In building our 4×4 adventure vehicle system, I learned a lot about the way people build dual battery systems in cars and trucks.

You build a dual-battery system into a car or truck for mainly one reason: to keep your accessories (fridge, chargers, epic sound system) on one battery, and your vehicle system (starting and operating critical electrics) on the other.

There are three main ways you can connect two (or more) batteries in a dual- or multi-battery system

• Direct parallel connection. This works, but is the worst solution.
• Voltage Sensitive Relay/Isolator (a.k.a. “VSR”). This works better, and isn’t very expensive.
• DC-DC charger. These are expensive but have multiple benefits.

Let’s go through the pros and cons of these in turn.

You might also enjoy my guide to buying cars from government auctions on Pickles and Manheim — this is how I bought our Hilux.

Dual Battery Systems — Direct Connections

• Cheapest
• Engine bay only, lead acid batteries only
• Gets 80% out of your accessory battery; doesn’t charge it too fast
• May leave you stranded
• Won’t work with latest cars with smart alternators

It’s tempting to think “Ah, I’ll just connect the batteries in parallel!”

This works, but only with a precarious set of assumptions:

• The batteries have the same capacity and chemistry. If not, then you may not charge one to its fullest level.
• The voltage that arrives at each battery is the same. A short distance difference of just one metre usually results in a few tenths of a volt difference from the current that we normally see in 12V systems.
• Batteries are at the same condition level. They usually aren’t.

This means that if you connect two batteries in parallel, you’ll never get full potential out of your system.

Secondly, if you connect two in parallel, you lose out on the two main benefits of dual battery systems:

1. You won’t drain your starter battery. If you have another battery to power your current-hungry accessories, you’ll be able to run your battery flat and still be able to drive home.
2. You can (maybe) use your second battery to start the car. If your starter battery fails, you can perhaps use current from your accessories battery to either start the car, or to charge your cranking battery.

So a dual battery system in parallel will work and won’t cause any huge problems, but it doesn’t cost much to go to the next best solution: a VSR.

Dual Battery Systems with a VSR (Voltage Sensitive Relay)

• Engine bay only, lead acid batteries only
• Prevents you from draining starter battery
• Gets 80% out of your accessory battery; doesn’t charge it too fast
• Cheapish ($50-150, plus wiring etc.)
• Won’t work with the latest cars (~2015 onwards) with smart alternators

The next best dual-battery system is one with a VSR. This is suitable for under-bonnet set-ups where cables are short and batteries are the same kind.

A VSR is a high-current relay that has simpler wiring than most relays.

A relay (for those who don’t know) is a switch that’s triggered by voltage. In cars and motorcycles, you use a relay on something like the accessories wire (that’s only live when the bike is on) to power other high-current accessories, like extra lights, a fridge, or heated grips. This lets you run those accessories without drawing too much current through a single wire.

A VSR is a relay that separates the car’s main cranking battery and the secondary accessories battery. The VSR is triggered by detecting when the alternator is running and charging the car battery. When the VSR detects that the car is on, it says “Hey the car’s on! OK let’s connect the two batteries”. Thus, your secondary battery starts charging.

Circuit diagram for installing a VSR into a car

The advantages of a VSR are that you get the potential full advantages of a dual battery system mentioned above, namely, that a) if the car is off and you’re powering accessories, you won’t drain your cranking battery, and b) if you need (in a pinch) to use your secondary battery to crank the car, you can override with the click of a switch.

The disadvantages of a VSR are that you’ll never fully charge your accessories battery, and thus never get full use out of it.

Why is that? Well, it varies by car. But it’s for the following reasons:

1. Cars don’t try to optimise to keep the cranking battery at 100%; you’ll only get about 80% out of it. They simply don’t need to, so why should they? A cranking battery will last longer if it’s not fully charged, and there’s no way you need all the stored power to start the car. So they won’t do the same for another battery, either. Firstly, a basic alternator + regulator/rectifier doesn’t optimise for stages of charging of a battery (you have to charge them at a different rate as they get fuller). Secondly, they don’t even try. So you’ll only get about 80% out of your secondary battery.
2. Modern cars (~2015 onward) have “smart alternators”. Smart alternators may detect that the cranking battery is at 100% and then turn off the charging system, even if the accessory battery is at less than 100%, and so may never charge the secondary battery.
3. If there is voltage drop between the cranking battery and the second battery (if there is any distance between them, or a thinner gauge cable), then the secondary battery won’t be fully charged.

