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Batteries in Parallel vs Series: Which is Best for Your RV?

The question of wiring your leisure batteries in parallel vs series is bound to come up at some point. Our articles on campervan electrical systems and Leisure batteries will give you a good understanding of the broader subject. This article looks into the specifics of wiring multiple batteries together. We'll review series and parallel wiring setups, wiring different kinds of batteries together, and more.


 

Index


 

Shane has been teaching people to convert campervans for years; he's the author of Roaming Home; The Comprehensive Guide for Converting Your Van Into a Campervan, writer of The Van Conversion Newsletter, instructor of The Van Conversion Course over at Udemy, and a full-time van lifer of 4 years!


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Note: Did you know that you can get a free electrical wiring diagram by signing up for The Van Conversion Newsletter?

 

Understanding the Basics of RV Electrics

To understand why we would wire RV batteries in series or parallel, we need to understand the units that define the parameters of those batteries. We have articles on our site that go into the details of campervan electrics, batteries, and wiring, but we'll give a primer here.


Volts, Amps, and Watts

When choosing batteries, you'll make your choice based on a few parameters that define the properties of the battery.


  • Voltage, measured in volts (V), is a measurement of 'electric pressure'. Think of it like the speed at which water flows down a pipe. The more energetic the flow of electricity, the higher the voltage. The voltage needs to be the same across your whole electrical system, or you'll need a DC-DC converter to change between voltages. The voltage of your batteries is the 'base' voltage that defines the voltage of the rest of your electrical system, so it's an important decision.


  • Amperage, measured in amps (A), is a measurement of electrical current flow. Think of it as the quantity of water flowing down a pipe. The more electricity is flowing, the higher the amperage.


  • Wattage, measured in watts (W), is a measurement of electrical power, calculated by multiplying voltage by amperage. Think of it as the combination between the quantity of water flowing down a pipe and the speed of that water. These two factors together would determine the power that the water had to turn a water wheel, and so it is with wattage. The higher the wattage, the more electrical power is available to do work.


You can remember the relationship between volts, amps, and watts using this conversion triangle:


Watts volts amps conversion triangle

Units next to each other are multiplied, and units above and below each other are divided. Thus we see that:


watts = volts x amps

volts = watts ÷ amps

amps = watts ÷ volts


Battery Capacity

When looking at batteries, we also need to think about capacity. It's not enough to just look at electrical power (wattage), as we also need to know for how long that power will be available. In other words, we need to know the total stored electrical energy of the battery. This value is quoted using the units watt-hours (Wh) and amp-hours (Ah).


  • Watt-hours (Wh) tell you the amount of power in watts that a battery can output for one hour. For example, a 1200Wh battery can output 1200W of power for one hour.


  • Amp-hours (Ah) tell you the amount of current in amps that a battery can output for one hour. For example, a 100Ah battery can output a current of 100A for one hour.



Whilst watt-hours are more descriptive of a battery's practical usefulness, most battery manufacturers quote capacities in amp-hours. Fortunately, we can easily convert from amp-hours to watt-hours by multiplying the battery's capacity in amp-hours by the battery's voltage. For example, Tesla sells two different kinds of home batteries:


  • A 12V battery with a 500Ah capacity

  • A 24V battery with a 250Ah capacity


We can work out the capacities of these batteries in watt-hours like so:


12V x 500Ah = 6000Wh

24V x 250Ah = 6000Wh


Thus we see that both batteries have the same capacity in terms of watt-hours. Because this is the measure that tells us the amount of work a battery will be able to do over time, it's the most useful unit to use. We thus see that these batteries have essentially the same capacity and that the difference is in their voltage. We'll get into battery voltages later.


Understanding voltage, amperage, wattage, and capacity is essential when deciding whether to wire your batteries in parallel vs series. Now let's look at the practicalities of these numbers in the context of wiring setups.


Wiring Batteries in Series

You get very different results when wiring batteries in parallel vs series. Wiring batteries in series combines their voltage, meaning the battery bank receives and outputs a higher voltage than the individual batteries would. The capacity (in Ah or Wh) of a battery bank wired in series doesn't change and is limited by the capacity of the smaller battery. To illustrate:


2 x 12V, 60Ah batteries in series → a 24V, 60Ah battery bank

1 x 12V, 60Ah battery and 1 x 12V, 45Ah battery in series → a 24V, 45Ah battery bank

2 x 24V, 120Ah batteries in series → a 48V, 120Ah battery bank


Wiring leisure batteries in series
We see here that wiring batteries in series produces a battery bank with a higher voltage and the same capacity as the component batteries.

