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Loki

Running the AC (and more) with Battery and Solar

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In a previous post, I described my 1750 Watt solar installation on the roof of my 29 ft. Arctic Fox 5th wheel trailer. Today, I am going to share the rest of my project: becoming independent of generators and shore power. Tu make optimal use of the solar power supply, I chose to install two Tesla Lithium battery modules instead of using a large lead acid battery bank. Each module weighs 55 lbs, has a storage capacity of 5.2 KW and runs with 24 V. A lead acid battery bank with a comparable capacity would weigh hundreds of pounds (400 lbs my best guess), but what is even more advantageous with lithium batteries is that they can routinely discharged down to 15% of their capacity without compromising longevity. With the discharging limit for lead acid batteries being around 50-60%, you would need even more batteries to match the usable power reserve of these two lithium batteries.

I decided to keep the 24 V system completely separated from the 12 V house system, in order to keep things simple and straightforward. The solar panels feed into a Magnum PT 100 MTTP charge controller, which powers  a 24 V bus that is connected to the batteries and a Magnum MSH 4024 RE inverter. I chose this 4 KW inverter because it is able to work in Hybrid mode: When connected to shore power and the AC load gets close to the maximum amps available,  the Magnum supports the AC load by inverting and prevents tripping of the shore breaker. I left the built-in 12 V system untouched, meaning I kept both the two 12 V batteries that came with the unit as well as their converter-charger. When disconnected from shore, the 12 V batteries are being charged with the built-in charger using the solar/inverter power. I installed, however, a switch that enables directing solar power from the PT 100 to the 12 V batteries, just in case the 24 V system fails to work, from whatever reason.

The only real downside with lithium batteries (besides the price) is their sensitivity to low and high temperatures as well as to overcharging and over-discharging, so I had to design and build some protection circuitry. The electricity to and from the batteries goes through two Victron Battery Protect units (basically unidirectional solid state relays) rated at 60 A and 200 A, respectively. They shut off at 20 V on discharging and 24.6 V on charging, and are connected to two temperature sensors that disconnect from charging below 35 and above 120 degrees, respectively. I also installed a heating blanket around the batteries that starts heating around 35 degrees, and a radial fan blowing air into the compartment from the outside if above 100 degrees in the compartment.

The first pic shows the installed units without the batteries. On the upper right side, the solar charge controller, and on the compartment floor the Magnum inverter. It also shows the control panel with the protection circuitry that I built. Upper left corner, two circuit breakers in line with the two blue Victron Battery Protect relais guarding the charging and the discharging flow. Also shown is the switch that directs the solar current to the 12 V or the 24 V system,  4 temperature control modules, and in the lower right corner the Magnum battery monitor unit. The second pic shows the compartment with the two Tesla battery modules in place. The blue unit above the control panel is a 600 W Samlex inverter that is powered by the 12 V batteries and is used as backup if the 24 V system fails. The entire installation is super-compact and uses less than half of the front compartment, just about 8 square feet.

My maiden voyage with this installation took me across part of the Southeast and included a week of boondocking. I planned a longer boondocking experience, but a week of rainy weather put an end to that - no sun, no solar! Bottom line is, on a sunny and warm day it is no problem to run the air conditioning and the fridge with the battery and the solar. And this was October, with the sun already pretty angled and the peak temperature still 88 degrees. Especially on long, hot travel days it's so nice to have the RV  air-conditioned when you stop or arrive at your destination.

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This looks to me more like a blog post than a forum post. Blogs are where we write large posts to share with people what we did when nobody asked. It's my understanding is that the forum is intended to give us a place to answer each other's questions.

Linda Sand

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Thanks Loki! I appreciate the info, as I am planning something similar  - off grid solar powered AC but with a seperate 48v battery bank (as I plan on installing a super efficient native 48v mini-split DC air conditioner like this one that only draws 560 watts to produce 12,000 BTUs of cooling.  http://www.geinnovations.net/solar-electricity-cost.html

You answered my question about how you charge your 12v house battery bank. I have been wondering exactly how I am going to do it. I have been considering a separate 300+ watt solar panel and controller for the task, as my 12v needs will be minimal - mainly lighting, slide motor, jacks and fridge/roof AC control boards. I thought that the inefficiency of converting high voltage dc to ac and back to 12v dc would be impractical.

Do you know how many amps your 12v battery charger draws? Mine is an older model so if I go this way I will need to replace it with a more efficient converter/charger. 

I'm afraid I can't afford the sweet lithium batteries you have, though I recognize their inherent advantages. I plan on using 8 GC-2 FLA golf cart batteries initially, maybe upgrading in a few years when they wear out and lithium's drop in price. Yeah, they are heavy, (500 lbs) but they are cheap - under $800 for the 10.3 kw (5 usable kw) bank vs $$$ for an equivalent amount of lithium battery power. It's hard to compare the two because lithiums charge and discharge more efficiently too, lacking the internal resistance of FLA, but I don't think Peukert will affect me much as the AC only draws 11.5 amps (at 48v) - close to the 20hr discharge rate for my planned bank.

Do you mind me asking where you got your Tesla battery modules?

I'd rather spend $2,000 for the super efficient mini-split AC that only draws 560 watts with no start surge than try to run one of my inefficient rooftop air conditioners that draw about 1,500 watts to run and 2,000 watts to start - meaning I'd have to have 3+ times the solar and battery power for the same amount of cooling produced. I'm planning on around  2,000 watts of solar now, so I wouldn't have the roof space or the weight capacity for that big of a solar system. Also the money I save on the smaller solar system will more than offset the ACs cost.  I'm sure if I used lithium batteries I could reduce the solar bank even more, due to their better charge/discharge efficiency But I'm not sure if I could afford their high cost (plus the cost of the BMS and temp management system like you installed) initially.

