The Ultimate Overland Dual Battery and Solar System

Introduction to Dual Battery Systems

Here’s an in-depth DIY (do it yourself) write up on a “simple” dual battery and solar system for your overland rig on a budget.  It includes layout, component selection, required sizing calculations, and recommended tools.  I’ve researched and compared with alternate methods to end up at this design.  After building a couple of these types of systems, I’ve put together a list of components I feel are the best compromise of cost, quality, and functionality.

A lot of these components come from the marine industry, where they have a long history of off grid power management solutions and high reliability. Perfect for an overland dual battery system. There are many other ways to achieve similar results but as a DIY’er starting with no prior knowledge this approach is the most logical to me.

Why go through the trouble and expense?

Why would you need a dual battery system in your car? It’s all because ice melts. When you take a 3 week camping trip with a cooler full of drinks and food you’re constantly having to buy ice to keep everything cold. Not to mention all that ice melts causing the labels to detach from the condiments, cold water dripping everywhere when you get a drink out, and even worse soggy food. Yuck. Ice takes up a bunch of space and weight too. What if you have warm drinks you want to chill? That ice will melt even faster and you’ll be buying another bag the next day again.

Having a 12 volt fridge on long overlanding trips (and even weekend getaways) makes life so much easier. Dry food held at a constant desired temperature. Rotate in warm drinks to chill no problem. No stopping for ice or spilling melted ice water everywhere. Ultimately it takes up less space than a bulky cooler too. Now you’ve gotta ask “really Fill, you’re going to spend over $1,000 (fridge, battery, wiring etc.) to avoid buying a few $3 bags of ice? That doesn’t sound very frugal!” You’re right. It would take me less than 200 bags of ice to cost justify this setup vs an expensive rotomold cooler. There’s nothing wrong with simple cooler camping either! But I can justify the convenience and value that a dual battery system brings to us.  More time with family in the great outdoors with less stress over food going bad or needing to get back to town. I also simply enjoy researching, building, and sharing these sorts of projects. Not to mention having power available to run and charge other devices. Plus solar to top off and maintain our batteries.

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Battery Charging Methods

To start on dual battery systems, the first decision you’ll commonly see is Automatic Charging Relay (ACR) versus DC-DC charger.  Without going too far into the pros and cons of each system type I’ll hit the highlights of why I prefer ACR. The ACR setup is overall cheaper and easier to understand. It’s easier to modify the system in pieces over time. Simpler and cheaper components can be replaced individually or function independently in the event of a failure.

A DC-DC system has its benefits of optimal charging profiles for different types of house batteries and smaller gauge wiring requirements. But DC-DC is an all inclusive controller making it cost prohibitive up front. The integrated solar charge controllers do not back charge the starting battery and require an additional relay to trigger the solar controller to turn on properly. Even then I’ve seen people frustrated with a lack of solar charging due to flakey on/off solar voltage trigger hysteresis. So much so they got to the point of adding a separate stand alone solar charge controller like we’d do on an ACR system anyway. So ACR it is!

Edit 2023: Since writing the article, the cost of LiFePO4 batteries and DC-DC chargers has come down significantly with increased competition in the market. There are still pro’s and con’s of each design. Ultimately, AGM is still less expensive up front and a great hardwired entry system.

House Battery Sizing Guide

The first calculation you need to make is how to size your battery bank. How much power do you really need? You’ll need to sum up the current draw of the electronics you currently power (and leave some room for what you may power in the future).  Then multiply by how long you intend to power those items before recharging the house battery bank.  For example, you want to charge a couple of cell phones at 2.1amps for 1 hour. Run a 12v fridge all night (12 hrs) with an average draw of 0.7amps. Some LED camp lights for a few hours 0.2amps. Then in the Tundra we either run a 12v electric blanket or the Maxxair RV vent fan on high 3.5amps for 8 hours (worst case).  This gets us to 34.6Ah (amp hours) required at a maximum.

