Important Note: There are always risks associated with being around and/or handling electricity. If you have any questions about your ability to handle these risks safely, please do not attempt to follow any of what I have outlined below and instead, hire a licensed professional to do this for you. I am only sharing what I have done. Neither I nor Bus Conversion Magazine assumes any liability for what you do or how you do it.
This is to chronicle the solar power build with The Electrodacus SBMS0 and Lithium Iron Phosphate (LiFePO4) Batteries for our Bus Conversion (1989 MCI 96A3). We do not currently Boondock a lot, but we plan to in the future. The few times we have boondocked this system has worked quite well for us. This unit provides the same level of service as if we were plugged into a 30A connection at an RV park.
Power connections are not available at the storage yard where we store the bus. It is nice to go work on the bus and have the option to turn on the air conditioning and plug-in power tools without running the generator.
What you need may differ. Please study your needs before jumping in and spending any money. The cost for these systems can easily get quite expensive.
I thought I knew quite a bit about batteries, electrical, and wiring, but when it comes to the current state of solar power I had quite a bit to learn, including some new rules.
Not all of it is spelled out in the electrical codes and there are differences when building a system in a moving vehicle. There are also different technologies and approaches to charging methods, battery arrangement, and wiring. We found the DIY Solar Power Forum to be an indispensable resource.
Something to keep in mind when reading and posting on any forum, there will always be negative comments on anything you post. There will be the perfectionists that say you have not crossed every “T” or dotted every “I”. There are also elitists that say if you have not bought the best and most expensive products on the market you will certainly fail catastrophically.
Some professionals insist this is rocket science and should not be attempted without a college degree in Electrical Engineering and certifications in solar technology. Some people have never worked on anything and never will, yet feel they are expert enough to tell you how to build yours and of course the trolls that are just there to start controversy.
There are also a lot of good people with good solid advice and knowledge willing to go the extra mile to make sure you understand what is needed. Many people have been where you are and can tell you what worked for them. Without information gained from the forums, I could never have built this system. Only you can know what is right for you. Weed through the information to find what you need.
The first thing to do is an energy audit. List everything you could possibly want to run off the batteries. Research the power demands. This could involve using a measuring device like a clamp meter to measure amps, a Kill-A-Watt monitor that you plug each device into to get usage, or reading the device specifications from the manufacturer. Use this information to determine how much draw you might need at peak times and how much draw you will need over a given time.
Determine what things you absolutely need to run, refrigerator 24/7 vs. things you might like to run, air conditioner vs. things that you could run on a different medium. A water heater could run off of propane and other things you could run at different times. For example, do not run the microwave, air conditioner, and hair dryer at the same time!
From this you can determine what size Inverter you would need to accommodate the load, the amount of battery power you would need to run this inverter, the number of solar panels (PV – photovoltaic panel), and the space for these panels to charge the batteries or help handle the daytime load, the space available for a battery bank and breaker panel for all the supporting devices and wiring.
We determined, that for AC loads, we need to run an apartment-sized refrigerator 24/7, the microwave, induction cooktop, and air fryer on occasion, and charge the phones, TV, laptop, and the mini-split A/C some of the time. We determined that for AC loads, we need to run an apartment-sized refrigerator 24/7, the microwave, Induction cooktop, air fryer and run the Mini-Split A/C some of the time.
For 12-volt DC loads we have numerous LED lights, the fresh water pump, and the tank level monitors. The TV, phone chargers and laptop can run on either 12V DC or 120V AC.
We settled on a 24-volt system. This includes a 3000W Inverter, 1500 watts of Solar panels, and 600Ah of batteries. We also have a second rooftop A/C that draws way too much power and the water heater also draws considerable power. If we want to run those appliances or run the mini-split A/C when the battery bank is low or if we are experiencing rain or cloudy skies for too many days, then we need to start the generator.
As a backup, we have a way overkill 20KW diesel generator. It will power anything in the bus and probably two or three other buses. We prefer not to run the generator as it makes noise and consumes diesel fuel. After much research, I decided I would stray a little off the beaten path and choose a system that is not quite as established as the mainstream high-dollar alternatives. We chose the Electrodacus SBMS0 (Solar Battery Monitoring System Zero).
