That is not a typo… our RV is now powered by TESLA! Well, Tesla battery modules that is! Oh, and the Sun! Yes, the Sun, too!

WARNING — This post is full of technical content. Here’s the TLDR;

Our original House batteries only powered part of our coach via the inverter, and could only power the coach in a boondock situation for about 4 to 6 hours. After that, the battery voltages were below 11.8vdc and we’d have to start the Generator.

Our new Tesla batteries can power ALL of the coach, not every load in parallel, but 2 of 3 ACs can be running at the same time! Given the same loads that would only last 4 to 6 hours on the old batteries, we can now last indefinitely on the batteries / solar combo!

Check the bottom of the post for loads of photos!

I’m not the first!

Nope… I’m not the first to have braved this path. I may be the first with this exact combination, but ultimately I stand on the shoulders of giants. Here’s a link to a YouTube playlist of videos I used along the way.

YouTube Playlist: LiPo and Solar Installs

And a special thanks to Tom of Mortons on the Move. It was his videos & posts that gave me the final push to get this project in motion!

Mildly Technical Stuff:

Old vs New Specs
Old System New System
Batteries: 6v US2000 FLA 24v Tesla LiPo
Maximum Capacity: 216 aH 200 aH
Nominal Voltage: 6.5 Vdc 22.5 Vdc
Configuration: 2s 3p 4p
Pack Maximum Capacity: 8.4 kWh 18.0 kWh
Pack Usable Capacity: 4.2 kWh 14.4 kWh
Inverting Capacity: 2.8 kW 8.0 kW
Solar Capacity: 145 W 1800 W

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FAQ

Q1: Why is Usable Capacity 1/2 of Maximum Capacity for the Flooded Lead Acid batteries?
Discharging Flooded Lead Acid batteries below 50% causes permanent damage and a reduction of future capacity and charge cycles.
Q2: Are there any limitations or downsides to the LiPo pack?
Yes, a few things that must be kept in mind..

  1. The packs *MUST NOT* be charged while below freezing temperature.
    • This will destroy the pack. They can be discharged below freezing, but not charged.
  2. The packs *MUST NOT* be charged above their rated Maximum Voltage
    • Doing so causes permanent damage and may result in a fire!
  3. The packs *MUST NOT* be discharged below their rated Minimum Voltage
    • Doing so causes permanent damage.
Q3: How long did this take to install?
The installation took place in a three main phases:

  1. Planning!
    I spent almost 2 full months researching every aspect of the system. Again, I have an extensive electronics and electricity background, but there was much to learn and many items that needed to go into the system. Each of those items had multiple possible options with varying costs, capabilities, and limitations.Even with all that research, I messed up on the DC-to-DC converter decision. The first one I bought was an Eaton 21100C00. It was an industrial unit rated for 100 Amps of 13.5 vdc output with an input range much wider than my maximum range of 18.0 to 25.2 volts. It was expensive ( $600 ), but I thought with the specs and industrial design, it’d be bullet proof. The thing I missed though, was a bit of fine print in the spec sheet that stated the 13.5 volt output was only valid for input voltages of 22 vdc or higher. Below 22 vdc, the output was 1/2 of the input voltage. This meant, for my pack voltage from 18.7 to 21 volts, the output would be 9.35 to 10.5 vdc. Let me tell you, RVs do NOT like 10.5 volts! Ultimately, I went with the Victron Energy Orion units, rated at 70 amps of DC output, and I put two in parallel to get 140 amps capacity for less than 1/2 the cost of the Eaton unit. The only down side is I cannot return the Eaton unit since it had already been installed.
  2. Solar Panel Install
    This took a full two days of work ( roughly 20 hours ). The first day I was aided by my nephew (Hi Aydan!) who was crazy enough to help me the day of his Senior Prom! The second day was actually a week or so later when more panels and mounting hardware arrived. Note that this time was ONLY getting the panels on the roof and wired down to the battery compartment. This did not include wiring or mounting the solar charge controller.
  3. Everything Else
    I worked a full 24 hours straight on this phase! It required completely de-powering the coach and I couldn’t leave things half done, so once this phase started, it had to be worked to completion so I could turn power back on inside the coach. This phase included removing the old inverter and batteries, grinding out all of the extra structure in the battery bay, installing the two new inverters and batteries, making up all the new power cables, installing the new DC distribution and protection system, rewiring the coach’s AC breaker panels, etc.
  4. Did I say 3 phases?
    Phase 4 was shaking down the new system. Testing different settings, seeing if things really worked as advertised, instrumenting the new system for easy monitoring, figuring out my mis-step with the first DC-to-DC converter, etc.
  5. And you thought 4 phases was it…
    I’ve still not added any temperature protections for the new modules. As noted above in the FAQ, the modules cannot be charged below freezing and really shouldn’t get excessively hot either. The Tesla modules have coolant loops running through them from the factory, so I intend to make use of these loops. Until I have time to set that feature up, we’ll just stay in good weather! 🙂
Q4: Anything to keep in mind with the dual hybrid inverters?
Yes… Magnum Hybrid Inverters do not support ‘stacking’. To make this work, you need the Magnum ME-ARTR ( Advanced Router) remote, both inverters must use the same configuration, and the ‘neutral’ output of the two inverters must be isolated from each other. To achieve this, you have to re-wire the RV breaker box with two neutral bars, and ensure that each load must be wired Hot+Neutral to one inverter. The mechanical ground can (and should) still be combined. The neutral isolation is to prevent upstream GFCI breaker trips when one inverter goes into load support mode before the other.