Obviously this leaves plenty of margin below 35Ah if we didn’t run the heated blanket at night or ran the fan on low. Any margin just means we’re keeping our battery at a higher level, lower “depth of discharge”, which is good for it’s long term health. More on AGM type batteries in the next couple sections. Battery bank size of a typical overland camping rig is usually around 100Ah, or a common group 31 battery. Some slip in campers will run two of these for a total of 200Ah. Larger 5th wheel campers up I’ve seen up to 6 batteries wired in parallel!

Lithium Ion vs. AGM

To find what size battery we need to hit that ~35Ah requirement comes our next decision split; battery type.  Lithium Ion or LiFePO4 can handle a much deeper depth of discharge, or percentage that you can use before it’s considered “dead”, than a standard lead acid battery. Some claiming up to 100% depth of discharge, compared to AGM at only 50%.  Meaning you can get away with a 50% smaller battery capacity.  They’re much lighter too about 1/3 of the weight of lead acid! A must for campers running large Ah battery banks.

The downside is the technology is somewhat new and still very expensive (although coming down in price!) They also require a very specific charging profile and you’ll hear of many failures caused by poor BMS (battery management systems). Sizing is only available for large campers and home solar battery banks as well. So even if I wanted one in the truck, it wouldn’t fit under the hood. You also can’t run a lithium ion battery as a starting battery so that makes it not possible on the simple ACR system (as is).

Again, this could be a long discussion of pros and cons but I’ve weighed them out and in my opinion the best frugal compromise is to run deep cycle Absorbed Glass Mat (AGM) starting and house batteries on the ACR system.  Deep cycle AGM can only handle a 50% depth of discharge. This means if we need 35Ah, we need to double that value to at least 70Ah for our battery capacity. They’re also heavy and require some maintenance to have a long life.  On the bright side, store brand AGM batteries are relatively inexpensive and readily available at any Walmart or auto parts store across the US. Great for getting a battery warranty honored when you’re thousands of miles from home. Even better when it’s easy to find a replacement in stock.

Battery Location Considerations

There may also be some size constraints for your house battery depending on what vehicle you’re mounting it in and where you’re mounting it.  I prefer to keep the house battery under the hood on the opposite side as the starting battery.  This ensures the large gauge cable run is short, weight is offset from driver to passenger side, and both batteries see similar ambient conditions.  I’m also not fond of the idea of having a battery in a passenger cabin area.  I know sealed AGMs technically don’t off gas (that much) but if anything were to happen I’d prefer to keep that extra risk contained elsewhere.

The 4Runner got 2x group 34R batteries at 68Ah each. As it turns out in the Tundra, the largest battery I could fit in the second battery location I chose was a group 24F at 77Ah. Perfect for our 35Ah overnight needs! The next thing to figure out is how each night of discharge will be supplemented with a recharge.  In our overland style of travel we typically only stay in a camp spot for one night then spend the majority of the next day driving along trails to get to the next camp spot.  This tops off our battery while driving for hours so we don’t really need solar on those days.

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The Benefits of Adding Solar

Occasionally we’ll be in a great location and have time to kill so we’ll just stay another day.  In that case solar is paramount. Sure we could run the engine but that burns fuel that we most likely need to get back to the next closest gas station when we’re really far out in the backcountry.  Solar is not 100% efficient so we accept some losses along the way. We use flat fixed roof mounted panels so we can’t aim them directly into the sun which is not ideal for output. We compensate by simply adding more solar arrays.  The Tundra is equipped with 2x 100 watt monocrystalline solar panels. Even mid day once they warm up we’re lucky to get over 100 watts total output to the battery.

Solar Array Sizing

Going back to circuits 101, P=IV.  Power = Current * Voltage so, Power/Voltage = Current. 100watts/14 charging volts = ~7 amps feeding into our house battery.  Needing 35 amp hours means we need 5 hours of good sunlight to top the battery off. Of course there are clouds, shadows, and bird poop on the solar panels.  Plus the fridge is still running all day, so really 8 hours of sunlight should get us back to topped off.  What if we left the truck parked outside and left the fridge running to go hike or take a side trip? The fridge would run for 50 hours with no solar! Add in the solar charging input and we could leave the truck indefinitely. Discharging the battery overnight and topping it back off every day. Coming back to cold food and drinks and two healthy batteries, perfect.