Dacian, the SBMS0 designer being an off-grid solar user himself and not liking the way the mainstream solar industry was headed, designed his own solution for his off-grid system. His system worked so well that he shared his vision. He builds and sells all his products and has great support on his website and forum for what he sells.
The tradeoff for Dacian’s solution is that it is not typical and you need to understand a little about electronics and how it all works together. Although not terribly technical, it is not a plug-and-play solution. Your choice of BMS (battery monitoring system) will affect the other components purchased.
When designing your system as a rule the higher the DC voltage the better. With higher voltage you have lower amps and can use smaller cable sizes. This also allows for a larger inverted AC amperage you could produce.
Standard house battery DC voltages commonly found in RVs are 12-volt, 24-volt with a few 48-volt. 12-volt can be less complicated in not having to use a 24 to 12-volt converter for 12-volt loads. We chose to use 24-volt. If you are using SBMS0 you will be using 12 or 24-volt.
Batteries/Cells
The batteries will probably be the most expensive part of your system. There are currently many choices in many different sizes and prices to choose from. There are good picks from Tesla, Lion, and Renogy. For those of us on a budget, most DIY LiFePO4 batteries are ordered as individual cells direct from China. Watch Will Prowse’s videos on YouTube for some excellent battery suggestions, builds, and reviews.
Alibaba and AliExpress are good places to look. On my first purchase of eight 3.2V 302Ah CATL cells, I went through a group buy on one of the forums. It took most of a year to receive them and they were less than perfect. Battered and bloated, one of them was completely unusable. The busbars that arrived with them looked like they had been handmade from discarded pieces of aluminum and had to be replaced.
My second try was through Amy Zheng at docanpower.com, although more money nine 3.2V 302AH CATL cells arrived in perfect condition in less than a month. These cells arrived with very nice tin-coated copper busbars. I ordered more of these bus bars to replace the substandard ones from the first cell purchase. Results may vary.
Amp-hours (Ah) is an interesting term. It specifies how much current can be pulled from a battery and for how long. IE: 10 amps × 10 hours = 100 Ah battery, which has come to standardize battery capacity and size.
Connecting the batteries in series (+ to -) will add the voltages together. Connecting the batteries in parallel (+ to + and - to -) will add the amps together. To create a battery pack, you can do both. With my sixteen 3.2-volt 302AH cells, I wired eight sets of two cells together in parallel and the eight parallel sets together in series. The short form for my pack is 2P8S, giving me a 24-volt 604Ah battery pack.
There are multiple ways to connect your cells together. Some people use a heavy cable with crimped-on ends. I chose to use tin-coated copper bus bars, stacked where necessary to connect multiple batteries. It took 23 busbars (lots of controversy on this on the forums). My cells had 6mm welded studs. Some cells have smaller studs. Others have threaded holes for bolts.
My cells did not fit together as well as I might have liked due to the bloated, battered cells, some with bent posts from the first batch. My original battery box design called for compressing the cells together using threaded rods and plywood end plates. This is rumored to increase battery life but was not an option with these cells. You have to play the cards you’re dealt.
Instead, I arranged them in the plywood Battery Box using pieces of plywood. I compressed them securely into position using a ratchet strap, then connected them with the busbars.
Battery Monitoring System
Another one of the main components of a LiFePO4 system is the battery monitoring system (BMS). Unlike your old-school lead-acid batteries, each individual cell (or parallel sets) of your LiFePO4 batteries needs to be monitored. LiFePO4 can be charged and discharged up to 90% of its rating without damage and will last for many thousands of cycles. Lead-acid can only be cycled to 50% capacity before they start to degrade, but LiFePO4 batteries can be easily damaged by over or under-charging them.
Thus, a BMS is used. In the case of the SBMS0, it is the brain of the system and controls all the charge or load devices connected to the batteries and monitors each cell. In my case look at the above picture and see the individual cell monitoring wires connected to the cells. The SBMS0 will shut off any charging source if any single cell gets to 3.56 volts for 5 seconds and resume charging when drawn down to a predetermined level.
On the other end, all load sources will be turned off when any single cell voltage gets to 2.8 volts for 5 seconds. The SBMS0 also includes multiple parameters that can be adjusted to your system and monitoring screens that are also viewable on your phone. Other BMS are less sophisticated, each cell will still be monitored, but the entire battery load will pass through the BMS and shut off if the voltage is too high or low.