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Benefits of the new system

MORE POWER!
When running on battery, we can now power almost our entire coach at the same time. We can power any load in the coach at different times if we choose.
Not only can we power any load in the coach, we can do so for an extended period of time without worrying about doing any damage to the battery packs.
Better charge profile
Lead Acid batteries can only be fast charged up to about 80% of their capacity. The last 20% must be charged quite slowly. Further, Lead Acid batteries incur charge cycle reductions if they are not charged to 100%. Thus, if you want the maximum life from Lead Acid, you must take all the time needed to complete that last 20% of the charge. Additionally, the deeper the discharge of a Lead Acid pack, the fewer charge cycles you get. Thus, for the ‘best’ balance of charge life and usable capacity, the general recommendation is to only allow the Lead Acid cells to go to 50% discharge.
For LiPo batteries, you can fast charge them practically all the way to their 100% mark. Further, they can be deeply discharged without drastically affecting their usable cycle life. Granted, doing 100% charge/discharge cycles isn’t the best though..(more on this in the next section)
Extended Pack Life
Lead Acid batteries typically need to be replaced every 3 to 5 years. Even with the best possible maintenance ( maintaining water levels, not overcharging, equalize charges as needed, etc ), 5 years is about all you will get. Lead Acid batteries aren’t cheap either! Even for the cheapest type ( Flooded Cells ), you can expect to pay $150 to $200 per battery, repeated every 3 to 5 years.
The Tesla LiPo cells are only rated to perform about 500 charge cycles at 100% charge/discharge. But, if you limit to the middle 80% of their capacity, that charge cycle count goes to an astonishing 28 THOUSAND cycles! So being even mildly conservative on the charge/discharge limits, I can extend the life of the pack well beyond any reasonable expectation for the life of the coach!
Hybrid Inverters?
Non-hybrid inverters do not supply any power to the Coach’s AC loads unless shore power is completely unavailable, or at least is severely degraded (severe brown out for example). This means that the power stored in your batteries is only ever used “in emergencies”.
Hybrid inverters are able to help sustain your Coach’s AC loads in two key situations.

  • Load support Amps AC: In this situation, the power requirements of the coach exceed your configured shore power limit. The inverter will take power from the batteries to supplement the shore power and allow your higher load requirement to be met without tripping shore power breakers.
  • Load support Volts DC: In this scenario, the battery voltage is higher than your configured charge maximum, so the inverter starts using the battery power to supply your coach’s AC loads so that the excess power does not excessively over charge your batteries. This has the added benefit of reducing your shore power usage, and thus, your electric bill.