Solar Charge Controller Selection and Sizing

Next up is selecting our solar charge controller.  The option split here is between Maximum Power Point Tracking (MPPT) and Pulse Width Modulation (PWM) controllers. The MPPT is more efficient and can usually be set to our ideal charging profile for battery type. PWM technology isn’t that much cheaper these days so we don’t need to accept the losses from it. To size our solar controller simply look at max power in watts and battery voltage (as low as 12 volts to be conservative). In the case of the tundra 200watts in solar panels is all we could fit on the roof.  200watts/12 volts = 16.67 amps, so a 20 amp MPPT solar charge controller will be perfect. However this leaves no room for future expansion. Consider going one step larger in solar charge controller amperage rating if you may add more panels later.   

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AGM Battery Charging Requirements

Now that we’ve got our batteries and solar capacity needs figured out, lets take a moment to discuss the AGM maintenance I mentioned earlier.  Deep cycle AGM batteries require a higher charging voltage than your normal automotive lead acid battery.  This is quite simple to get around on the starting battery side of modern Toyota vehicles (and possibly others).  All we need to do is trick the engine control module into thinking it’s getting a lower voltage from the smart alternator than it really is. This forces the alternator to output the higher voltage needed for the AGM batteries. We can achieve this by adding a diode to the “alt-s” fuse.  If this is all gibberish to you don’t worry. All you need to know is that if you’re running an AGM starting battery you should change the alt-s fuse out with one of these options:

• GM part number 12135037 diode fuse <— Thanks Chi-town 4×4 for this tip! (used on the 4Runner)
• HKB Alternator Voltage Booster Fuse
• Ebay “mini-fuse alternator voltage booster” diode soldered to an add a fuse (used on the Tundra with same results as the GM fuse)
• DIY the above ebay components

Deep cycle AGM batteries are also happiest when kept topped off. For that we have a few options too.

• Drive the vehicle more than 20 minutes daily (not backed by science, just good AGM practice)
• Manually hook up to a high-quality battery charger/maintainer at a minimum of once per month
• Plug into a preinstalled battery maintainer anytime the vehicle is parked at home (we us a Noco 2D on the Tundra and have been extremely pleased with it)

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Wire Gauge and Sizing

With that PSA out of the way, the next thing we need to consider is wire sizing. We want to size the wires large enough that they can transfer the current we’re sending through them without too much loss.  But oversizing wires is a waste of weight and cost on expensive copper conductors.  Losses from too small of wire diameter will cause too high of resistance, leading to heat. Heat in wiring causes more resistance, making it a bad cycle of performance loss and possibly even melting. A melted wire will cause a short, rendering the system useless.  I always go just a bit oversized on wiring to be sure that I don’t ever have to worry about wiring capacity being the limiting factor. I commonly reference this chart by Blue Sea Systems to look up appropriate wire gauge (diameter) for 12 volt systems in non-critical applications depending on the length of the run.  This is required for EVERY wiring run in your vehicle.  This is why vehicle manufacturers run such small wires to your radio but big thick wires to your battery and alternator.

Blue Sea – DC_wire_selection_chart

For linking two batteries together, they will have short bursts of very high current but will even out quickly. Were concerned with the highest average continuous current we think we’ll see. Consider your alternator max charging amperage for calculating cable size ampacity between your two batteries. You can look up factory alternator max amperage rating easily by looking at replacement alternators for your vehicle on rockauto.com.

In the 4Runner and Tundra I ran 1/0 aka “one aught” or zero gauge or 0 awg cable.  Do not get this confused with 1 gauge, or similarly 2/0 with 2 gauge they are not the same.  By this chart 0 awg can be ran up to 20 feet in critical applications or 65 feet in non-critical applications for 150 amps of constant current. In most cases you could get away with running 1 or 2 awg as I’ve seen in some pre-packaged dual battery kits. I go slightly heavy here just in case we want to run the winch off both batteries or something like that.