Charge Controllers and Solar Panels
There are a few different types of SCC (Solar Charge Controllers), and your choice of SCC will also dictate what type and size of solar panels you buy. The main types are MPPT and PWM. The industry standard these days is MPPT. The most common (and most expensive) brand is Victron.
You will not see many bad reviews on Victron. If you truly want a mostly plug-and-play system, Victron is probably the way to go, but you will pay dearly for it. MPPT is supposed to be the best where you connect your panels in series to obtain a higher voltage and the MPPT SCC will convert this to a lower voltage high amp charge closer to the requirement of batteries. It is supposed to maintain a higher charge rate in lower light conditions.
SBMS0 has the charge controller built into the BMS and does not use either MPPT or PWM. What they do is match the panel voltages to the battery voltages and basically charge directly from the panels using their version of a charge controller, a DSSR20.
They are controlled by the SBMS0 and switch the DSSR20 on or off depending on the state of charge. This is a simple but effective concept. No current is wasted converting voltages. The DSSR20 is rated for 20 Amps. Dacian will soon be adding a new version, the DSSR50 that will accommodate 50 Amps. The SBMS0 can accommodate up to 30 DSSR20 and 600 Amps of solar panels.
After deciding on the SBMS0 I needed solar panels. I needed 60 cell panels putting out approximately 30 Volts in full sun with no more than 20 Amps for two panels in parallel.
After some measurements of the bus, I decided to put as many panels as would fit between the escape hatches, giving me an area 20 feet long and 8 feet wide.
I found a guy locally selling what he termed as slightly used Trina 60-cell 250-watt panels for $50 each. These panels are approximately 5ft. 6in long, and 3ft 2in wide. If I mounted them crosswise, I could fit six panels between the hatches.
I bought six panels at $300 for a total of 1500 watts. Purchasing four 10 ft. pieces of Unistrut at Home Depot and having some brackets made to account for the curvature of the roof I mounted the Unistrut to the roof of the bus using Plus Nuts (like rivet nuts only stronger), drilled through the aluminum roof, and into the steel bus ribs. I Installed the Plus Nuts, then bolted the brackets down and glued them with liberal amounts of Sikaflex 252.
Then the panels were mounted to the Unistrut using solar panel clamps sourced from Amazon.
To get the power from the panels to the batteries, I used a junction box glued to the roof with Sikaflex 252 and a hole drilled to accommodate a 1-½ in conduit down through an inside wall into the luggage bay. Three runs of 2-conductor 10-gauge wire were run from the panels that are wired in parallel using MC4 Y-connectors, connected through 20-amp breakers to the DSSR20s. This created six parallel panels with the potential for almost 50 Amps of solar power.
Inverter/Charger
The Inverter will probably be the second most expensive component you buy. It is what takes the DC power from the batteries and changes it to 120-volt AC power to run your appliances. The Charger will allow you to plug into an external source like the power pole in an RV park (also called Shore Power) to charge your batteries. Some inverters will allow the external power source to pass through and directly run your AC loads.
As with batteries, there are many different types and sizes, some with built-in chargers, some with Bluetooth, Wi-Fi, and wired monitoring. Some put out 120 Volts, some will do 240 split-phase. There are pure sine and modified sine. There is also an all-in-one, which includes inverter/solar and shore power charging. I will not get into the differences here. The energy audit, your appliances, and your budget will dictate what you buy.
For my purposes, I needed a 3000W
pure sine inverter preferably with an onboard charger on a limited budget. A friend, Ted had just upgraded to a new 6000WQ 240-volt inverter and had older AIMS 24-volt 3000W Inverter/charger PICOGLF30W24V120VR available. After some negotiation, the unit was mine.
Although the AIMS inverter was 3000W and pure sine, this is an older inverter, and it did not have a specific charge profile for LiFePO4 batteries. It also did not have any external controls to turn the unit on/off or turn the charger on/off separately. The SBMS0 needs to be able to control all charge and load sources externally.
I communicated with AIMS Support several times through email. After asking the right questions I was told that the best charge profile for LiFePO4 batteries on the older hardware is to use #4 sealed lead-acid, 28.8V bulk setting.
The next topic I asked AIMS Support about was the rotary dip switch that sets the battery charge profile and includes a setting for charge-off. On the dip switch, I asked, “If I set the charge type to #0, charge off, and soldered wires to the dip switch that could make the connection as if #4 is selected? When connected, would this turn the charger on, or off when disconnected, and would this hurt the operation of the unit?”