Why would you want to wire your batteries in series? The only reason would be to create a battery bank with a higher voltage, in order to power higher-voltage components and appliances.


Do You Need a Higher-voltage Electrical System?

Some campervan owners opt for a 24V electrical system instead of a 12V one. A 24V system makes good sense if you have a high-capacity electrical system. If you'll be running an inverter rated at 3000W or more with more than 1500W of solar, increasing the voltage of your electrical system probably makes sense. This is because such high wattages at low voltages result in very high currents.


12V System Example

Remember the conversion triangle earlier? Imagine a 3000W inverter running on a 12V system, which at peak power can pull an overload current of 4500W:


Current (A) = power (W) ÷ voltage (V)

Current = 4500W ÷ 12V

Current = 375A


Wires, fuses, breakers, and busbars should all be rated slightly above the maximum current the system can pull. Cabling that can handle 400A of current is thick, hard to work with, and expensive (around £15 or $19 per foot). 400A fuses, breakers, and busbars are also expensive and harder to come by than their lower-capacity counterparts.


24V System Example

By increasing the voltage of our system, we get a much more reasonable current draw:


Current = 4500W ÷ 24V

Current = 187.5A


Wiring rated to 200A costs about half as much as wiring rated to 400A, and is much easier to work with. The same can be said of 200A amps, fuses, and busbars. In general, the cost-benefit calculation works out in favor of increasing voltage for high-power systems. The other situation in which higher voltage makes sense is installations with very long cable lengths, like bus-sized RVs, boats, and tiny homes. This is due to electrical resistance increasing with current and cable length. Electrical resistance reduces the efficiency of a system and produces dangerous heat. Reducing current by increasing voltage is a good way of mitigating this effect in long cables.


Below is a useful table comparing 12V systems with higher voltage systems. To reiterate, higher voltage RV electrical systems should only be considered if you are running high wattage outputs:



Standard 12V System

High Voltage System (e.g., 24V or 48V)

Current Draw

Higher current for same power (e.g., 3000W @ 12V = 250A)

Lower current for same power (e.g., 3000W @ 24V = 125A)

Wiring

Requires thicker, heavier, and more expensive wiring due to higher current

Requires thinner, lighter, and less expensive wiring due to lower current

Components Cost

Higher cost for components like fuses, breakers, and busbars due to high current ratings

Lower cost for components as lower current ratings are sufficient

Battery Cost

Generally cheaper batteries

Generally more expensive batteries

Efficiency

Less efficient over long cable runs due to higher current and resistance

More efficient for long cable runs due to lower current and reduced resistance

Market Availability

Wide variety of batteries and components

Smaller variety of batteries and components



Should You Create a 24V Battery Bank by Wiring 12V Batteries in Series?

When looking at 24V batteries, you'll find that there's a smaller variety in capacities, types, and brands on the market. You might also find that a single 24V battery is more expensive than two 12V batteries of the same capacity. On the face of it, the wider choice and lower price of 12V batteries would seem to argue in favor of making a 24V battery bank using 12V batteries wired in series. However, there are some very good reasons to avoid this.


Batteries wired in series always receive the same current, but the voltage they receive differs slightly depending on their state of charge and internal resistance. Batteries can be slightly different out of the box, meaning that as you charge and discharge these batteries in series, they receive different voltages and end up with uneven states of charge. Seeing as state of charge affects voltage, you wind up in a feedback loop with a serious risk of overcharging and over-discharging your batteries. At best, this drastically degrades their lifespan, and at worst it causes battery leakage, fires, and explosions.


Preventing uneven charging and discharging in batteries wired in series requires sophisticated monitoring and balancing components. Rather than using a simple shunt to monitor the batteries' state of charge, you'll need a battery balancer or battery management system. This involves more wiring, more components, and ensuring that everything is wired and set up perfectly: no room for error. Even with a perfectly set up electrical system, failures can happen. In the event of a fault that results in one battery receiving less voltage than the other, the imbalance can be catastrophic for the electrical system.



Further to these issues, a battery bank comprised of batteries wired in series can never be added to, as adding a battery changes the voltage of the bank as a whole. This means an inflexible electrical system that's hard to upgrade.


