Off-Grid Mobile Power Supply - a.k.a. "The Battery Cart"

My last post on October 4, 2014 shows a used, but still working, Deka deep cycle marine/RV battery that was given to me by my boss at the battery company where I work part-time. In that post I discussed plans to build a power supply using that battery. Herein I show what I did, and the rationale for doing so.

The photo above shows the 90% completed unit - it just needs the lid and side mounts painted and some closure hardware installed.


First of all - why this took so long:
It has been nearly eight months since I first obtained the battery and formulated plans to build this power system. This project has been in the works nearly that whole time due to time constraints posed by a busy work schedule, family issues, a relative nearly dying, as well as a medical scare of my own - which fortunately was a false alarm. During that time the various parts and pieces sat around for weeks, unassembled.


Rationale:
I wanted a semi-portable off-grid power system to run my amateur (ham) radio equipment and other items such as lighting, small power tools, security systems and to recharge cell phones and tablet PCs or laptops during power outage emergencies, amateur (ham) radio Field Day events or camping trips. To fulfill these needs, it needed to have the following:
1) Be self-contained.
2) Have wheels, since this battery is heavy - weighing in at around 45 lbs. Two of them would of course weigh 90 lbs.
3) Supply 12 volts DC at up to 200 ampere-hours capacity; the unit can hold two of these 100 ampere-hour batteries.
4) Safety - be properly fused with any terminals or connections protected from casual contact with wiring or other objects. Also contain the battery(s) in such a way that any rupture of the case and resultant leakage of acid is contained and does not damage floors, car interiors, or pose a hazard to people or pets.
5) Provide low voltage shut-off protection for the battery in the event it is left unattended with a load connected, thereby preventing damage to the battery through over discharge. This circuit shuts EVERYTHING down when the battery voltage falls to 10.8 volts.
6) Provide standard Anderson "Power-pole" type connectors for interoperability with emergency crews from CERT, ARES and RACES. These connectors, located on the left-hand front of the unit are in the standard configuration for quick match-up with other people's equipment in a field situation.
7) Provide standard 12 volt automotive "cigar" lighter receptacles for convenient use with commonly available 12 volt accessories as well as a pair of 5-way binding posts for use with alligator clips or even stripped wires.
8) Be rechargeable with a standard automotive battery charger, car electrical system, solar panels, wind power, etc.
9) Provide 120 volts AC for running small appliances, small power tools, soldering irons or powering "wall-warts" for laptop computers, etc.

This system achieves all these requirements and provides convenient operation.


The "battery cart" measures 21" w X 24" deep X 28" tall. It is on wheels removed from a moving dolley bought from Harbor Freight tools for $8 during one of their sales. This was far less expensive than buying the same wheels individually at a hardware store. It provides room for 2 Deka DC31DT 12 volt/100 ampere-hour deep cycle batteries to be connected in parallel for a total of 200 ampere-hours of capacity. Automotive "blade" type fuses protect the separate power inverter and 12 volt outlet circuits. The blue plastic boxes seen in the next photo contain screw-on post type battery terminals for rapid connection to an automotive type battery charger with standard charging clamps. Since the terminals are recessed deep within the blue boxes, which are standard electrical wall outlet boxes purchased at the local home improvement store, they CANNOT be accidentally touched or shorted against anything else. The battery in the third photo is enclosed in a standard ABS battery box designed for the purpose; this case has vents for the hydrogen gas to escape from the battery so it doesn't become trapped and pose an explosion hazard. The Anderson power-pole connectors are on the far left-hand side of the cart and are mounted on a Radio Shack black plastic project box. To the right of that are the control box with its 5-way binding posts, a "Harbor Freight Tools special" 400 watt power inverter for supplying 120 volts AC, and two 12 volt cigar lighter sockets purchased from an auto parts store.

The photo below shows the innards with the cover removed. Note the USB charging devices plugged into the cigar lighter sockets. I routinely charge my cell phone and my pocket MP3 player this way. A relative of mine recently was quite impressed at being able to plug her cell phone directly into the battery cart, using a USB cable borrowed from me, after leaving her charger at home :)

The photo below this one shows the battery box - located in the compartment below the wiring, outlets, inverter, etc.

