Build Your Own Battery Box
You've decided on the minimum Amp hours and battery type for your battery box. Now you want to build it. Where do you start?
First, the Batteries
You first need to decide if you plan to buy one large battery or two or more smaller batteries connected in parallel to give you the Amp hours you need. If you are planning to connect several batteries in parallel, they must be the same voltage and charge level. If you use two different voltage batteries, the higher voltage battery will drain into the lower voltage battery while attempting to equalize the voltages. This current flow may burn your circuit and destroy the lower voltage battery.
If two equal voltage batteries have different charge levels, the same thing might happen. The lower charged battery may not be at full voltage so there will be a voltage difference between the batteries again. If that difference is large enough, you may destroy your circuit and battery as before.
Now you need to find batteries that fit your specifications and get their dimensions. Knowing the footprint of potential batteries is a must before you move to the next phase. The batteries will be the most expensive part of your box so everything will be designed around them.
Some popular deep cycle battery suppliers are: Amped Outdoors, Bioenno, Mighty Max, Dakota Lithium, and Power Sonic.
The Case
Start searching for a case that will fit the batteries you have chosen. If the retailer doesn't have the interior dimensions of the case listed, try another retailer or the manufacturer's Website. Copy any pictures of the case you can find to help later with your design. You are looking for an easy to carry, plastic case. You can use metal but it will be heavier and harder to drill.
Having the dimensions of two or three potential batteries that match your needs can really help here as one battery might not fit a case while another will. Try orientating the batteries length–wise and width–wise to get a fit. If you can get a snug fit, all the better. You can then use industrial Velcro to secure the batteries to the case sides. Using a paper or cardboard template of the case bottom and the batteries can help you visualize how they will fit.
Getting a good balance in the case is also important. Having all the battery weight at one end can make carrying the case very awkward especially with heavy lead acid batteries. This is also a good place to check the weight capacity of the case. Will it hold that heavy lead acid battery safely?
Where do you find cases? Whether you are searching via a general search engine or a search box on Amazon.com, Walmart, Harbor Freight, etc., getting the right key words will maximize your results. Besides the obvious Battery Box, Battery Case, Power Box/Case, Portable Battery Supply, etc., other terms might surprise you. Include Ammo Can/Case/Box, Marine Dry Box/Case, Portable Telescope Power, Field Box/Case, and other similar terms. MTM Case-Gard and Plano make popular cases for battery boxes.
Choosing Components
Once you have a good idea of what case you want and how the batteries will fit, it's time to lay out the components for the case. Make a list of the types of ports you wish to have and how many of each. This step is likely where you will make the most compromises. Use the case dimensions and pictures to create a template of the case lid and sides to help during this step. To get ideas about how to lay things out, look at several pre-built boxes. You can find several manufacturers' Websites on the Purchase a Pre-Built Battery Box page.
If you plan to put all the ports in the case lid but they will not fit, perhaps you can move a few to the sides (avoiding the batteries) or do without a few ports, or even go back and choose a larger case. Everything in life is a trade off.
When planning to put components in the lid, be sure to check the underside. Often, it will have support ribbing hidden there. You need to avoid those areas or else plan to cut sections of it out to give clearance for components.
Here are some common battery box components:
- 12V 10A Cigarette Lighter Port
- USB-A 5V, 2.4A Port
- USB-C PD 5V, 9V, 15V, 20V, 3A / Quick Charge 3.0 Port
- 12V, 7A (6.5mm/1.4mm) port
- LED Lights
- Volt Meter
- Power Meter (Volts, Amps, Watts)
- Digital Thermometer
- SAE Port
- Anderson Port
- Binding Posts (Banana Jacks)
- 120V AC, 60 Hz Pure Sine Wave Inverter
- Solar Controller
In addition to the items above, you will need room for switches (at least a master switch). Do you want a fuse box protecting each component or just a single large fuse near the battery? If you are adding an AC inverter, will you also need a fan to dissipate the heat? Many cases are sealed quite well to maintain water resistance so the heat can build inside.
You will need a port to charge the battery. If you are planning to use solar and have lithium batteries, make sure you choose a solar controller designed for them. The same caveat applies to an AC charger.
Where do you find all the components? Look in the usual places like your local electronics store (if any are left), Harbor Freight, Walmart, Ace Hardware, Lowes, Amazon.com, etc. Some companies such as Ice Hole Power sell DIY kits. You can purchase a bag of components with or without a pre-drilled box and with or without a battery.
Drilling Your Box
You've now purchased your battery, case, and components. Now it's time to drill the box to install the components. What you will need will depend upon the components you've chosen for your box. You can cut the holes with a utility knife but a drill will sure help. If you are installing switches with a circular footprint, a step drill bit will do a nice job. Usually a 3/4 inch bit can be found for $5 or $10. Hole saws can be used for larger circular components such as a 12V Cigarette Lighter port. A 1.25 inch hole saw will often do the trick.
