Load testing my pack overnight... Should be interesting.

[willo]

Today I kicked of a test of my system with a few ideas in mind.

• Discharge testing of my new 14Kwh battery pack
• Energy consumption and offset when using my Absorption fridge
• Getting used to the discharge numbers and behavior of my new pack
• Evaluate just how off-grid I can be with the current configuration
• Validate my SoC info - I just calibrated the 100% value on my BMV, and I want to get used to the new charge/discharge curves.

So what am I up to here:
This afternoon I put a gallon of water in my freezer and most of a case of bottled water into the fridge section of my Dometic 1350 fridge.
It was sitting at room temperature when I kicked it on. Normally, I'd toss some ice into the freezer to help kickstart the cooling process but I wanted a full performance test of the fridge.
I'd originally wanted to start this earlier today, but actually the late start will be a better test of my battery capacity.
I started up my inverter (Victron Multiplus 3k/24)
I kicked the fridge on in auto mode, which will run it on A/C while there's power.
And then I left.

Meanwhile, I've been keeping an eye on it via the VRM portal.

I normally burn about 50w with my dometic cooler and monitoring, so discount that from the DC side.
Figuring on 500w, I'm estimating that I'll use 5-6kwh overnight before the sun comes out tomorrow.
Then I'll get to watch how well the system can both offset load and charge the pack.

What I'm really interested in watching is the SOC:

The other number I'm keeping an eye on is the AC wattage. It's been a solid 420w since I kicked this off.
I'm really curious to see if the fridge will tail off once it hits a target temperature.
If it does, I want to understand the average duty cycle and how much it'll burn in real life.
The mass of the water inside should help it stabilize. At that point, I'll have some interesting data.

Anyhow, I wanted to see how well this works - and how realistic it is to use my solar to precool the fridge before trips.
I'm pretty excited to actually challenge the battery like this. So far my testing has been short times of running a space heater or A/C unit to use up a Kw or whatever.
It's kinda of reassuring to measure how the system does with a nice consistent 21A discharge.

Realistically, I'm probably not going to mod or replace the fridge until later this year. That doesn't mean I can't experiment with cheap mods - like a fan on the HX system and seeing if there are ways to improve the efficiency of the fridge itself. Obviously there's nothing to be done for the heating element wattage, but I may be able to improve internal cooling or external heat exchange...

 

[Boondock Saint]

Thanks Willo I really liked this post. I like your plan and your ability to explain and execute it.

Do you have anything more to share, such as part numbers for "Victron smart solar, multiplus 3k, 24/12 converter, 12/24 tow charger, raspberry pi venus, SBMS0" ?

Btw I never received my Flat Earth Society Member Welcome Kit.

 

[willo]

I have made many threads about various topics of my build... I chucked a bit of a cohesive update onto my original design thread tho.

Finishing up my design for my 1200w RV build

Welp, I finally wired in my multiplus! I used Tom Morton's wiring idea for a contact switch. The switch disconnects shore power and bridges L1 and L2 for the multiplus to feed. I ended up buying 6/3 wire at HD and extending the main shore feed over to my battery bay, and a return run to feed...

diysolarforum.com

Looks like my web host is busted for my personal site... I'm just gonna ignore that tonight.

I still need to work on the tow charger a bit - it works fine but it over-runs my tow connector which is known for being lame (thanks GM) I'm thinking of adding an anderson plug with a dedicated run.

 

[willo]

Testing overnight showed my expected results. That makes my happy since the battery seems to be working as planned.


No change at all in AC load. I am starting to suspect it'll remain a constant.
Of course, it looks like it's going to rain later so it'll be a bit of a shabby solar day.
The test continues....

 

[willo]

This is the first time that I've tried this with so much battery capacity. Previously, I only had around 3.5kwh of storage and I rarely ran it down more than a couple of Kwh. It's also the biggest constant load I've run... ever.
That means that in general, my battery was topped off part way through the day.
This morning I'm finding flaws in some of my presumptions - mainly that I'd already be banking power back by this time in the AM.


I expected to use around 6kwh overnight - which I did. What I didn't expect was the extra couple of Kwh used at dusk and dawn.
OOh, and just now, I'm FINALLY back to charging my battery again!

 

[willo]

Trying to log some updates as I go along here.
It's been a poor solar day so far. Only 1.5Kwh generated and none of that time actually exceeded the load. Bleh.