For these reasons, we can't recommend wiring batteries together in series at all. Whilst the variety of 24V batteries on the market is more limited and can be more expensive, this alone doesn't justify the extra cost, complexity, and safety issues that come with such a setup.


Wiring Batteries in Parallel

Considering the above, let's go over what happens when you put batteries in parallel. The crucial difference is that a battery bank comprising batteries wired in parallel keeps the same voltage as the individual batteries. What changes is the capacity; the capacity of the battery bank is the sum of the capacities of the individual batteries. This allows you to create a battery bank with a bigger capacity which you can add to down the line.


batteries wired in parallel
Here we see that wiring batteries in parallel produces a battery bank with a higher capacity and the same voltage as the component batteries.

It's much easier and less risky to set up and run a battery bank wired in parallel. This is because, unlike batteries wired in series, batteries in a parallel system receive the same voltage, whilst current is divided among the batteries depending on their capacity to receive it. Despite possible differences in state of charge, internal resistance, or capacity, the system balances out as each battery contributes a different amount to the overall current draw. No sophisticated battery balancing is needed. This also means that a partial or complete failure of one battery will be compensated for by the other batteries in a parallel system, avoiding further damage. This all means that a parallel system is far easier to wire, set up, and monitor.


Finally, adding capacity to a parallel battery bank is as simple as adding another of the same battery in parallel.


Differences, Pros, and Cons: Batteries in Parallel vs Series

Batteries wired in series

Batteries wired in parallel

Voltage is cumulative

Voltage stays the same

Capacity stays the same

Capacity is cumulative

Batteries always draw the same current

Batteries balance current draw between them

Batteries receive different voltages

Batteries receive the same voltage

Requires external balancing

Self-balancing

Failure more likely to damage other batteries

Failure more likely to be compensated for by other batteries

Fixed capacity

Capacity can be added to

Voltage can be added to

Fixed voltage

No room for error in wiring

Allows for imperfections in wiring

List of Materials to Connect RV Batteries in Parallel or Series

Whether you decide to charge batteries in parallel or series, the list of components and tools you'll need is the same.


Batteries: we can comfortably recommend Victron batteries for your van conversion. Victron makes lithium, AGM, and gel batteries in a wide variety of capacities. They also have a good selection of 24V batteries. Victron consistently produces high-end components with high efficiencies and long lifespans. For more information on battery types and how to calculate your required capacity, check out our long-form article.


Victron AGM leisure battery

Battery terminal clamps: battery terminal clamps are the connection points from which you'll wire the battery to the rest of the electrical system. Make sure they're sized correctly to the terminals on your batteries, and make sure you buy your clamps in pairs of one positive and one negative. More on how to attach these here.


Busbars: a busbar is a connection point for multiple components to be powered by your battery bank. Think of them like the power strip you use to connect multiple appliances to one wall outlet in your house. More on busbars and how they're used here.


Fuse holders and fuses/circuit breakers: fuse holders, fuses, and circuit breakers work the same as the ones in your home. They're an essential safety component in any electrical system. More on fuses and breakers here.



Battery monitor/BMS: at minimum, it's a good idea to have a battery monitor (or shunt) in your electrical system for monitoring battery capacity and charge and discharge rates. 'Smart' versions are Bluetooth-enabled, allowing you to monitor your battery using your phone. Most systems won't need anything more than this, as long as you're wiring in parallel. A more sophisticated battery management system (BMS) will also provide active balancing, extra safety features, and more sophisticated monitoring. This is optional for parallel-wired battery banks, but essential for series setups (and highly recommended for lithium batteries). Read more about these devices here.


Isolator switch: an isolator switch allows you to cut power safely through your entire electrical system. This is very important when touching wires or connectors for inspection, maintenance, troubleshooting, and upgrades.


Wiring: wiring comes in varying diameters (gauges) rated to different maximum current flows. You'll likely have a handful of different wire gauges in your electrical system. Make sure you understand how to select your wire gauge and buy more than you think you need. Good quality, tinned, stranded wire is flexible and corrosion-resistant, and will stand the test of time.


Wire cutters/splitters: for cutting wires and stripping the insulation from the ends.



Tips and Safety Precautions for Battery Wiring

When wiring batteries to your campervan electrical system, there are a few essential safety features that must be included.


Identical batteries: whatever you do, batteries in parallel have to be the same. Parallel systems are self-balancing to an extent, but can't account for large differences in current flow. Differences in capacity, construction, and internal resistance between batteries would result in an unbalanced battery bank where the current flow and state of charge across the batteries would be different. As we discussed above, this at best degrades the condition of the batteries, and at worst causes leaks, fires, and explosions.