But a VSR works in a simple situation where you have two batteries under the bonnet and you want to provide a degree of protection of the cranking battery from being drained. It actually will work for most people, despite the above limitations.

The main downsides of a VSR are that

• It won’t charge the secondary battery as efficiently, or fully, as mentioned above.
• Your battery pretty much has to be a heavy one, under the bonnet. This means a) ~30kg of extra weight (vs a lithium), and b) you can’t use lithium with this system.
• You need a separate solar controller. DC-DC chargers often come with high-tech in-built solar controllers (much better ones than the solar panels ship with), which saves you from doubling up on purchases.

DC-DC chargers — the Gold Standard

• More expensive — $250+, $400+ for a good one
• Get 100% out of your battery, and charge it faster
• Lets you use lithium, keeping the battery outside the engine bay
• Better for solar (with a better DC-DC charger)

A DC-DC charger takes DC from one source (usually the alternator/existing battery, or a solar panel), optimises it to charge a battery depending it on its type, condition, and level of charge, and then outputs DC to charge the battery.

What does a DC-DC Charger Do (Redarc)

DC-DC chargers are what you need if you

• Want to use a lithium battery (saving weight, and increasing usable charge), or another battery type. Charging a lithium battery means applying different voltage in different stages to a lead acid battery.
• Want to charge the battery faster, and/or get 100% out of it (rather than 80%). DC-DC chargers have multi-stage charging.
• Want to keep the battery outside the engine bay in your canopy, trailer, caravan, or camper. You place a DC-DC charger near the battery, and it doesn’t matter if there’s voltage drop before then — it takes care of it.
• Use a solar panel as an alternative charger. Most solar panels come with PWM regulators, but these leave about 20-25% of potential power output on the table, and you need an MPPT regulator anyway. A good DC-DC charger comes with one built-in.

There are cheaper DC-DC chargers from vendors like Jaycar and Adventure Kings that don’t have an MPPT solar regulator in them.

Cheap DC-DC Chargers lack what I think is one of the best advantages of a DC-DC charger input. If you get one of them, you’re not saving money, because you’ll need to buy an MPPT regulator anyway.

The best ones that do have an MPPT regulator are from

• Redarc
• Intervolt
• Kickass (a cheap distributor, but their products aren’t bad)

Generally, people seem to love Redarc — there aren’t reports of them failing after a few months to a year of use.

Combined Chargers Vs Separate Chargers

7 April 2022  |  Paul

In this article we'll be looking at combined DC-DC, Solar and Mains charging systems versus separate components.

Both have advantages and disadvantages that need to be considered before determining which solution is best for your set-up and needs.

Triple Charger Systems

Triple Chargers are single units with DC-DC, Mains & Solar charging all built-in, enabling you to charge from the alternator, shore/hook-up power or solar panels. These units have become very popular where space is tight and you want to keep wiring to a minimum as you will only need one set of cables from the battery to the unit for all three types of charging. The downside to this, however, is that if one component fails you may need to replace the whole unit. Another downside is that they are inflexible in terms of sizing each charging source to your requirements. The manufacturer has to design the triple charger to suit a 'typical' system, but two systems are rarely the same so you might, for example,  find that the outputs of the DC-DC charger and mains charger are about right for your battery size, but that you need much more solar capacity than the controller allows for. You may find you need to accept a compromise in one area if your system isn't the 'typical' one the manufacturer has deigned the charger for.

