Let’s break down the difference between series and parallel wiring
When you first start wiring batteries in a campervan, it can feel like you’ve stepped into a new universe of terms: series, parallel, 12V, 24V, Ah, Wh, cable sizing, voltage drop and suddenly you just want to go back to thinking about curtains and cabinet doors instead.
But breathe. The difference between series and parallel is actually straightforward once you understand the relationship between power, voltage, and current. And if you’re running more than one battery in your battery bank, you do want to wire them correctly, so your system is safe, stable, and doesn’t wear your batteries out faster than it should.
So let’s untangle this in a way you can actually use in your DIY campervan build.
Why it matters: series vs parallel affects the entire electrical system
If you’re building a standard 12V campervan system with one or more leisure batteries, parallel wiring is often the obvious choice. But as soon as you start talking about a bigger inverter, higher loads, long cable runs, or “I want to run the coffee machine and the air fryer at the same time” (no judgement), then system voltage suddenly becomes very interesting.
System voltage is what decides how you should wire your batteries.
Watt: the formula that makes everything less confusing
There’s one equation that basically explains this whole topic:
P = I × V
- P (watt) is the power a load uses. A fan might draw 20W, a hair dryer could draw 1500W.
- I (amps) is the current flowing through your cables.
- V (volts) is the voltage from the source like 12V, 24V, or 48V.
So if a device uses 10A at 12V:
P = 10A × 12V = 120W
That also means: for the same power, if voltage goes up, current goes down. A classic example is an inverter. Let’s say you’ve got a 3 000VA inverter:
- At 12V, current is roughly 250A
- At 24V, current is roughly 125A
Same type of inverter, but by doubling the voltage you halve the current and that impacts everything from cable size and fuses to how easy it is to build a tidy, sensible system.
Series vs Parallel Battery Wiring:
What Does it Actually Mean?
Parallel battery wiring (leisure batteries in parallel)
When you wire batteries in parallel:
- Positive to positive
- Negative to negative
What happens:
- Voltage (V) stays the same
- Capacity (Ah) adds up
Example: 2 batteries in parallel
2 × 100Ah, 12V in parallel = 200Ah, 12V
To calculate energy (Wh), use:
Wh = Ah × V
So: 200Ah × 12V = 2400Wh
Parallel wiring is how you get more capacity and more energy without changing your system voltage.
Series battery wiring (leisure batteries in series)
When you wire batteries in series:
- Positive from battery 1 to negative on battery 2
- With more batteries, you continue in a chain
What happens:
- Voltage (V) adds up
- Capacity (Ah) stays the same
Example: 2 batteries in series
2 × 100Ah, 12V in series = 100Ah, 24V
Energy in Wh: 100Ah × 24V = 2400Wh
Here’s the key point: in both examples you get 2400Wh, i.e. the same amount of energy, but delivered at different voltage.
So the question isn’t “which gives more energy?”
It’s: Which system voltage do you want and why?
Campervan system voltage: 12V is standard, but 24V has real advantages
Most campervan builds run 12V, and there are good reasons for it:
- Many campervan accessories are 12V (lights, pumps, fans, fridge/freezer boxes)
- Your van’s starter battery is 12V
- You avoid extra converters and specialist solutions
The exception is anything mains/AC (230V) and for that you’ll use an inverter anyway to convert DC (12V or 24V) into 230V.
When 24V is worth considering
A 24V system can be smart if you have:
- A large inverter (for example 3kVA or more)
- High loads and high power draw for longer periods
- Long cable runs between battery bank and inverter
- A system where cable size would otherwise become seriously thick and expensive
This is where the other important relationship comes in:
I = P / V
When power (P) goes up, current (I) goes up. If you want to reduce current, you increase voltage (V).
How voltage affects cable size, components, and the whole build
It’s easy to think voltage is just a number printed on a battery, but in a real campervan electrical install it has very practical consequences.
1) Cable size and voltage drop
High current needs thicker cable. And the longer the cable run, the thicker it needs to be so voltage drop doesn’t become an issue.
In bigger 12V systems, the cables between batteries and inverter can become:
- Expensive
- Heavy
- Hard to route
- Difficult to keep neat
If you move up to 24V, you halve the current for the same power, which often makes cable sizing far more reasonable.
2) Busbars, switches, fuses and other components
High current also affects the sizing of:
- Busbars
- Main battery isolators
- Fuses
- Charge controllers and other “in-between” components
Increase system voltage, and you can often reduce the current rating you need. That can make component choices simpler and sometimes cheaper.l enklare och ibland billigare.
Maximum discharge current:
The detail that is often miss when choosing batteries
This is especially important with lithium batteries: every battery has a maximum discharge current, often limited by the battery’s BMS.
For example:
- One 230Ah lithium model might have a max discharge of 250A
- Another version might handle 300A
So it’s not enough to only look at “Ah”. You must know whether the battery can actually deliver the current your system may demand. And again: if you want to reduce current, you can increase voltage (V).
What happens to max current when you wire batteries?
- Parallel wiring: increases possible max current because multiple batteries share the load
- Series wiring: increases voltage, but the max current per series string is basically the same
Example:
- Two 24V 230Ah batteries with 300A discharge in parallel could, in principle, give a bank capable of higher total current (e.g. 600A as a simple illustration).
- Two 12V batteries with 300A discharge in series become 24V, but the max current in that string is still 300A.