After a few emails they reluctantly admitted this would turn the charger on/off and should not have any negative effect on the unit, but to understand this is not what it was designed to do and had not been tested.
I opened the unit and soldered wires to the switch connections on two small boards (I make very sloppy solder connections) paralleling the on/off switch for the unit and paralleling #4 on the dip switch, running the wires to an external terminal strip. The charge profile on the charger would not matter now as the charge would be controlled by the SBMS0.
If you had chosen to purchase a Victron Multiplus Inverter/charger, these external control connections are included as part of the design.
SBMS0 Connections
Here is where it gets technical. I will try to explain this as non-technical as I can.
The SBMS0 has connections to control external devices when certain parameters are met like when you are charging, using either solar or shore power. Any single cell gets to 3.56 Volts for 5 seconds (100% state of charge). The SBMS0 uses the external connection EXTIO4 to switch all charging sources off and will switch them back on when the SOC (state of charge) gets below 96%.
If you drain the batteries all the way, when any single cell gets to 2.8 Volts for five seconds, external control EXTIO3 will switch all loads off to protect the batteries. The power used through the EXTIO switches for the external devices is very low, no more than 6 Volts at 50mA. As a result, a small 22-24 AWG wire can be used for the control circuits. Dacian recommends using Cat5 or Cat6 cables. The four sets of twisted pairs can be used or broken out for other connections.
The power switch connections for charging, the AIMS charger side, and the DSSR20 were hooked to the EXTIO4 on the SBMS0. The power switch connections for loads, the main power switch for the AIMS, and the 24 to 12-volt converter were connected to EXTIO3. Other required external connections on the SBMS0 are to each cell. These are used to read the voltages of each cell and provide balancing between cells.
There are also connections to current shunts that are connected between the batteries and the positive bus bar and between the solar panels and the positive bus bar. The current shunts are used to measure the amount of current in and out of the batteries and the amount of current coming in from the solar panels.
The Shunts should be matched to your system size using the formula found in step 4 of the SBMS0 manual. I am using a Battery shunt of 200A 75mV and a PV (Photovoltaic Panel) shunt of 100A 75mV. The values of these are entered as parameters in the SBMS0.
I would recommend reading the SBMS0 manual on the Electrodacus website at least twice before attempting the installation.
24 to 12 Volt Converter
Most of the DC devices I need to power are the standard automotive 12 Volts. I needed to drop the 24-volt battery voltage down to 12 Volts. Again, there are many options. After doing some calculations it was determined I would not be using much over 25 amps and purchased the one piece of Victron hardware that I own. The Orion 24/12 – 40-amp converter. The main deciding factor was the included external on/off connections that can be directly connected to the SBMS0 EXTIO connectors and 40 Amps would give me room to increase the load over time.
Connections and Wire Sizes
Most of the electrical systems I have worked on, have the main fuse or breaker as the first component from the positive side of the battery. Dacian insists the first thing off the battery should be the battery shunt and then the PV shunt with its connections to the SBMS0 and then the fuse or breakers. The reason is, that if the connection between the battery and battery shunt is severed under the right conditions the SBMS0 would be damaged. I don’t know enough about electronics to know why, but I followed this rule.
For the main cables between the battery and inverter, I used 1/0 AWG pure copper cable with tinned copper lugs crimped on the ends with a TEMPCo hammer lug crimper tool and then I applied heat shrink tubing.
There are many opinions and much controversy on the forums surrounding how large of a gauge wire should be used and how the lug crimping should be done. I took the wire size that was recommended by the AIMS inverter manual. The hammer crimper tool has worked well for me. I have tested maximum amperage through the cables and have detected zero heat build. A good rule would be to always go larger on wire size. For more information on crimping large cables see the excellent article titled Tools for Cutting and Crimping Ends on Large Electrical Cable.
I used a 150-Amp Blue Sea Systems breaker between the battery current shunt and the positive busbar any other DC loads connected to the positive busbar are independently fused. The idea for any fuse or breaker is to protect the wire and the components connected to it. Always use a breaker or fuse with an amp rating that the wire or device is rated for, or lower.