The key things to take into consideration when looking at Triple Chargers for your build are:

• Charging capacity - Do the Mains & DC-DC chargers provide the right output in Amps for your battery's overall capacity (around 10-20% for lead-acid technology; higher for lithium) and can the unit handle the size of your solar array? If you pick a unit with a charger output that is too high then it might damage your battery over time, and a charger with too low an output might take a long time to charge (see our mains charging guide for more info). If you pick a unit with a solar charger capacity that's too small, you might not be able to harvest all of the energy that your solar panels can produce.
• Open Circuit Voltage - Check that the solar controller in the Triple Charger can handle the VOC (Open-Circuit Voltage) of your intended solar array. If the solar array VOC is higher than the solar controller can handle, it can be damaged. If you are ever unsure of your panel's VOC, we advise contacting the retailer who sold you the panel to get confirmation. All of the panels available on our website have their VOC stated for easy reference.

DC-DC Chargers + Solar Controllers

DC-DC + Solar Controller chargers are similar to the Triple Chargers but do not have mains charging capability. Again, these combined units enable you to save on space and cable required for your project but suffer similar limitations as per Triple Chargers. Making sure that the solar charging capacity (as well as VOC tolerance) and charging output match your intended setup are the key things to watch out for with these units.

Separate Systems

Of course, all three charging systems can be purchased separately which can allow you to pick and choose each unit so that it best meets the needs of your setup. You can mix and match components by selecting a solar controller that has sufficient capacity for your intended array and mains/DC-DC chargers that have an output that's optimised for your battery type and capacity. With separate components you also have the opportunity to select models with features you want (i.e. Bluetooth communication, trickle charger features, etc), which might not necessarily be available on a Triple Charger. These systems, however, take up a lot more space which can be difficult to fit in smaller areas, and you will need significantly more cabling and fusing for the installation. It can also prove tricky to fault find should an issue develop, as you need to take into account all three systems when you look into the cause.

Key things to take into consideration with a separate system: 

• Space & heat - As all these units produce heat it can be tricky to find an area for them that has a good airflow to help keep them cool. Gaps around the units should also be planned as having them too close can restrict cooling, potentially leading to loss of performance. 
• Cable connections - Having multiple units feeding power to one battery can lead to a lot of connections having to be made. Items like busbars and power posts can help, however, they can cause some slight drop in performance due to an increased number of connections and therefore an increased resistance in the circuits.

Inverter Chargers

Combined Inverter/chargers are increasingly popular as a space-saving solution where your system needs a mains charger and an inverter, and can even help reduce costs in some cases. Most Inverter/chargers can detect whether you're on mains shore power or off-grid, and will automatically and seamlessly switch between supplying mains power from the shore supply or from your battery (via the inverter), without interrupting the 240V device being powered. Although this reduces the number of cables required, because the same set is used for the charger and the inverter to the battery, you will need to ensure you use large enough gauge cable to cope with the power requirements of the inverter when off-grid.

As with Triple Chargers, these units are designed for a 'typical' system, so you might find you need to compromise on either the charger or the inverter output. For example, you might ideally need a 20A battery charger and a 1200W inverter, but you might only be able to get a 20A charger with a 500W inverter, or a 1200W inverter with a 50A battery charger.

Key things to watch with Inverter/chargers

• Charging capacity - Again, it is key to make sure that your mains charger is providing approximately the right output for your battery type and overall capacity. Typically with Inverter/chargers, as the inverter output increases, so does the battery charger output. If you're having to compromise too much on battery charger output just to get the inverter output you need, we'd recommend looking at separate components.
• Automatic AC changeover - Victron inverter chargers will automatically switch between shore power and inverter AC but other brands might not have this useful feature, so it's worth finding out before deciding which to buy.

 

So, if you're trying to decide between a combined unit or separate components we hope this article has helped, but if you have any questions please don't hesitate to get in touch. 

Disclaimer

The information contained in this article is provided in good faith and we do our best to ensure that it is accurate and up to date, however, we cannot be held responsible for any damage or loss arising from the use or mis-use of this information or from any errors or omissions. The installer is ultimately responsible for the safety of the system so if you are in any doubt, please consult a qualified electrician.

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