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Fusing and Power Distribution

The next consideration is fusing.  You should install a fuse at the power source end of every “hot line”.  Because both of your batteries positive terminals are “hot” you’ll need to have an inline fuse at BOTH ends of the battery linking positive line. As well as a terminal fuse on your direct lines out to accessories or other power distribution blocks. The fuse’s purpose is so that if something happens to a power wire; it gets cut in an accident, chafes through, melts etc. the fuse will blow creating an open circuit and that power wire will become dead. This is for your safety, shock and fire prevention. As well as the safety of first responders in the case of an accident. Your fuses on hot lines should be sized around the max wiring current capability we just pulled from the Blue Sea Systems wire sizing chart above.

I like to run a smaller gauge wire (4 awg) to a power distribution panel somewhere central in the vehicle.  This enables me to wire a bunch of accessories easily to one power source while protecting each one with difference fuse requirements.  It keeps the installation clean and safe. I find this to be a good location for a circuit breaker as well. These are resettable in case of an accidental trip from the panel, and also a quick single shut off for all accessories if need be. Great for making sure there’s no parasitic draws if leaving the vehicle parked for a while.

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Making Your own Power Leads

Lastly, we need to put all of these items together in a clean and secure manner. Wires of this size are bolted to the components with barrel lug style ring terminals.  The lugs come in different sizes based on wire gauge and terminal diameter. We can look up these dimensions ahead of time to know how many of each size lug we need on hand. These lugs are so strong that they require a hydraulic crimping tool to install. Finally we cover these crimp connections with a high quality marine grade adhesive lined heat shrink.

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Component and Tool Lists

The only other things left are add on accessories for battery monitoring, switches, lighting, charging, etc. These are highly subjective to your needs and vehicle layout so I won’t go into too much detail on what I’ve used.

The remainder of this post is a list of components that I’ve selected, tested, and prefer with a short review of why I selected them. 

*This post contains affiliate links, meaning that I receive a small commission at no extra cost to you when you make a purchase through these links. Thank you!*

*disclaimer – modify a vehicle at your own risk. I’m not responsible for any damage or loss of warranty incurred from your component selection or installation. Know your DIY limits and hire it out if need be.*

Dual Battery System Components

Batteries – Subject to your size and application requirements. You’ll have to select these on your own using the calculations above.

Blue Sea Systems 7610 Automatic Charging Relay (ACR) – This is the heart of the system functionality wise. Most overlanders will run the ML-ACR, it’s got an ugly switch that’s hard to fit into a clean factory looking switch location. The only functionality you’re losing by going with the much cheaper 7610 is monitoring (which we can add anyway) and manual linking switch which can be replicated by carrying an extra 6” cable to jump across the 7610, or standard jumper cables. Alternatively, carry a lithium ion jumper pack like the Antigravity XP-10 (don’t buy from Amazon due to risk of counterfeits). The ML-ACR also has a manual cut off switch. We added a winch cutoff switch on the Tundra anyway so we got this capability back.

Battery Terminals – Military style. They bolt onto standard battery posts tight and allow easy addition of 5/16” terminal lugs onto the bolt on the other end. They come with nice rubber boots to complete the install clean and safe.

ANL Fuse Holders – These come with the base, fuse, small fuse cover, and rubber boots.  They are 5/16” terminal size and come in various amperage ratings to suit your needs based on wire sizing table above.

Terminal Fuse Holder and Terminal Fuse – Easily add an inline fuse to the main power take off line(s). Many amperage ratings available. A bit pricey but easier to mount than adding another circuit breaker at the battery.

Wiring – I prefer this brands “extra flexible” welding cable wiring for easy routing through vehicle interiors and engine bays. It does require more zip ties to keep it tight on long straight runs but that’s okay for keeping it secure.