I used three CHTAIXI 20-Amp Circuit Breakers mounted on a DIN rail one for each parallel group of two solar panels. I don’t believe these breakers will ever get tripped, but they make a good switch to disconnect the panels from the system for maintenance. Also mounted to the DIN rail are three more CHTAIXI 20-Amp circuit breakers to switch the 12-volt DC loads coming off the Orion Converter going to the lights, tank monitors, and freshwater pump.
Now it gets a little more technical. Before I installed the system in the bus, I prototyped it and assembled it in my back bedroom.
It became apparent that I would be unable to connect both the DSSR20 and the AIMS charger together on EXTIO4, or the main power switch for the AIMS and the Orion converter together on EXTIO3.
Any one device alone on the EXTIO connections worked as expected, but two in parallel would not work. The solution was to get a PC817 Optocoupler board, this board is four small solid-state relays together on one small inexpensive board, designed to work with the small voltages that are used to turn these devices on and off. As a bonus, the control side of the relay is electrically separated from the switch side and one EXTIO can trigger more than one of the relays on the PC817 board.
The control side of two relays was connected to EXTIO4 with the DSSR20 and the AIMS charger connected to the switch sides. The control side of two relays was connected to EXTIO3 with the main power switch for the AIMS and the Orion converter connected to the switch sides, a little more complex but worked as designed. As a bonus LEDs on the Optocoupler board light when the relays are active and turn off when the relays are off making it easy to tell when charging is active or the inverter is on.
With all the cells top balanced and the entire system bench tested it was finally installed in the bus, and a section of the luggage bay wall was allocated as the power wall.
I see a lot of builds place the power wall and the batteries in the living area of the bus conversion, probably a better idea, unlike lead-acid batteries, LifePO4 batteries do not give off any gases or vapors and are perfectly safe in the living environment. Some builds make a showpiece out of their power wall, mine is more built for functionality to be close to the battery pack and to fit in the space available.
Although my solar system is essentially complete, there are still pieces to be added and settings to be tweaked.
All Lithium batteries require an ambient temperature above freezing when charging. The SBMS0 has connections for an external thermometer to suspend charging when the temperature gets too low. I live in Florida so this has not been a priority, but I do plan to install the thermostat and a heater to keep the batteries functioning in cold weather.
The SBMS0 also has a function called diversion, when the batteries are completely charged for the day, the panels can be diverted to another task, in my case that will be the water heater.
The SBMS0 has many different onboard data screens, stores a year’s worth of data, and a couple of different methods to retrieve the data. You can also access the usage screens through Wi-Fi to display on your phone. A few people have created elaborate systems to monitor, display, and graph solar usage using this real-time and archived data.
In summary, I would say if you are not comfortable working with electricity then the SBMS0 is probably not for you, but if you are willing to do the research and work you can certainly create a very capable solar system using new and used equipment at a fraction of the cost of the high dollar plug and play alternatives, with a side benefit of knowing all about the system you built, if (when) there is an issue you will be able to diagnose and fix the system yourself.
This is a multifaceted project, you will need some electrical for the wiring, some metal working to build and install the solar panel mounts, and a little carpentry to build and install the battery box and power wall. Do not underestimate the cost, even with used equipment and doing the work yourself this will not be an inexpensive project, we jokingly refer to $1,000 as a Bus unit, we used two bus units on just the batteries!
In my opinion, the results justify the expense, it has opened opportunities we would not have considered before, stopping for the night in a rest area or Walmart is not an issue when you have all the comforts of home. When driving, we can run the refrigerator and air conditioner. We are now looking at boondocking campsites with no utilities, something Darla would not have considered before.
Going to the yard where we store the bus to work on it and being able to run power tools and the air conditioner, all without running the generator is great. Diesel fuel is expensive, and the sun is free.
Your system might not need to be as robust as ours. Maybe you need more capacity, or maybe you need to start out small and add to it as money becomes available. The SBMS0 is quite adaptable.
Thank you for reading. Contact me with any questions and I will answer to the best of my knowledge.
Resources I used in this build:
DIY Solar Forum, Lots of good information for anyone, https://diysolarforum.com/
Dacian’s Website to purchase Electrodacus products or download manuals, https://electrodacus.com/
Google forum For SMBS0, ask Dacian a question, https://groups.google.com/g/electrodacus
Oberon’s Beginners Guide to SBMS0, https://diysolarforum.com/resources/beginners-guide-to-electrodacus.174.
