Big Power in a Small Camper – Going Lithium Cheap

If you were unable to attend the 2021 Truck Camper Adventure Rally you missed out on some great classes and discussions. Everything from boondocking, tire repair, winching, batteries, and solar power were taught. Steve Hericks, who holds a mechanical engineering degree from the Massachusetts Institute of Technology (MIT), taught two classes: one on battery charging, the other on his amazing do-it-yourself (DIY) truck camper. Not only were we impressed with the build of Steve’s camper, that he affectionately calls Maximus, but also his camper’s 24 volt electrical system, which features an 1,100 amp hour lithium battery, 950 watts of solar, a DC-DC alternator charging system, and a 4,000 watt inverter. For those who missed the rally, we thought it would be great to interview Steve to get this thoughts about the massive lithium battery he built for his camper and how his camper’s electrical system works. Like the title says, Big Power in a Small Camper – Going Lithium Cheap.

Thanks, Steve, for taking the time to talk with us. Can you first tell us about your background, where you’ve worked, and what you’re doing now?

Steve Hericks: I grew up in a small town in south central Montana with a love to “make things.” Army ROTC paid for an MIT mechanical engineering degree after which, I served as an active duty combat engineer officer for nine years. After leaving the Army, I worked in a number of small manufacturing companies making Class A RV’s, portable and fixed power equipment, aerospace connectors, and large air conditioners. I retired in 2017 and have been working more than full-time on my own projects, including building the small apartment in which I live, five kitchen renovations, and the chassis-mounted camper currently nearing completion. My kids call me, the “fountain of useless knowledge,” so I tend to get into details when explaining things.

We enjoyed the two classes that you taught at the TCA Rally. We also enjoyed our tour of Maximus, your DIY camper. Can you tell us about the electrical system that you built for it?

Steve Hericks: I designed a very large battery and power system for a 6×6 military expedition vehicle, but built it into a 2-inch spacer under a 2000 Lance 1130 to test. The details on how I did that are in our blog at workingonexploring.com. I postponed the 6×6 build and repackaged into a specially-made battery box of my new chassis-mounted camper. It is designed to run a mini-split air conditioner for two days and the camper’s microwave, induction cooktop and all other needs for a week with no additional charging. It’s a 24 volt, 17.5 kilowatt hour (1,166 amp hour/12 volt equivalent) lithium manganese oxide (LiMn2O4) battery constructed from 35 Nissan Leaf electric vehicle (EV) battery packs.

This “mega” battery supplies power to multiple equipment: a 4,000 watt Samlex inverter/charger for A/C and cooking, a 800 watt Reliable Electric inverter running 24/7 to supply the AC refrigerator and small needs, 40 amp/13.8 volt and 12 volt DC-DC supplies “automotive 12 volt” (designed to operate at  around 14 volts) and “true” 12 volt equipment. It’s charged primarily by five 190 watt/24 volt (950 watt) solar panels and backed up by a 120 amp/24 volt second engine alternator or up to 105 amps from the charger in the 4,000 watt Samlex when connected to shore power.

Where did mount such a large battery?

Steve Hericks: I first tested it in the driver side electrical bay, which I found unacceptable because it made other equipment inaccessible and created a weight distribution problem. It has taken a considerable amount of thought, and fabrication to resolve those problems. The battery and case weigh 340 pounds. Because of the three-point mount, placing that much concentrated weight in any one location above or below the camper floor caused balance problems. It needed to be attached somewhere “inside the truck frame” of my Ford F-350 to prevent it from creating a large twisting force. As it happens, there is just such a place, opposite the 38-gallon diesel tank, on the passenger side of the rear driveline, between the rear axle and the cab. That space was in use by the exhaust system. The 4-inch diameter exhaust pipe was shortened and the catalytic converter and muffler were moved under the cab with the tail pipe coming out at the rear corner of the cab. I also needed to move the tail pipe as it hung low out the rear interfering with the 30-degree departure angle clearance. This solution, surprisingly, allowed the shortest possible battery connections, filled the available space completely and placed it in the best position to minimize weight and balance issues.

Why did you decide to go with a 24 volt system when 12 volt systems are the norm in recreational vehicles?

Steve Hericks: Because it’s not possible to make an efficient 12 volt equivalent battery with lithium oxide chemistries. Oxide and phosphate lithium chemistries have different operating voltages. All LiMn2O4 chemistries range from 3.0 to 4.2 volts per cell, while lithium iron phosphate (LiFePO4) ranges from 2.5 to 3.65 volts per cell. Oxide batteries, when combined either with three or four in series to make a “12 volt” compatible battery, do not fit well in the operating voltage range of 12 volt system designed equipment (typically 10 to 15 volts). Three cells in series operate 9 to 12.6 volts (under range) and four cells in series operate from 12.0 to 16.8 volts (over range) but they do make perfect 24 or 48 volt systems (seven or 14 cells in series respectively). My seven cells in series, 20 cells in parallel (7S20P) battery operates from 21 to 29.4 volts, which is perfect for 24 volt equipment (typically rated from 21 to 30 volts). An added benefit of higher voltage systems is that electrical cable can be smaller and therefore less expensive because it carries less amperage. Higher wattage conversion equipment is somewhat less expensive, but the high volume of 12 volt equipment is still pretty competitive.