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Deep Technical Stuff

Tesla Module Specs
Source Crashed Tesla Model S 60 kWh pack
Each Module Contains 384 individual 18650 cells in a 64 parallel x 6 series config
Module 1C Capacity Rating 200 aH ( 64 Cells Parallel * 3.125 aH per Cell )
Charge Current ( 0.5 C ) 100 Amps
Module Nominal Voltage 22.5 Vdc ( 6 Cells Series * 3.7 Vdc Nominal per Cell )
Module Maximum Voltage 25.2 Vdc ( 6 Cells Series * 4.2 Vdc Maximum per Cell )
Module Minimum Voltage 18.0 Vdc ( 6 Cells Series * 3.0 Vdc Minimum per Cell )

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Tesla Module Charge Profile math

Reference Material:

Recommendations for NCR118650B:

Charge Rate: 0.5C
Constant Current: Until 4.2 Vdc per cell
Constant Voltage: 4.2 Vdc per cell until charge current drops to 65mA

Extrapolating to the 60kWh pack module:

Charge Rate 0.5C * 1C Rate ( 200 Adc ) = 100 Adc Max
Charge Voltage Max 4.2 Vdc * 6s = 25.2 Vdc
Hold Charge Voltage until pack charge current of 0.065 Adc * 64p cells = 4.16 Adc

For my use, I want to extend the pack life by using the middle 80% of the pack ( leaving 10% margin on charge and discharge ).

Let’s do some more math:

80% Pack Calculations:

Cell: Vmax – Vmin ( 4.2 Vdc – 3.0 Vdc ) = 1.2 Vdc delta for 100% usable range
Cell 10% Delta 0.12 Vdc
10% Voltage level ( 3.0 Vdc + 0.12 Vdc ) = 3.12 Vdc
90% Voltage level ( 4.2 Vdc – 0.12 Vdc ) = 4.08 Vdc

Extrapolating to the 60kWh pack module:

Charge Voltage Max 4.08 Vdc * 6s = 24.48 Vdc
Low battery cutoff 3.12 Vdc * 6s = 18.72 Vdc

Limitations of my specific Inverter/Charger ( Magnum MSH4024M ):

Charge Current range 20 -> 110 Adc ( in 1% steps )
Charge Voltage range 24.0 -> 32.0 Vdc ( in 0.2 Vdc steps )
Low Battery Cutoff 18.0 -> 24.4 Vdc ( in 0.2 Vdc steps )
Charge Current Cutoff 0 -> 110 Adc ( in 1 Adc steps )

Thus, for the 60kWh pack module, I’d expect settings of:

Max Charge Current 400 Adc (limited to 110 Adc for Inverter)
Constant Voltage Setting 24.5 Vdc ~90.3% State of Charge
(( 24.5 Vdc – 18 Vdc ) / 6s ) / 1.2 Vdelta
Low Battery Cutoff 18.7 Vdc ~9.7% State of Charge
(( 18.7 Vdc – 18 Vdc ) / 6s) / 1.2 Vdelta
Recharge Voltage 24.0 Vdc ( minimum recharge voltage of the MSH4024M )

Note: The above calculations make a horribly wrong assumption… that the discharge voltage curve for LiPo batteries is linear. It is in fact, not linear, but I’m OK with this level of inaccuracy.

Turns out, I’m not OK with that level of inaccuracy. In a recent video by Jack Rickard, he provides graphs of actual charge and discharge curves for Tesla battery modules. Now, his were from an 85kWh pack, and mine is from a 60kWh pack, but the voltage levels of inflection should be practically the same. Given this new data, I’ve adjusted my set points.

LBCO: 19.7 Vdc
CV: 24.6 Vdc

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Solar Panel Specs

Model #: Renogy RNG-100D
Max Power at STC (Pmax) 100 W
Open-Circuit Voltage (Voc) 22.5 V
Optimum Operating Voltage (Vmp) 18.9 V
Optimum Operating Current (Imp) 5.29 A
Short-Circuit Current (Isc) 5.75 A
Temp Coefficient of Pmax -0.44%/C
Temp Coefficient of Voc -0.30%/C
Temp Coefficient of Isc -0.04%/C
Max System Voltage 600 VDC
Max Series Fuse Rating 15 A
Fire Rating Class C
Weight 16.5 lbs
Dimensions 47 x 21.3 x 1.4in
STC Irradiance 1000 W/m^2, T=25C, AM=1.5

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Solar Panel Calculation

I had initially thought based on quick measurements, a drone photo of my roof and some room design software… that I would only be able to fit 15 solar panels on the roof of our Coach. Once I got those 15 panels in, I found I could add up to 19 total! Unfortunately, 19 is a prime number, so there wasn’t a good way to split them up into the same number of panels per string, so I had to drop one and go back to 18 panels total.