Terminal Lugs – I’ve tested tinned vs. bare copper and didn’t see a difference in corrosion for automotive use so just bare copper is fine.  I like this brand because of all the different options for wire gauge, terminal diameter, and package quantity.  Slight hack, if you order lug terminals that are too small for your terminal bolt size you can drill them out slightly to fit. Not preferred but can save you from having to order 10x of a size you only need 2 of when you have some leftover from your other 10x of the next size down.

Hydraulic Terminal Crimper Tool – To crimp these lugs on you’ll need a hydraulic crimping tool. I think all of them on Amazon in the yellow case are made by the same overseas supplier and are of similar quality.  Since I’m using it as a hobbyist it works for me to then just pick the cheapest one.  I found this chart for die size reference.

Heat Shrink – The key to heat shrink is the 3:1 ratio and internal adhesive.  This creates a tight seal leaving no areas open to corrosion or rolled edges. This marine grade stuff is awesome! Works great on other exterior wiring projects too.

Heat Gun – Harbor freight with hi/low settings gets the job done. Been using one like this one for years. Frugal hack, everything at harbor freight is always 20% off!

Bussman Circuit Breaker – Located just before the power distribution panel. There are other brands but this one is reputable and affordable. Comes in many amperage ratings depending on total of your power distribution panel accessory load.

Power Distribution Fuse Block – Choose more terminals than things you plan to wire to it, you’ll always find more things to wire in.  I like the panels with ground terminals available, most things you wire up will need a ground wired anyway. They also come with nice covers and labels.

Solar Components List

Solar Panels – Highly subjective based on the real estate you have to fit them and preferred quality.  We used Renogy Eclipse series on the 4Runner but HQST on the Tundra. HQST are basically Renogy B grade parts and that’s good enough for me!

Solar Charge Controller – MPPT 20 amp from HQST again.  Good for up to 260 watts of solar panels.

Solar Charge Controller Display – This was a free add on to the charge controller at one point so might as well. (check the “promotion” section above the item description, amazon hack)  It shows a plethora of info about the solar state of charge. It’s not backlit and is difficult to read in direct sunlight so be careful on where you mount it. UPDATE 2023: This is no longer needed, the updated charge controller has built in display.

Solar Panel Wiring Splitters – to link multiple panels together, since these are kind of a commodity cheaper brands are fine.

Solar Panel Cabling – for plug and play solar panel install. Size these appropriately for your current calculations and run length.

Extra system add on components

Battery Maintainer – this one is hard mounted. Just plug the truck in when it’s parked at home and let the maintainer do the rest.  It’s low current do it does take a few days to top off two “dead” batteries (yes it will open the ACR and back charge the starting battery). But as a maintainer it works great. It’s nice to have everything at 100% every time we leave on a trip.

Battery Voltage Monitoring – Mounted in the center console on a 3 position switch to flip between starting, house, and off.  Confirm either battery status quickly from the drivers seat and diagnose if there’s any issues with charging of either battery. Also convenient addition of quick charge USB ports.

Buck Boost Converter – Used to regulate voltage to the sensitive maxxair vent fan. Also another source of current measurement for this line.

12 Volt Fridge – I’ve tried name brand and the cheapest of the cheap. Here’s one that’s in the middle, decent price, functions well, quality compressor, decent warranty. I’ve not tested it enough yet to comment on durability so test at your own risk.

Power Inverters – You may notice I didn’t mention running any 110v outlets in this system.  While they’re often included in photographers and vloggers setups to charge camera and drone batteries, our setups simply don’t require them. We can power almost everything off of USB and 12v outlets. This is more efficient than converting (inverting actually) to AC power. For our limited 110v needs we have a small single outlet inverter from Target that plugs into a 12v port. It won’t run a toaster or a blender and that’s okay with us!

Solar Charging Relay – If you want to only charge with either solar OR alternator but not both, this is installed with key switch power to turn off your solar controller output.

RV Vent Fan – Another reason we needed dual batteries was to vent our truck cap with this bad boy. 4 Speed exhaust only, manual lid and fan speed controls.

Sealant and Butyl Tape – For proper waterproof fan install

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System Diagrams

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