Since your battery puts out 24 volts, do all of your loads—lights, fans, water pump, USB ports, etc—run on 24 volts or 12 volts using a DC-DC step down buck converter?

Steve Hericks: I use six different voltages in my camper:

• 5 volts DC for USB charging is produced locally by integral ports in four 120 volt AC “always on” power strips.
• Regulated 12 volts DC is distributed throughout the camper, produced by a 24 volt/40 amp DC-DC buck converter to operate non-automotive LED lights, computer fans, and non-automotive resistance heaters (freeze protection).
• Regulated 13.8 volts DC is distributed throughout the camper, produced by a 24 volt/40 amp DC-DC buck converter to operate “automotive” 12 volt equipment such as four Maxxair ceiling fans, the water pump, and automotive seat heaters (battery warmer).
• Regulated 19.2 volts DC is produced locally by a 12 volt/3 amp DC-DC boost converter to replace the AC “power brick” used to operate the LCD TV.
• Unregulated 24 volts DC (21 to 29.2 volts DC) is distributed to specific equipment to operate the automotive diesel air heater and two inverters.
• 120 volts AC (108 to 126 volts AC) is distributed throughout the camper in two systems, one produced by an 800 watt pure sine inverter, always-on for the refrigerator, USB supplies and four outlet strips for small loads. A second 4,000 watt pure sine inverter is only on intermittently and supplies the air conditioner, the induction cooktop and the microwave plus several power strips for high power portable equipment (because every girl has a blow dryer and blender).

This seems like a different kind of crazy so let me explain. Running everything on unregulated 24 volts DC would be the best for efficiency’s sake but the LiMn2O4 battery voltage ranges from 29.4 volts to 21 volts, so this large range definitely affects operation of equipment. The lithium battery’s 24 volt range is significantly larger (higher and lower) than what is considered “24 volts” by the automotive equipment manufacturers (typically 23 to 28.8 volts). Although “automotive 24 volt” equipment exists it is nowhere as plentiful as “automotive 12 volt” equipment. There is also a large amount of regulated 12 volt (electronic) equipment that is also desirable to use. Desired 24 volt equipment was either not economical, not available nor would operate within my battery’s voltage range. Once I needed equipment from different voltage categories, separate power systems emerged.

It is important to note that there are two categories of equipment voltages in the 12 and 24 volt DC world; that which is designed to operate on the “automotive” range and that which is designed to operate on regulated voltage. Automotive equipment is designed to operate directly from a system powered by battery and alternator. A lead-acid battery ranges from about 11.5 to 12.6 volts DC and when the alternator is operating, system voltage goes all the way up to 14.4 volts DC. Newer (smart alternator) vehicles can even produce system voltages as high as 15.5 volts DC. Automotive equipment operates safely over this large range and is designed optimally for 13.8 volts DC. Different supplies for powering equipment designed to operate on “automotive 12 volt” versus regulated 12 volt is important. Operating non-automotive 12 volt equipment on an automotive system is likely to shorten equipment life if not destroy it immediately. Conversely, operating automotive equipment at regulated 12 volt usually results in poor operation (hence the availability of 13.8 volt power supplies).

Which make of DC-DC buck converter did you go with?

Steve Hericks: It’s Chinese made. It’s sold by a large number of distributors under their name. The 12 volt was bought recently on amazon. It’s sold by a company called Valefod. My guess is they are made by one company who is nameless and sold by dozens of distributors. They are made of about five large electronic components and potted in the cast aluminum heatsink, making them very waterproof. They are available in 5, 10, 20 and 40 amps in voltages of 5, 12, 13.8, and 24 volts. Prices range from around $6 to $35.

Board level converters are more plentiful and available in smaller sizes, often with adjustable voltage outputs and often selling between $2 and $10. Some have current regulation as well. There are three types, “buck” or voltage reducing, “boost” or voltage increasing types need a supply voltage that is at least 1.2 volts different than the delivery voltage. A third type called sepic” can be used as a voltage regulator/stabilizer, which can accept any variable input voltage that can be above or below the desired output voltage. Conversion efficiencies are around 90 percent and are better the closer input and output voltages are. These devices are the basic building blocks of solar charge controllers and battery to battery chargers. A key to using them is not to parallel connect the output of two or more devices. If you need more amps than one device gives, your only option is buy a bigger single unit. They need to sense their output voltage and will be “confused” if in parallel to another source attempting to do the same thing.

Where did you buy your “damaged” EV cells? That isn’t something you can normally find in your local classifieds.

