Hot Water Heater on Battery Power?

Although the boat is in the tropics and we don't have a/c - we still enjoy a hot/warmish shower at night for some reason.

There are several ways to get hot water on Yoto:

1. Plug into shore power
2. Run the generator
3. Run the engine for 20min or so (coolant loop runs through hot water heater)
4. Power hot water heater from inverter (boats general wired to keep heavy loads like HWH off when on battery power)

We did not visit a dock from November 2023 through the end of May 2024. So, option 1 was never available.

Our generator has never worked properly since we've purchased the boat. So, option 2 was never available.

Our inverter was wired (correctly) to not run heavy loads while on battery power. And we were on battery power 100% of the time. So, option 4 was never available.

We did have a working engine with the hot water coolant loop. So, option 3 was technically our only option.

However, the engine that heats the water is directly under Olivia's bed. And, running the engine just to make hot water never felt good. Which means, hot water was very hit or miss for the beginning of our cruise.

Then one day, the clouds parted in my mind and after realizing we had quite a bit of excess solar power many days - I started working on plan to install some kind of switch so I could turn the hot water heater on/off manually from the inverter. Meaning, we'd use the battery power to get hot water on days when we had excess solar energy.

Great, now how to do this?

My idea was pretty simple:

1. Move the hot water heater load wiring on the inverter to behave like all the other AC powered equipment on board.
2. Then add a switch in-line so I could manually turn on the heater when there was excess power.

As I was reading through the Victron user manual, I came across section that described "programmable switches" using VEConfigure Assistants. Curious, I read more and then got online and started researching what might be possible.

The functionality provides several triggers for different actions. My new plan started to materialize as:

1. Create a program/rule on the inverter that would turn on/off the hot water heater load
2. Trigger that on/off switch based on battery state of charge

I didn't have any physical cable or dongle to connect to the Victron inverter, so I incorrectly thought I'd have to order one and wait.

However, I found that changing configuration can be accomplished via the VRM online portal by downloading and then subsequently uploading a configuration file. I had many apprehensions about using that method as our inverter/charger is a key component of our boat's electrical system.

After downloading the configuration file and making the changes, I held my breath and uploaded. The whole boat shutdown. After a minute or so, things started to come back online. All seemed good.

My battery SOC strategy was to turn the heater on when the SOC was 93% or above. The heater would turn off if the SOC dropped below 90%. My logic was that even if the sun disappeared right when the SOC hit 93%, I was willing to sacrifice 3% of the battery charge to get some warm water.

We all waited and watched the SOC rise throughout the first day. When it hit 93%....nothing happened.

Now what? I logged in via the VRM portal and saw that the resolution on SOC was different than on the physical display in the boat. On the cerbo gx screen - the SOC was rounded to 93% but the actual SOC was something like 92.7%.

After waiting a bit more, the VRM SOC hit 93% and within seconds, the AC load was up almost 1800watts. A sure sign that the hot water heater was now on and heating up our shower water!

It has continued working perfectly and there have been very few days where we've taken cold showers ever since.