As mentioned above, watch for any support ribbing on the underside of the lid. If a component must go through that, a utility knife or a dremel can remove the ribbing. Beware of removing too much as that handle on the top of the box has to support the weight of your battery.
Wiring Your Battery Box
Now you have your components mounted in their holes, so it's time to wire the box. Decide on a wire gauge to use. Don't just think about the current your radio draws. While transmitting, you might also be charging your cell phone and since it is night, you're powering a set of LED lights too! Think about the total current you might use and use the appropriate wire gauge. Regardless if you fuse the individual components or not, make sure you ALWAYS fuse the positive line near the battery. You don't want some cheap $2 component to fry your $300 lithium battery.
There are many YouTube videos available to show various ways to wire a box. Some are good and others less so. Ice Hole Power has at least five or six videos that can be helpful. Jon from JDS Outdoors has one of the most popular battery box builds. Many boxes have been based upon his video.
I want to mention an issue you may see in some of the videos. It involves how to wire multiple batteries in parallel. In figure 1 on the left, four batteries are connected in parallel as a bank. The load is connected to battery 1. Jumper wires connect battery 1 to battery 2, battery 2 to 3, and 3 to 4. The reason this bank is unbalanced is those jumper wires (like all wires) have some resistance. It's not much but it is there. Thus, there is a voltage drop across each set of jumpers. The load sees the full voltage of battery 1 but for battery 2, it sees the full voltage minus the voltage drop from the first set of jumpers. For battery 3, it sees the full voltage of battery 3 minus TWO voltage drops (two sets of jumpers). For battery 4 it sees three voltage drops. Thus, the load pulls more current from battery 1 and progressively less from each of the other batteries.
Figure 2 shows the correct way to wire a bank. The negative from the load connects directly to battery 1 but the positive passes through three jumpers before reaching battery 1 for a total distance of three jumpers. Battery 2 is one jumper away from the negative side of the load and two jumpers away from the positive side for a total of three jumpers again. Battery 3 is one jumper on the positive and two jumpers on the negative making three. Battery 4 is connected directly to the positive lead of the load but is three away from the negative. Thus, each battery is three jumpers away from the load making things balanced.
Adding Solar
You've been passing emergency traffic for a few days now and your batteries are nearly out of power. The grid is still down so what do you do? You can charge your batteries from a car battery but a car might not be available. Solar is a great option to have. In southern California, we have a lot of sunny days.
If you do choose a solar option, your solar system needs to be connected (or turned on) in a specific order. The solar controller must be connected to the battery before being connected to the solar panels. This allows the controller an opportunity to synchronize with the battery. You would normally wire a switch between the battery and the controller and turn that switch on before attaching the solar panels. After the panels are attached, you can then attach a load to the battery. You disconnect in the reverse order — first the load, then the panels, and finally the controller from the battery. You should never connect the panels directly to a battery or to the load.
Solar controllers have three ports. There is a port to connect the solar panels, a port to connect the battery, and a port to connect a load. Under most circumstances, your load circuitry should not connect to the solar controller load port. It should connect directly to the battery. Bioenno Power, an Orange County lithium battery manufacturer, makes this statement:
For most end users, do not use the load port. We recommend all customers attach their load directly to the battery. Based on extensive tests and field experience, we have determined that the optimal setup is for the current to flow from the solar panel to the solar charge controller to the battery and from the battery directly to load. This ensures that your load is supplied with smooth and continuous voltage and current for your battery's rated capacity and serves to mitigate a solar panel's current output variance.
If you're using lithium batteries, make sure the solar controller is rated for the type of batteries you are using. A lot of the cheap solar controllers are only useful for lead acid batteries. They could damage your expensive lithiums.
Solar regulators come in two different types: PWM and MPPT.
PWM Controllers
Pulse-Width Modulation (PWM) controllers are simpler and cheaper than MPPT controllers. As the battery approaches capacity, the controller slowly reduces the amount of power going into the battery. When the battery is full, a PWM controller maintains a constant trickle of power to keep the battery topped off. When using a PWM controller, your solar panels and your battery must have matching voltages.
MPPT Controllers
Maximum Power Point Tracking (MPPT) controllers are more expensive and complex than PWM controllers. Like PWMs, MPPTs also reduce power going into a battery as it nears capacity. Unlike PWMs, MPPT controllers can pair non-matching voltages from panels and batteries. They can adjust panel input to maximize the power from the panel and also vary their output to match the battery. They are more efficient than PWM controllers. Usually, you will see this increase in efficiency only if you are using a large panel array like a fixed installation. For portable operations you usually do not need MPPT.