With 18 panels, that meant I could have 3 strings of 6 panels, or 6 strings of 3 panels. I had already purchased 8 awg wire for running power from the roof to the solar controller, so I did some math.

Knowing 8 awg wire has a resistance of 0.63 ohms per 1000 ft:
( 0.63 ohm / 1000 ft ) * 50 ft * 2 (round trip) = 0.063 ohms for 50 ft pair of 8 awg wire

Having this data as well as the panel specs lets us calculate the panels’ operating voltage and current, as well as the power loss for the wire run:

3s6p:

Vmp: 18.9 Vdc * 3 = 56.7 Vdc
Imp: 5.29 Adc * 6 = 31.74 Adc
Vdc Loss: 31.74 Adc * 0.063 Ohms = 2 Vdc
Watt Loss: 2 Vdc * 31.74 Adc = 63.5 W

6s3p:

Vmp: 18.9 Vdc * 6 = 113.4 Vdc
Imp: 5.29 Adc * 3 = 15.87 Adc
Vdc Loss: 15.87 Adc * 0.063 Ohms = 1.0 Vdc
Watt Loss: 1 Vdc * 15.87 Adc = 15.9 W

Conclusion:
Thus, I went with 6 panels in series per string to increase the voltage, but lower the current and thus, lower the power loss of the wire to the solar controller. Admittedly, the loss difference isn’t drastic, but every little bit helps!

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Parts List / Cost

Description Qty Unit $ Ext $
Renogy RNG-100D 100w Solar Panels 18 136.38 2454.90
Tesla 4.5kWh LiPo Modules 4 1161.50 4646.00
Magnum MSH4024M Hybrid Inverter 2 1758.13 3516.26
Magnum PT-100 MPPT Solar Controller 1 865.00 865.00
AM Solar Tall Solar Panel Mount Kit 18 82.29 1481.28
Magnum ME-ARTR Router Controller 1 289.99 289.99
Magnum ME-ARC50 Advanced Remote Control 1 181.03 181.03
ECO-WORTHY ECO-PV6 Solar Combiner Panel 1 169.99 169.99
GE TL270SCUP Load Breaker Box 1 22.24 22.24
Blue Sea 3003500 600A HD Disconnect 1 59.99 59.99
Bogart Eng 1000ADC/100mv DC Shunt 1 75.93 75.93
Victron Energy Lynx Power In 1000ADC Power Rail 2 141.10 282.20
Victron Energy Orion 24/12-70 DC-DC Converter 2 141.10 282.20
Miscellaneous Wire, Connectors, Fuses and Holders 1 1000.00 1000.00
Approxmiate Total: 15,300.00

Note: Prices above were what I paid and are likely to change. Further, the prices include tax and shipping where applicable.

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Considerations and Design Decisions

There are a lot of intricacies to consider in this type of conversion. You should not take this project on just based off the details of this blog post. Instead, you or someone working with you, should have a deep understanding of electricity, electronics, LiPo batteries, Solar power, etc. I did loads of research and have a background in electricity and electronics. I poured over specification documents for so many pieces and parts, and still missed some fine print on one part that ended up costing me another $600 which I can’t easily recoup.

Here are the thoughts and decisions I made in no particular order.

