Solar and Li-ion - Fiberglass RV


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Old 07-25-2017, 12:52 PM   #1
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Solar and Li-ion

Folks,
Because we have a 12V-only (50W) Dometic fridge and live in the NE where it *is not* sunny 365 days of the year, I want to upgrade the 2014 Trillium (1-100W solar panel and a cheapo G24 lead/acid deep-cycle battery). From the very handy ervsolar calculator (ervsolar.com/sizing-your-system/) I determined that we need ~110W of solar (1 portable to add to the rooftop fixed) and 530Wh of battery. Before we dive in and get these, has anyone done this? Success stories? Horror stories?
Thanks!
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Old 07-25-2017, 01:05 PM   #2
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Someone posted these former RV'ers info. Maybe it will help you crunch the numbers.
Basic intro to Lithium VS AGM
https://youtu.be/J-kND4PaZm8
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Old 07-25-2017, 03:24 PM   #3
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With your 50W (approx 4 amp) Dometic fridge, at a 50% duty cycle (running 50% of the time), the fridge will consume 600 watt hours (50 amp hours) per day. A 110W solar panel will produce maybe 500 watt hours per day using a PWM charge controller on a good day. So the 110W solar panel will not keep up with the fridge power consumption.

Is there a second solar panel?

Concerning the battery, with an electric fridge and other accessories typically a 200 amp hour (2400 watt hour) battery is used to keep the everything running on overcast days. For your fridge, the 200 amp hour battery would run down to 50% charge in two days.
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Old 07-25-2017, 03:30 PM   #4
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OK I missed that you actually will have two solar panels, a 100 watt and a 110 watt. That should be sufficient for running the fridge and other accessories (lights, water pump, etc). As I mentioned, typically a 200 amp hour battery is needed to carry you for 2 days of overcast.

Disclaimer: I have a solar setup, but its only 100W panel, and I'm not running an electric fridge. I have met two people on the road that run small electric fridges and accessories with the 2 100 watt panels and 200 amp hour battery setup. That seems to be the typical starting point.
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Old 07-25-2017, 04:06 PM   #5
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more thoughts

Re batteries, my above analysis assumes AGM or wet lead acid deep cycle batteries with a max depth of discharge of 50%. With a lithium-iron-phosphate battery, the max depth of discharge is 20% so you can get by with a smaller battery. In my 2-days-of-overcast example, a 120 amp hour (1440 watt hour) battery might be sufficient.

In my analysis, it appears I might have been overly optimistic in assuming the fridge "and accessories" could be run for 2 days of overcast. As you can see, you really have to have a realistic estimate of how much power will be used. More or larger solar panels may be needed if you want to run more accessories. More or larger batteries may be needed if you need to handle more days without sun.
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Old 07-25-2017, 04:20 PM   #6
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lithium battery solar charge controller? converter charger?

I'm not sure there are solar charge controllers available designed to charge lithium-iron-phosphate batteries or other lithium technology batteries. (anybody know?)

Also not sure whether any converter has a built in charger designed to charge lithium batteries.
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Old 07-25-2017, 04:53 PM   #7
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You'd want a controller that has settable voltage for each stage.
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Old 07-25-2017, 05:29 PM   #8
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John in Michigan,
Well, I obviously got you thinking about this.

I used a data logger on my current electrical system to see what the net flow of energy in/out of the battery is. I came up with a figure of 170Whr/day. (Sitting out in the sun in the back yard in Rochester, NY.) Just the fridge and a couple small things (USB charger--unloaded--and indicator LEDs, i.e., essentially just the fridge. The lighting is LEDs.) Conclusion: the avg duty cycle is about 170/(50*24)=0.14. I'm figuring on up to 3 no-charge days for a total storage requirement of 530Whr. (I've derated for temperature.) So a 12V, 50Ahr battery ought to be enough. I'm also assuming that in the case of lots of non-sun days we throttle back on listening to NPR since the car stereo is an amazing energy pig, much to my regret. No other big energy drains. Even the water pump makes a negligible load.

And yes, I'm concerned about the marriage of solar and Li-ion. I haven't done the research yet to find a suitable PV regulator. (The current one is geared to Pb-acid.) And I'll probably remove the built-in power center, which is also a Pb-acid charger. That also means coming up with a replacement for the incorporated fuse box, too.

My conversations with a couple of RV solar/battery vendors were unimpressive. They were not able to explain to me why they were choosing particular components. So I'm doing my own digging, and hoping to hear about real-life experiences.

Thanks for the link to the RVers-turned-sailors.

Regards,
Bill
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Old 07-25-2017, 06:02 PM   #9
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Bill,

That's great that you've already done some testing/evaluation. 14% duty cycle fridge is great vs. the 50% that I assumed.

Good luck!
John
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Old 07-25-2017, 06:12 PM   #10
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Bill,

Also I'm curious. Are you considering lithium iron phosphate (LiFePO4) or lithium cobalt oxide (LiCoO2) batteries?

-John
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Old 07-25-2017, 06:17 PM   #11
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my solar setup

Bill, I bought my 100W solar panel and 30 amp charge controller at amazon. So far used it for 17 days of continuous boondocking in TX in January and it worked great. The load was: 8 1156 led bulbs, LP detector, charging of tablets and phones...

EDIT: and a little Optima 38 amp hour AGM battery
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Old 07-26-2017, 12:26 PM   #12
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Gel cell batteries

Haven't seen anyone mention gel cell batteries. I used one that was purchased second hand...found it on Kijiji for 40$...group 27...removed from a backup system for a provincial telephone company, i believe. Worked well and felt "safer" in terms of off-gassing considerations.
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Old 08-07-2017, 01:29 AM   #13
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Honestly, for only 530Wh of capacity (~44Ah), Lithium is probably a waste of money. You can get something like a 90Ah lead-acid battery (so you don't have to worry about discharging below 50%) for a fraction of the price, and not have to worry about voltage settings (or, more importantly, a desulfation mode) on your charge controller(s). If you want a bit of extra margin, something like a pair of Trojan T105 batteries will get you 225Ah (though at that point, I'd up the solar capacity to match so you can fill them up most days)
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Old 08-28-2017, 12:57 PM   #14
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Folks,
My apologies for not keeping up with the replies to my post.

Given what I know now, the direction re batteries is a Lithionics 52Ah (12V, 624Wh) battery (available this Fall, ~$650). Since this thing contains its own charge/discharge controller (or Battery Management System--BMS--gotta love the lingo) I believe that a simple, no-nonsense AC2DC charger would suffice. Lithionics claims the advantages at http://lithionicsbattery.com/wp-cont...g2017final.pdf, p3. It's expensive, but with 6-8X the number of charge/discharge cycles, the overall cost of ownership (and the maintenance) should be less.

The solar controller is another question: the controllers I have seen so far are geared to Pb-acid, i.e., their maximum output voltages conform to Pb-acid requirements. For sure, I don't want to burn out anything else in the Trillium, so I have to look into what the fridge, car radio, lights, etc, can handle. The current electrical system is a common 12V bus--everything connects to that.

I'll update this as I learn more.

Thanks to all,
Bill
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