A Word About The Control Box:
The control box looks a bit lame right now. Originally I had intended to have the low voltage cutout board, its relay, and power ON/OFF switch on one metal "dual-gang" electrical outlet box. When I finally got around to building the low voltage cutout board and got an appropriate 30 amp relay (salvaged from a defunct air conditioner electronic board), I realized that everything would NOT fit into one box. These boxes are normally designed such that they can be stacked and screwed together with the screws that normally hold the covers on. Meanwhile the home improvement center in my area had completely changed its product mix and I could no longer buy the original type I had started with. In the interest of expedience, I bought the current issue item and vowed to eventually re-spin the whole design for the control box. So there it sits looking really funky - for now.

The low voltage cut-out board is of my own design and uses an op-amp comparator circuit with a zener diode as a voltage reference. There is some hysteresis built in to prevent chatter, or oscillation, at the 10.8 volt trip point as well as to provide some electrical noise immunity. The board has a 30 amp relay as mentioned earlier; there is another 40 amp relay, salvaged from the same air conditioner the other relay came from, which is used to operate the power inverter. This additional relay is mounted in a gray single-gang box which can be seen behind the inverter in the "cover off" photo.

The low voltage cut-out board is connected with a 5-pin "Molex" plug connector - making it an easy, solderless field replaceable module.

The top blue light is the main power ON indicator. The bottom green lamp and toggle switch are for the power inverter. The inverter is controlled by the toggle switch to prevent it being powered when not needed; it draws close to 0.8 amps even in standby mode. The top left button is the main "power ON" button; the upper right-hand RED button turns the main power off. Since the inverter relay's coil circuit is interlocked through the main power/low voltage cutout relay circuit, the inverter CANNOT run if the main switch is OFF and/or the cutout board has tripped.


What Remains To Be Done:

1) Paint the cover
2) Attach the side rails the cover mounts to; put cap screws or wing nuts to hold the cover to the side rails
3) Add a solar charge controller and an Anderson Power-pole connector on the rear of the unit for solar panel hookup
4) Re-spin the design of the control box - and the cutout board itself. I'm thinking of going to surface mount components on the next version. The current one is a through-hole "prototype" board. A properly designed printed circuit board could replace a considerable amount of the wiring in the box.
5) Eventually upgrade the wheels on the cart - these work but tend to bog down in heavy carpet.
6) Add the second battery.
7) Add volt/ammeters on the front panel - this should be easier with a re-spun control box.
8) Use an Anderson SB50 type plug connector to quickly connect/disconnect the battery for removal or service. Right now I have to unscrew the battery terminal nuts to disconnect the wires.
9) Replace the "blade" fuses with self-resettable thermal breakers. The low voltage cut-out board, inherent in its design, provides the added benefit of locking out the system after a breaker trip until somebody comes along and manually turns the system back on. Therefore the system could NOT just sit there with a short or overload oscillating - turning on and off ad infinitum.

What I've Learned/What I'd Do Differently:

1) This thing is big and heavy - IF I had it to do all over again I'd build it for ONE battery and have any additional batteries as separate plug-in "modules". My two battery system would then be in two pieces, but each would be MUCH easier to carry and store. From the photos you can see it takes up a lot of floor space; when done it will go under a work table. But after months of stepping around this thing during its construction, I like the idea of being able to maybe keep the main unit available in the room and the second battery tucked away somewhere - but connected by a suitable cable to the main unit. A modular system might also lend itself better to carry in a smaller vehicle - the two pieces could be squeezed into whatever nooks and crannies were available, whereas a one-piece cart like I have requires a decent sized cargo space in the vehicle to carry. This CURRENTLY is NOT a problem for me since I drive a pickup truck, BUT if that changes it could be an issue later.
2) Go with a 700 watt inverter - I went with the smaller 400 watt one to discourage using too much power, as even a 200 ampere-hour system will be quickly depleted using a 700 watt inverter. That said, it would be useful to be able to run a larger tool - like my heat gun - for SHORT PERIODS of time.


I hope this write-up is helpful to someone and gives folks some creative ideas for designing their own off-grid electrical systems.