reason it causes issues it's an easy swap. I had to lower the power in our travel trailer water heater to keep the 30a main breaker from tripping when the AC was running in high temps.

Thanks for the help!

 

[nebulight]

You will have to have two assistants: one to turn the relay on when loads are below 500w as AC2 Out is always off by default and one to turn the relay off at loads OVER 500w:

You can set the time offset to whatever you works. You should have something built it so it's not fluctuating on and off rapidly based on your loads.

Also a third may be needed if you plan to use shore power as well to force the AC2 Out relay when shore is available. I've never tested this though as I rarely am on shore power:

You can try this order:

 

[jracca]

I have a Cerbo and I didn't look in the generator start assistant for a way to do it. That's exactly what I was looking for. If something triggers an overload, turn off the relay.

I think the system will be okay, it's spreading the load across 4 batteries so each one is discharging at .15C and less than 1/2 the BMS's rating. I would rather hit it hard while we are driving and heat the water while we have alternator power and save the batteries for running the AC. If for some reason it causes issues it's an easy swap. I had to lower the power in our travel trailer water heater to keep the 30a main breaker from tripping when the AC was running in high temps.

Thanks for the help!

If you ever decide to modify, the isotemp water heaters that heat the water with using engine coolant is the way to go for people that plan to be driving often. I have had two class B RVs (1989 Dodge factory built, 2018 Promaster Self Built) and have had heat exchanger water heaters on both. I will never have an RV with an engine without one.

The electric element on them is only 750W (at least on my model) but if you drive every two days you will never need the electric element anyway because it heats the water to engine temp and keeps it hot for quite a while.

As Nebulight put three assistants may work, I haven't ever set up ones that conflict so I don't know how it behaves when one tells it to turn on the AC2 relay because it is connected to shore power and the other says to turn it off because of >500W load. He might know more than me on this one.

 

[nebulight]

He might know more than me on this one.

Negative. I typically never stay places with shore power so this is unknow territory for me. I can get my multiplus unpacked and run some tests this weekend.

 

[el Jefe]

If you ever decide to modify, the isotemp water heaters that heat the water with using engine coolant is the way to go for people that plan to be driving often. I have had two class B RVs (1989 Dodge factory built, 2018 Promaster Self Built) and have had heat exchanger water heaters on both. I will never have an RV with an engine without one.

The electric element on them is only 750W (at least on my model) but if you drive every two days you will never need the electric element anyway because it heats the water to engine temp and keeps it hot for quite a while.

As Nebulight put three assistants may work, I haven't ever set up ones that conflict so I don't know how it behaves when one tells it to turn on the AC2 relay because it is connected to shore power and the other says to turn it off because of >500W load. He might know more than me on this one.

Can you use Demo mode? I have never used demo mode for assistants.

 

[dog_house]

I have the system mocked up in my garage, I'll mess with it and see how it reacts. Stand by.

@jracca I have used water heaters with coolant loops on the boat and they work great but I don't want to have to mess with the PM cooling system if I don't have to. I considered it, but since I am this deep into batteries and inverter I can spare the 2kWh for us to take hot showers. What I don't want is any reason for Ram to deny the warranty. If this was a used van without much powertrain warranty left I would be using the engine to heat the water without a doubt.

 

[dog_house]

You will have to have two assistants: one to turn the relay on when loads are below 500w as AC2 Out is always off by default and one to turn the relay off at loads OVER 500w:

View attachment 93244
View attachment 93245

You can set the time offset to whatever you works. You should have something built it so it's not fluctuating on and off rapidly based on your loads.

Also a third may be needed if you plan to use shore power as well to force the AC2 Out relay when shore is available. I've never tested this though as I rarely am on shore power:

View attachment 93246

You can try this order:

View attachment 93247

This worked like a champ with a minor adjustment. The Relay OFF assistant needed to be increased by the amount of the load so that it does not cycle over and over. I could have set it to 2500w but instead I set it to turn the ACOut 2 OFF when the overload pre-alarm LED comes on and that seems to work well. Adding the AC1 trigger to turn it ON when we were plugged in was the ticket.

The inverter seems happy running a 1500w resistive load (oil filled space heater) and the heat pump at the same time so I think the idea of lowering the wattage of the HWT is the right way to go. I am going to swap the 2000w element out for 1125w one and 90% of the time the assistant won't be needed. I doubt the extra 30 minutes it will take to recover will ever really matter.

Thanks again!

 

[jracca]

I have the system mocked up in my garage, I'll mess with it and see how it reacts. Stand by.

@jracca I have used water heaters with coolant loops on the boat and they work great but I don't want to have to mess with the PM cooling system if I don't have to. I considered it, but since I am this deep into batteries and inverter I can spare the 2kWh for us to take hot showers. What I don't want is any reason for Ram to deny the warranty. If this was a used van without much powertrain warranty left I would be using the engine to heat the water without a doubt.

I hear that!, It was nerve wracking to splice into my promaster heating loop with 25 miles on the odometer. But Ram sells the kit to do it, so I figured it must be allowable.

I added ball valves right off the T fittings to be able to shut it down in the event of a heater hose puncture, but those hoses are pretty tough.

Of course cutting holes in the roof of a brand new van was also a high stress event. Once the first hole was made it got easier.

 

[nebulight]

I could have set it to 2500w but instead I set it to turn the ACOut 2 OFF when the overload pre-alarm LED comes on and that seems to work well. Adding the AC1 trigger to turn it ON when we were plugged in was the ticket.

that’s a much better idea for how you intend to use it! And the AC1 for anyone reading this in the future is AC1 INPUT (the assistant screen doesn’t show that in the summary screenshots).

Glad you got it working!