Should I convert all my coaches loads to 24v?
Absolutely NOT! That would be a massive undertaking and is completely unecessary. I changed out my Inverter/Charger to be 24v compatible, then ran all the 12v loads another way.
Ok, so.. I have a coach full of 12v loads, what did I do with them?
These loads really boil down to two categories – High current loads and Low current loads.
For High current loads ( Starting the generator, running the hydraulic pump ), I switched them over to operate off of the Chassis batteries. For my coach, these are two massive cranking batteries which easily power the two high current loads.
For the Low current loads, I installed 2 x 70amp DC-to-DC converters. They run in parallel for a combined availability of 140amps @ 13.2vdc. This is plenty to run everything else in the coach.
Uh… How do I maintain the charge state on my Chassis batteries now? Before, they would cross connect periodically to the House batteries to charge up!
When we ordered our Coach, we had Tiffin install their ‘Solar’ package. This consisted of a single 145 watt 12v panel and low end charge controller. Since the new system is 24v, the existing panel was useless for our House batteries, so I switched it over to maintain the Chassis battery. That single panel is MORE than capable of maintaining the charge on the Chassis batteries!

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Time for some Photos!


Michael

Resident Tesla nut and polymath. Raised in eastern Kentucky, joined the US Navy at 19 to operate a Nuclear Reactor on a Fast-Attack submarine. After finishing his enlistment, Michael has continued to follow his passions in technology, astronomy, and of course, traveling the country.

5 Comments

Robert Weiss · 2018-06-17 at 19:35

Oh, I see now 🙂

Statement: Your connectivity/connections may be the best I’ve seen.

Questions:
1) How do you charge 10kwh of storage with just 1.8kwh of solar?
2) Why 100w panels?
3) What has been your experience with these used Tesla modules. The idea of replacing my LA bank with this light weight and powerful solution really intrigues me, but I’m concerned about what I’d be buying for $1200~ per module.

    Michael · 2018-06-18 at 11:31

    Hey Robert,
    Thanks for the interest! Here’s the answers inline to your questions:
    1) How do you charge 10kwh of storage with just 1.8kwh of solar?
    – We actually have 18 kWh of storage. We are normally plugged in at campgrounds. So, we use the Solar / battery more as grid offset than off-grid. If we get tight while off-grid, we also can fire up a 10kw generator to quickly recharge the pack ( in under 2 hrs, compared to 8+ hrs for the tiny FLA pack we had ).
    2) Why 100w panels?
    – They were the best for density on our roof due to existing obstacles. Higher output panels were considerably larger and didn’t allow for as efficient use of space ( like between the AC’s and the roof mounted awning for example ).
    3) What has been your experience with these used Tesla modules. The idea of replacing my LA bank with this light weight and powerful solution really intrigues me, but I’m concerned about what I’d be buying for $1200~ per module.
    – Oh man.. they work SOOO well. For a pretty good explanation of the capacity to cost to weight comparison.. check out this video: https://www.youtube.com/watch?v=pnCJFXWEVYs <- it's the most recent addition to my LiPo & Solar YouTube playlist.

      Robert Weiss · 2018-06-18 at 17:27

      I know it’s too late now but I spent a ton of time finding the 190 Toppoints I’m using. Same issue as yours – panels had to fit. The older 6×12’s were the way to go, although i am concerned about replacement. They’re just not made anymore. I weep everytime hail is predicted 🙁

      The Tesla modules would resolve two of my issues: 1) current bank adds 1/2 ton to tongue weight 2) bigger bank is impossible given my space (duh, I already have 12 6v’ers). If i could resell my bank for acceptable buck, I’d take your approach in an instant. So mad that i missed it in my build research.

Robert Weiss · 2018-06-18 at 17:30

Also, I missed this the 1st time: “in under 2 hrs, compared to 8+ hrs for the tiny FLA pack we had”. The Tesla’s recharge fully 4x faster than FLA? Really?

    Michael · 2018-06-18 at 19:53

    Ya… FLA’s can only fast charge to 80% capacity, then nearly trickle from there to 100%. LiPo charge at full current ( 0.5C — 100 amps per module * 4 modules in parallel, 400 amps charge current capable without cooling needed ) up to about 95%. Since I only charge to around 90% anyway, I never throttle my charging. If I could generate 400 amps @ 24.7 vdc, I could top up in 2 hrs.. I can only get around 300 amps total w/ my solar and 2 x inverters, so I’d technically need a bit longer if going from empty to full. Bottom line though, FLAs take considerably longer to charge (if done properly) and have decreased cycle life if you don’t go to 100%. LiPo’s charge super fast and actually behave better if you don’t go to 100%… but don’t care where you stop below that.

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