How to calculate the maximum current in your system
A simple approach:
- Take max current from the inverter (at full load)
- Add your DC loads
- Account for inverter efficiency
Example at 12V:
DC loads: 50A (12V)
Inverter: 3000VA, efficiency 87%
Inverter current:
(3000VA ÷ 0.87) ÷ 12V = 287A
Total max current:
287A + 50A = 337A at 12V
Same scenario at 24V:
(3000VA ÷ 0.87) ÷ 24V = 144A
If the 50A loads were 12V, they’re roughly 25A at 24V (same power, higher voltage):
144A + 25A = 169A at 24V
That’s why 24V can be so handy for high-power setups: you end up with currents that are simply easier to manage.
DC-DC converter: if you choose 24V but still have 12V accessories
If you choose a 24V system, you’ll usually need a DC-DC converter to run 12V consumers reliably.
Yes, extra cost, extra component. But in bigger systems it can still work out cheaper overall because:
- You save money on cable size
- You can downsize certain components
- The install becomes “calmer” electrically (lower current stress)
How to wire your leisure batteries: practical scenarios
Now that you understand the logic, let’s turn it into simple choices.
If you’re running a 12V system
Almost always:
12V system + 12V batteries = wire in parallel
You keep 12V, and you build more capacity (Ah) and more energy (Wh).
If you’re running a 24V system
You have more options because batteries are often sold as 12V units. You can use:
- 1 × 24V battery
- 2+ × 24V batteries in parallel
- 2 × 12V batteries in series (gives 24V)
- 4 × 12V batteries in series-parallel
A common confusion here is the Ah numbers. Remember:
12V and 24V batteries with the same energy (Wh) can have very different Ah.
Examples:
- 628Ah at 12V can be around 8.0 kWh
- 300Ah at 24V can be around 7.7 kWh
- 300Ah at 12V can be around 3.8 kWh
That’s why Wh or kWh is often the fairest comparison not Ah.
What is series-parallel wiring, and when do you need it?
If you have 4 batteries and want 24V, but the batteries are 12V, then series-parallel is often the solution.
The basic idea:
- Build two series strings, each giving 24V
- Then wire those two strings in parallel for more capacity
So with 4 × 12V batteries:
- 2 batteries in series (string 1 = 24V)
- 2 batteries in series (string 2 = 24V)
- Wire string 1 and string 2 in parallel
One important detail (and it really matters):
Try to keep cables symmetrical and similar length where reasonable, so resistance stays equal.
4 batteries in parallel: wire it so current shares evenly
When you parallel multiple batteries, you need to wire it so each battery has roughly the same “resistance path” to both the load and the charger.
Otherwise you can end up with:
- One battery working harder all the time
- Another battery always lagging slightly behind
- The bank ageing unevenly and losing performance faster
Practical rule:
Aim for the same total cable run (positive and negative) to each battery so resistance is as equal as possible.
Quick memory rule: How to remember which is which
If it feels messy and hard to remember, here’s an easy rule of thumb:
- Parallel leisure batteries give you more amp hours (Ah) capacity over time:
12V 100Ah + 12V 100Ah = 12V 200Ah - Series leisure batteries increase voltage (V) the “push” or electrical force:
12V 100Ah + 12V 100Ah = 24V 100Ah
A helpful analogy:
Ah is like the size of a car’s fuel tank. Volts is like how strong the engine is. Higher Ah gives you longer runtime, but not necessarily more power.
And when you convert 12V DC (like a battery) to 230V AC (like a wall socket) using an inverter, you lose some energy as heat. Expect roughly 5–10% of battery capacity to disappear as pure heat when running an inverter.
FAQ: Common questions about campervan electrics
What is maximum discharge current, and why is it important?
Answer: It’s the highest current the battery can deliver, often limited by the BMS. If your inverter and loads can draw more than the battery can supply, the system may shut down or be damaged.
When should I choose 12V instead of 24V?
Answer: 12V is most common and simplest, especially for normal loads and a smaller inverter. 24V becomes interesting once power draw and current get high.
Do I need a DC-DC converter if I run 24V?
Answer: Yes, if you have 12V consumers. Many people run 24V as the backbone and create a stable 12V supply via DC-DC.
What’s the difference between series and parallel?
Answer: Series increases voltage (V) but keeps Ah the same. Parallel keeps voltage the same but increases Ah.
Can I mix batteries of different sizes or types?
Answer: It’s generally not recommended. Ideally use the same type, same capacity, and preferably the same age to avoid imbalance during charging and discharging.
What is a leisure battery?
Answer: A leisure battery is a separate battery commonly used in motorhomes, campervans, and boats to power living-area electrical loads.
Summary
Series and parallel aren’t “good or bad”. They’re tools.
- Parallel wiring gives more capacity (Ah) at the same voltage. Perfect for most 12V campervan builds.
- Series wiring gives higher voltage while keeping Ah the same. Useful when you want to build 24V from 12V batteries.
- In bigger systems, 24V can make a huge difference because current drops, making cable sizing and component choices easier.
- Don’t forget: maximum discharge current from the battery’s BMS must match what your system could potentially draw.
Want to read more?
- Build a Campervan: www.camperpals.com/bygga-campervan
- Interior installation: www.camperpals.com/invandig-installation
- Campervan electrical system & wiring: www.camperpals.com/elsystem-i-campervan