Grounding: ensure that your system is properly grounded to the chassis of your RV to prevent electric shocks. The grounding of your electrical system starts at the battery. Read more about grounding here.


Fusing: use the right fuses for your battery bank. Fuses should be rated slightly higher than the maximum current draw and output of your battery bank. Many people opt to use a specific fused battery terminal, which combines the functions of a battery terminal clamp and a fuse.


Wire gauge: wire gauge depends on current draw and the length of the wire between two connectors (cable run). Use this handy wire gauge calculator to help you get it right. With batteries, you'll likely use one wire gauge for connecting your batteries to your battery charger, and another for connecting your batteries to the rest of your electrical system. As we've said before, skimping out on wiring isn't worth it in the long run.


Routing wires: route your wires away from heat sources and sharp edges. Neat wiring using cable ties, conduits, and sheathing makes for a well-organized system that's easy to troubleshoot and upgrade.



Fire extinguisher: you should have a fire extinguisher in your RV anyway, but it's equally important to have one to hand when you're working with electrics. Make sure your fire extinguisher is rated for electrical fires.


A Word on Balancing

Whilst parallel-wired setups are self-balancing to an extent, they can't handle large differences in charge draw. It's possible to wire your batteries in parallel such that one battery is 'earlier' in the circuit than another. Take a look at the diagram below:


Diagram showing unbalanced charging of a battery bank wired in parallel

You can see that the battery on the right is connected to the first positive and negative wires coming from the battery charger. The battery in the middle is connected to the second set of wires, and the battery on the left is connected to the third set. Whilst all three batteries in this parallel setup receive power directly from the charge controller, the length of the circuit is different for each battery. The battery on the right has the shortest circuit length, whilst the battery on the left has the longest.


Wires and terminals provide some resistance to electrical flow, impeding the electrical current. By wiring the batteries so they have different circuit lengths, we're putting them in circuits with different levels of resistance. This means that the batteries all receive substantially different current flows, more different than what the self-balancing nature of parallel setups can compensate for. This leads to imbalances in the state of charge of the batteries which, as we discussed earlier, isn't a good thing.


To mitigate this, we need to wire our batteries up differently. Take a look at the next diagram:


Diagram showing balanced charging of a battery bank wired in parallel

You can see that the battery on the right is connected to the first positive wire and the third negative wire coming from the battery charger. The middle battery is connected to the second positive wire and the second negative wire, and the battery on the left is connected to the third positive wire and the first negative wire. The batteries in this setup all have the same circuit length. Slight differences in resistance in the cables, terminals, and the batteries themselves can never be eliminated, so the current draw won't be perfectly even across all the batteries. However, by wiring them in this way, we can reduce the difference in current draw enough that the self-balancing parallel setup will compensate.


If you only have two batteries in your battery bank, wiring them in a balanced parallel circuit is quite simple. If you have three or more, it may be a good idea to use two common busbars to organize your wires and make sure the cable lengths are even, as in the diagram below:


Final Thoughts

Is it better to wire batteries in parallel or series? The difference between batteries in series vs parallel is clear, and quite simple once you understand the key terms. A battery bank in parallel is flexible, easier to install, and self-balancing (to an extent). It allows you to create an expandable battery bank with a bigger capacity. On the other hand, a battery bank in series gives you a higher voltage battery bank that has the same capacity as its component batteries. This battery bank is not expandable, will be more complex to install, and requires precise monitoring and balancing. In our view, the possible cost benefit of a higher-voltage battery bank comprised of batteries wired in series does not outweigh the complications. We therefore recommend wiring leisure battery banks in parallel.


As with many aspects of van conversion, it's worth remembering that you can hire a specialist electrician to consult and wire your batteries for you. This may be worth the associated cost if you're still not sure what you need or how to wire it all up. If you're choosing your batteries yourself, Victron batteries are a good place to start and will form an excellent base for any electrical system.

 

Don't forget to subscribe to The Van Conversion Newsletter for everything you need to get started with your own van conversion (we'll send you a free wiring diagram when you join).


If you're looking for some guidance with your van conversion, you might be interested in Roaming Home; The Comprehensive Guide for Converting Your Van Into a Campervan. In the 380-page book (or ebook), you'll learn directly from Shane how to convert a van into your dream home - no prior experience needed!


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Until next time.

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