How many days of autonomy should I plan for? Is 2 days good?

[magicbus]

This will be for an RV application where power isn't a luxury; it will be needed to work remote (online / internet / IT stuff).

I'm still torn between AGM & LFP, but with either, my planned configurations will give me ~ 2 days autonomy based on my best liberal power use estimates. Specifically, 2.0 days w/ AGM @ 50% DoD, 1.6 days w/ LFP @ 80% DoD, & 2.0 days with LFP @ 100% DoD.

6 180W panels (3 parallel strings of 2 panels each) work out to ~ 3.0 peak sun hours required for daily charging (a bit more for AGM, a bit less for LFP). Since we're mobile I can't say where we'll be at any given time, but as we're based out of AZ most of the time I'm thinking we'll exceed those figs.

We will have generator power for backup, but would obviously prefer to not use it during normal operation unless necessary.

Do you think we'll be well-served w/ the amount of battery planned?

Thanks!

[Mike90250]

If you have a generator, it's fuel (when available) is going to generally be much cheaper than trying to cram more and more solar PV (unless you have lots of roof area) & battery bank into an RV

If you would tolerate only 24 hours, and then recharge, you could use a small eu2000 style inverter generator and I think you would be very happy, but FIRST you must make your load budget. That is the first step, then battery size, then adding enough charging sources to keep the batteries happy. anything less than full sun from 9am - 3pm will become increasingly expensive with out using a generator - remember, you are running off batteries at 3pm thru 9am the next day, 18 hours on batteries, then you have "maybe" 6 hours to run loads & recharge the depleted bank


What will the battery bank location / temperature be ? Below 40F, LFP starts to loose capacity faster then most lead acid, and below 33F, LFP cannot be recharged without destroying the battery.

https://electronics.stackexchange.com/questions/263036/why-charging-li-ion-batteries-in-cold-temperatures-would-harm-them

Do not charge lithium ion batteries below 32°F/0°C. In other words, never charge a lithium ion battery that is below freezing.

Doing so even once will result in a sudden, severe, and permanent capacity loss on the order of several dozen percent or more, as well a similar and also permanent increase in internal resistance. This damage occurs after just one isolated 'cold charging' event, and is proportional to the speed at which the cell is charged.

https://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

Many battery users are unaware that consumer-grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the pack appears to be charging normally, plating of metallic lithium can occur on the anode during a sub-freezing charge. This is permanent and cannot be removed with cycling.

[magicbus]

Thanks Mike!

I have a generator. Two (parallel capable), to be precise. But the entire reason I want a solar system in the first place is to avoid running one or both whenever possible. The economics aren't my driving concern.

I have made a detailed, comprehensive energy budget. That's how I determined approximately how much no-sun autonomy I have with my proposed banks.d

My question isn't how to do the math. It's more along the lines of "For those of you living in an RV, would 1.6 - 2.0 days of no-sun autonomy in actual use significantly limit your generator use to be the exception, rather than the norm"?

Batteries will be inside, which will be a conditioned space in use. If using LFP, it will be with a comprehensive BMS w/ battery shut-off capability to account for temp & other factors. I am familiar with both chemistries. My indecision is based on logistical factors (price, weight, suppliers, etc)

[RichardCullip]

I can't answer anything specific to your RV off-grid situation since my solar experience is limited to a grid-tied home solar installation. However, we just went thru a 5 day period down here in sunny San Diego where, due to cloudy rainy weather, my roof top solar didn't provide near enough power for our needs. I'm glad I had the SDGE grid as my back-up electrical source rather than a bank of batteries. If I wasn't tied to the grid but dependent on battery for my back-up I would have needed at least enough battery capacity to provide the equivalent of two full days worth of our normal household electric use. That's how much of a deficit my net energy use was at the end of the 5 day period. We normally run a surplus of solar energy each day but not during this 5 day stormy period.

[zamboni]

Late reply, hope it's still helpful.

Taking your numbers and estimates at face value, I'd expect you to do pretty well in the desert SW. We get 3 days of "normal living" autonomy, and have gone to 5 or 7 in 'conservation mode'. Being fulltime, and with a penchant for visiting stupid places like the PNW, we still get pretty good freedom, enough that we're content. Sometimes we run away from weather; usually we just hunker down (we're a very stubborn couple). Only once in nearly 5 years have we actually used "plan C" and plugged in at an RV park, and you know what? It was fine. That's a great "plan C" (Plan B is using our camping gear such as backpacking stoves, etc.). It depends on your work, family, travel preferences, but your design sounds pretty comfortable (3 sun hours is very conservative in AZ). Once you're used to things, you can probably stretch it.

Folks who don't live in RVs often underestimate exactly how aggressively it's possible to conserve (once you know what you're doing). We use electricity for absolutely everything, including cooking and fridge, and are currently on day 4 of rain and overcast...not a problem. We don't usually do much space heating, though ; )

Mike's warnings above are worth heeding, but I have some perspective to add: Our batteries (LFP) are in the same compartment the propane used to be in, e.g. exterior, uninsulated. I have temp sensors on them, and they have never gone below freezing, not even when outside temps were in the teens. That is, in very large part, because they are always in use. Charging/discharging generate heat. So yes, freezing is a limitation, but perhaps not as strict as it may seem. Regardless, I'd still advise against LFP for starters. If AGM is even still on the table (sounds like it is), your "need" for LFP is probably not strong enough to justify the egregious expense. It's your money of course, that's just what I'd do. LFP can be super nice...but you'll appreciate it so much more after living with lead for a while first : )

Yes, it gets expensive (vs. generator fuel) to up-size a system for more autonomy, weather, and flat-mounted panels. But you also have to look at what you're getting. We don't even own a generator. Don't have to store one, keep fuel in it, pull it out or put it away, worry about it being stolen, or *listen to it*. And expensive as it is, our solar install paid for itself in avoided RV park fees in our first year on the road. That was almost 4 years ago. Expensive up front? yes. Long-term, though? Big winner. The important caveat is you have to design/use your system intelligently. If you trash your batteries every year due to abuse or poor design, that ROI goes through the floor ; )

- Jerud

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1220W array / 1000Ah LFP house bank
MidniteSolar Classic, Magnum MS2812
ME-RC, Trimetric, and JLD404
Full-time 100% electric boondocking (no propane, no genny) since 2015
2001 Fleetwood Prowler 5th wheel 25 foot, self-rebuilt
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