Steve Hericks: I was in the process of building a different 24 volt battery from 840 18650 NCO cells when an entire Nissan Leaf battery appeared for sale on Craigslist. It took some time for the seller to remove it from the wrecked car and when they did, they discovered the sheet metal case was penetrated in two places. They dropped the price considerably when they made this discovery. Because I was going to disassemble it and use less than the 48 packs it contained, it did not matter to me that some packs might be damaged. I bought it for $300. Packs sell on eBay or EV repair shops for around $100 each. It turned out that the penetrations of the case only touched one pack and all 48 were still usable so it was a tremendous bargain.

What are the inherent advantages going with that type of lithium cell over the standard LiFePO4 cell?

Steve Hericks: Price and capacity. The electric vehicle industry provides a mass market of lower cost, high capacity cells. All EV batteries are oxide chemistries which have from 45 to 120 percent more capacity per pound than phosphate. A large quantity of EV cells can be relatively easily found and will become easier as more EV’s are decommissioned. My cells (purchased in 2017) came from a 2015 wrecked leaf so all cells were at nearly new capacity. They are typically about $100 per pack or $4,000 for the whole battery. Each pack is a flat “sardine can” with each can measuring 8 inches wide by 12-inches long by 1-3/8-inches high with each containing four pouch cells rated at 7.5 volts/.5 kilowatt hours/62 amp hours (or 40 amp hours at 12 volt equivalent). Each can weighs about 8 pounds.

Do you have any advice for our readers on how to go lithium in their campers?

Steve Hericks: I see three ways of “going lithium” in recreational vehicles:

• Lithium ion replacement batteries—like those made by Expion360 and Battle Born—have similar battery terminals and sized cases to lead-acid are drop-in equivalents. They are easily accommodated in existing battery space, but do require changes to charging equipment. They are expensive, but simpler.
• Assemble your own 12 volt LiFePO4 battery from four prismatic (rectangular) cells and a battery management system (BMS). Construction is pretty simple. Cost will be around about 40 percent less expensive than replacement batteries. They still require charging equipment changes. Cells are different size and shape depending on capacity. The biggest challenge will to design it in a size and arrangement that will fit in existing battery spaces. While it seems intimidating, I think most recreational vehicle owners are at least marginally handy, which is all it really takes to succeed with this option.
• Recycled EV batteries are all oxide chemistries and require a change to 24 or 48 volt systems to use the full capacity. Making an upgrade when all equipment has to be changed is a big expense and is only justified if a no current system exists or a very large system is the goal.

Tell us about your camper’s solar power system. You just completed it, right?

Steve Hericks: Yes. I have five 190 watt/24 volt panels on the roof for 950 watts total of which four will tilt electrically to either side (a later improvement). I expect to be able to get as much as 3,000 kilowatt hours a day from the four that tilt and about 500 watt hours per day from the fixed panel.

Can you tell us about the DC to DC charger that you built for your camper?

Steve Hericks: I pursue double or triple redundancy in most of my systems, but solar, even a lot, is not completely reliable and I almost never plug into shore power. I want a significant ability to directly recharge from a truck engine alternator because I don’t intend to carry a generator set. This will be a separate topic all its own, because it is a complex topic in design, but fairly simple in implementation. In the meantime, I have a detailed explanation of what I did on our blog.

We will do a follow up on your DC to DC charging system in another article, for sure. How do you keep your lithium batteries warm in winter?

Steve Hericks: The battery box is well-insulated using 3/4-inch of Poly-Iso, R4 with four 18 watt “seat” heater blankets, each measuring 12 inches by 18 inches, two on each side. A dual-channel thermal controller in the control center monitors temperatures in front and rear of the box and turns on all four heaters if the temperature at either end gets below 4C/38F. The BMS has a temperature sensor in the center of the pack that will disallow charging at 0C/32F or discharging at -16C/4F.

We noticed you went with a residential AC refrigerator rather than a DC one for your camper. Why?

Steve Hericks: We get the best features with the residential AC model being competitive in a large market, equal overall efficiency using a high-efficiency AC compressor coupled with a small, efficient inverter even though we need to run an inverter full-time. You are probably thinking a DC refrigerator is super-efficient when in reality they are not any better than average AC units. DC is only comparatively super-efficient when compared to an RV absorption refrigerator. Thermodynamics is a pain. By fundamental principles, compressor refrigerators, whether AC or DC are eight to ten times more efficient than absorption refrigerators. RV DC refrigerators are a small market and do not have the market incentives (or governmental mandates) driving efficiency. AC refrigerators have both. Most of the “efficiency” in the DC space is achieved by super-insulating the compartments (“solar” refrigerators), not because of compressor improvements. AC compressors have much greater potential for efficiency improvements, have much more incentive and research than DC. In particular, European efficiency requirements are higher than US and have begun reaching the market. Our refrigerator is German. We put out a video on the refrigerator and why we chose it.

Thanks, Steve, for talking with us. We’ll talk again soon.

Steve Hericks: It was my pleasure, Mike.