Chip

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1 hour ago, Sehc said:

Is that a rule? Sounds like an opinion.

Yeah, I think I missed the "no sharing information you weren't specifically asked about" rule.  This internetting is some serious business!  :)

Thanks for sharing OP!  Looks like a nice setup!

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Thanks for sharing your setup Loki - I can be camped in “short days” and “cloudy” days and shaded by those annoying trees and buildings sometimes so I have a question: is it feasible to use a transformer to connect the vehicle alternator for charging the lithium bank? I understand from my learning that Li batteries are not fussy about reaching a full state of charge etc. 

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Thanks for sharing your setup Loki. I like seeing the non traditional systems people have put in and how they are working for them. It's always cool to get new ideas. 

8 hours ago, sandsys said:

This looks to me more like a blog post than a forum post. Blogs are where we write large posts to share with people what we did when nobody asked. It's my understanding is that the forum is intended to give us a place to answer each other's questions.

Linda Sand

It's also just as easy on a forum post like this to skip over the post that may have no interest to you instead of feeling like you need to comment on it to share your opinion on what type of post someone should share on the forum. These types of Lithium battery setups (especially EV batteries) are interesting more and more RV'ers as Lithium batteries become more popular and all of us that are interested in learning more about using Lithium appreciate when others share how they setup their RV. 

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On ‎12‎/‎5‎/‎2018 at 11:16 PM, Loki said:

In a previous post, I described my 1750 Watt solar installation on the roof of my 29 ft. Arctic Fox 5th wheel trailer. My maiden voyage with this installation took me across part of the Southeast and included a week of boondocking. I planned a longer boondocking experience, but a week of rainy weather put an end to that - no sun, no solar! Bottom line is, on a sunny and warm day it is no problem to run the air conditioning and the fridge with the battery and the solar.

Question: If you chose to do without air conditioning on this maiden voyage, would your power supply be enough for you? 

Edited by rm.w/aview

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Why don’t you get a 24 volt to 12 volt converter to run you 12 volt lights. You can then get rid of the lead acid batteries and not use the converter. When my converter burned up, I decided not to replace it as I had an inverter/charger. Are your lights led? If som they won’t draw hardly anything. Your furnace will be your biggest draw. Do you have pictures with the lithium batteries mounted? Where did you get them and how much?

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Just a thought. Being as the type technology Telsa uses, they can become dangerous if too hot. A simple cooling system could be installed. A bottle of antifreeze, tubing and a small pump would be sufficient. 

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3 hours ago, Ronbo said:

Why don’t you get a 24 volt to 12 volt converter to run you 12 volt lights. You can then get rid of the lead acid batteries and not use the converter. When my converter burned up, I decided not to replace it as I had an inverter/charger. Are your lights led? If som they won’t draw hardly anything. Your furnace will be your biggest draw. Do you have pictures with the lithium batteries mounted? Where did you get them and how much?

I thought about using a converter, but the slide motor and jack motors draw lots of amps (up to 100 amps) even though they just run for a short time. A pair of these would do it: https://www.ebay.com/itm/48V-to-12V-60A-Step-down-720W-high-power-DC-Transformer-Vehicle-power-converter/142174939521?_trksid=p2047675.c100009.m1982&_trkparms=aid%3D888007%26algo%3DDISC.MBE%26ao%3D1%26asc%3D20131227121020%26meid%3D88db938dd9ee4b35ad8fe00510af09f5%26pid%3D100009%26rk%3D1%26rkt%3D1%26sd%3D222127847212

This transformer or a simple buck converter cannot be used as a 12v battery charger as their output is stabilized at 12v, not the 13.7 one would need to charge a battery. Plus they are pricey, at $200  for the pair. What I haven't found is a relatively inexpensive 12v DC multi stage battery charger powered by a 48v dc source.

I think a more direct approach would be to install an additional solar panel, wiring and charge controller for my existing 12v battery bank (basically a pair of GC-2s). That way you would be collecting more power as well. A single 305 watt solar panel can be had for 33 cents/watt ($100.65) a decent 30 amp MPPT controller kit will cost about $150  https://www.amazon.com/dp/B071XP39BK/ref=psdc_2236627011_t3_B07DNVNV7B

So for under $300 You can have another 300 watts of solar charging your 12 v bank with an additional 220 amp hrs of power for 12v incidentals, saving all the power in my main 48v bank for the heavy lifting, ie. air conditioner/heat pump and 110v plug in devices. Plus I still have my 5.5kw onan generator back-up to charge both 12v and 48v battery banks (12v through the RVs converter charger and 48v through an add on inverter/charger like this https://www.sigineer.com/product/2500-watt-48v-100v-110v-120v-off-grid-battery-inverter-charger-65a-mppt-solar-controller-pure-sine-wave-w-transformer/) on those rare occasions when solar proves to be inadequate. 

Chip

 

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Thank you for the link to the transformer. Saved that. I looked hard at the 48v mini splits. I would need 3 of them in my unit. Nowhere to put the outside units. As efficient as they are, just can't use them. I might could crowd 2 on rear but then where to put the other. I 240v unit on rear and 2 cassettes in ceiling works for me. Now a smaller unit they are the ticket.

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