On the 'bright' side (sorry, I'm a dad) I get to continue capacity testing my battery pack. This is great for validating my pack's performance.
I'll have to run down below at some point because I don't actually want to do a full discharge right now. (I could but I'd have to go clean out my deep freezing dometic cooler)

 

[willo]

Allrighty, I headed down and switched my fridge over to gas for tonight.
The fridge would have absolutely killed my battery charge overnight if I'd left it on.



I probably should have taken some temp readings, but I did find that my gallon of water in the freezer was mostly frozen, but not quite solid.
Water in the fridge was cool as expected. I am thinking that the RV fridge is just gonna be a constant power draw no matter what, but I will test that theory in another day or two.

Now, for some more graphs....




Hmm, that's an interesting dip there. interesting - maybe it did cycle off for a short time?

VRM has a nice mouseover feature, I checked and the battery current spike coincides with the dip in the AC load.
Looks like I did manage to hit a heating cycle break.

OK, I'll definitely test for the fridge duty cycle later this week.

 

[willo]

AHA!

I zoomed CSI style and found that it was actually off for 30 minutes! Interesting.

 

[willo]

And to round this out...
My pack dropped a few percent over a couple of bad solar days. Today was much better (with rain later on in the afternoon)
For today, My pack went from 44 to 72% (so up 28%). It probably would have it 75 or so if I hadn't been there for a while.
I ran off propane for a day, but then decided I didn't have a need to cool the fridge right now and turned it off. I can figure out the duty cycle question later on when I'm actually keeping food cold.

Takeaways from my testing:
If I'm using power heavily, I can definitely drain the pack in a day. That said:
I've got enough power to run all my day to day items with enough left over to afford to run an A/C for an hour or six or run the fridge (some of the time) to reduce my propane dependency. (I didn't cover A/C usage, but for me it's about 1.2kw. Acceptable for a few hours if I need it)

While I can run the fridge all day from solar, it's a burden on my system. Even IF I had say, 2kw instead of 1.2kw of panels, I'm still at risk of a bad solar day if I run the thing 24x7 from my batteries. Of course, if I'm going to be off-grid for a day or three and I know that I'll have access to shore power, I can definitely save some propane.

For towing, I have a 240w charger. That will offset nearly half of the fridge load, so IF I'm towing all day in the pouring rain, I will be well within my power budget. I don't think that propane works very well while towing, so that's actually a really nice thing. I think I can definitely afford the power budget to run the fridge off electricity while I'm towing.

I can definitely see a use case for optimizing by switching between propane and electric. If the propane is mildly more effective at heating, it may be even better to alternate in 1 hour or 1/2 hour blocks of electric/propane.

Beyond that, for me this test drove home the importance of optimizing your electric loads and being aware of my charge state. It was really helpful for developing my own power budget planning concepts as well.

 

[time2roll]

Need a compressor fridge to run on electricity.

 

[willo]

I have found a decent use case that leverages my solar system and the absorption fridge.
I’ve always thought that running the fridge on propane while going 70mph down the road seemed a little sketchy - depending on the trip I’ve certainly seen fridge temps rise.

It comes down to this. On travel days, as long at my battery has over 50% and I’m driving to a location where I have power, then I run the fridge from my inverter. Between getting some solar and the tow charger, I’m usually still up a bit of charge by the time I arrive.

Until I replace the fridge with a compressor unit, this at least lets me reduce my fossil fuel usage. d
My only real complaint right now is that I need to swap L1 and L2 in my RV breaker panel - the fridge is on the non-inverter side and I have to remember to flip the combiner switch if I’ve been running on 50a. (And I have to turn the A/C off, since it IS on the inverter side.

 

[svetz]

What's consuming 240V that you need both L1 and L2? Or is it that the inverter setup for split phase? Just curious! Thanks for posting your data!

 

[willo]

Been busy playing.... Anyhow.

What's consuming 240V that you need both L1 and L2? Or is it that the inverter setup for split phase? Just curious! Thanks for posting your data!

It’s more about load division and management. The start surge of A/Cs units is hard on inverters. I am willing to run one, but both of them wouldn’t be a good idea without soft starts on both.
Once in a while, I manage to trip the main breaker at a site when I’m running too many appliances.
(The other day I popped it when I had the absorption fridge, water heater, 3D printer, A/Cs, computers, coffee roasting and the espresso machine was still on from earlier.)

Hmm, I guess I should add a load sensor to the other leg so I can get things balanced better across the legs.