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Parallel versus series batteries: How to wire your leisure batteries

When it comes to wiring your campervan leisure batteries, you might be wondering what the difference is between wiring in ‘series’ or ‘parallel’. What does this even mean? If you’ve opted to use more than one battery in your campervan electrical system, you need to ensure they’re wired up correctly. So, that means choosing between parallel and series wiring.

Let’s dive in, and find out which battery wiring option is right for you.

The electric power formula

The key to understanding this subject is in just one simple equation. The equation, called Watts Law, explains how electrical power is a product of both current and voltage: 

P = I x V

P (measured in watts) is the amount of power used by an electrical load. For example, a fan might draw 20W and a hairdryer might draw 1500W.

I (measured in amps) is the current running through the wires. 

V (measured in volts) is the voltage (sometimes called potential difference) generated by the power source. For example, a leisure battery might output 12V, 24V or 48V electricity.

So, if a device uses 10A at 12V, it consumes 120W of power.

This also means that for the same power, when when voltage is higher, current will decrease.

For example, a 3000VA inverter at 12V will have a current of 250A. In comparison, that same inverter running at 24V will have a current of 125A.

We can use this relationship to decide on a sensible electrical system voltage. In turn, we can decide how we should wire our campervan leisure batteries. But first, let's take a minute to understand what parallel and series wiring actually is.

A 12V and a 24V 3000VA MultiPlus. The 12V model will have a maximum current of 250A, while the 24V version will have a maximum current draw of only 125A. By doubling the voltage, the maximum current draw is halved.

Wiring batteries in parallel

To wire leisure batteries in parallel, you connect the positive terminal of one battery to the positive of the other. Then, you do the same with the negative terminals.

In this instance, the current (A) increases and the voltage (V) stays the same.

Example:

2x 100Ah, 12V batteries wired in parallel = 200Ah, 12V battery bank

To work out the watt hour capacity of the battery bank:

P = 200A x 12V = 2400Wh

Wiring batteries in series

To wire leisure batteries in series, you connect the positive terminal of one battery to the negative of the other. If you have more than two batteries, repeat this process until all batteries are connected in a chain.

In this instance, the voltage (V) increases and the current (A) stays the same.

Example:

2x 100Ah, 12V batteries wired in series = 100Ah, 24V battery bank

To work out the watt hour capacity of the battery bank:

P = 100A x 24V = 2400Wh

So in both instances, the total capacity of both battery banks is the same in Wh. However, one battery bank is 12V, and the other battery bank is 24V. So, why should you use one and not the other? In other words, when should you use series versus parallel battery wiring for your campervan electrical system?

Electrical system voltage

The answer comes in deciding the voltage of your electrical system. Usually, a campervan electrical system will be 12V. This is, in part, because many campervan appliances are 12V and your vehicle starter battery is 12V. So, this means you can avoid using any converters to convert the voltage in the system. The only exception to this might be if you need to power AC appliances, in which case you would use a campervan inverter. A campervan inverter converts your DC voltage (e.g. 12V) to AC voltage (230V).

However, there are some instances when it makes sense to use 24V in a campervan electrical system. For example, if you have a high power system that utilises a large inverter, for example a 3kVA model, you might want to opt for a 24V system.

If we rearrange Ohm’s Law to find current, we get:

I = P / V

So, when power increases, current will also increase. To decrease current, we can increase voltage.

The affect of voltage on cable gauge and component sizing

Current directly affects a few areas of an electrical system, such as cable gauge. The higher the current, the thicker the cable we need. In very large systems with longer cable runs (as cable length also affects cable gauge due to voltage drop), we may need extremely thick cable to cope with our high current. To decrease the cable gauge, we can increase voltage.

And it’s not just cable gauge. Higher current also affects other areas of the system, such as charge controllers, busbars and switches. By increasing the system voltage and decreasing the current, we can decrease the size required for each of these items within a system.

By increasing the system voltage, you can decrease the rating required for ancillary items such as busbars.

Maximum discharge current

Finally, system current directly impacts your battery's maximum discharge current. Every lithium leisure battery has a maximum current it can be discharged by, usually due to its BMS rating. For example, the 230Ah Fogstar Drift has a maximum discharge current of 250A. In comparison, the 230Ah Drift PRO has a maximum discharge current of 300A.

So, you need to ensure that your total system current is less than the maximum discharge current of your batteries.

By wiring batteries together in parallel, you increase the maximum discharge current and the system voltage stays the same.

For example, two 24V 230Ah batteries with a 300A discharge current wired together in parallel create a 24V battery bank with a 600A discharge current.

By wiring batteries together in series, you increase the system voltage and the maximum discharge current stays the same.

For example, two 12V 560Ah batteries with a 300A discharge current wired together in series create a 24V battery bank with a 300A discharge current.

As well as choosing a battery with a higher discharge current, you can wire batteries in parallel to increase their maximum discharge current. Remember, wiring batteries in parallel keeps the system voltage the same.

To calculate your maximum system current, add together the maximum current draw of your inverter and the current of all DC loads. Make sure you factor in the efficiency of the inverter - divide by this to work out the power it will actually use.

Example

If your system has 50A of 12V DC loads and a 3000VA inverter with an efficiency of 87% running at its maximum:

(3000VA ÷ 87% efficiency) ÷12V = 287A inverter discharge current

287A + 50A = 337A max discharge current @ 12V

So, you would need a battery which has a discharge current of at least 337A at 12V. In this instance, you would need to connect multiple batteries together to achieve this. For example, two 460Ah Fogstar Drift batteries wired in parallel would have a max discharge current of 400A.

Alternatively, if you opted for a 24V system voltage:

(3000VA ÷ 87% efficiency) ÷ 24V = 144A inverter discharge current

50A of 12V loads = 25A @ 24V

144A + 25A = 169A max discharge current @ 24V

In this scenario, you would need a battery with a discharge current of at least 169A at 24V. So, this could be achieved with a single battery, such as the 24V 280Ah Fogstar Drift. This has a max discharge current of 200A.

DC-DC converters

If you think you need a 24V system, you will also need a DC-DC converter to power any 12V appliances. Even with this additional cost, in a high power system (with a 3kVA inverter or larger), it’s usually cheaper to increase to 24V.

For more information on system voltage, read our article: 12V, 24V or 48V – what’s the best voltage for your campervan electrical system?

A DC-DC converter converts power from your 24V electrical system into 12V electricity to power any 12V DC appliances.

So, how should I wire my leisure batteries?

Now that you’ve learnt about electrical system voltage and you understand the relationship between power, current and voltage, you can decide how to wire your leisure batteries.

If you’re sticking with a 12V system, wire your 12V batteries in parallel. On the other hand, if you’re opting for a 24V system, you have a few options. Most leisure batteries are 12V, but there are some 24V options available. So, if you have a 24V system, you can install:

  • 1x 24V battery
  • 2 or more 24V batteries wired in parallel
  • 2x 12V batteries wired in series
  • 4x 12V batteries wired in series parallel

Remember, 12V and 24V batteries that have the same capacity will have a different 'amp hour' (Ah) rating. A 560Ah 12V leisure battery has a total capacity of 6.7kWh. A 280Ah 24V battery also has a total capacity of 6.7kWh. In comparison, a 280Ah 12V only has a total capacity of 3.4kWh.

What about series parallel?!

If you have 4 leisure batteries, you may still be feeling confused. But don’t worry, the answer is relatively simple:

  • 12V system, 12V batteries - wire in parallel
  • 24V system, 24V batteries - wire in parallel
  • 24V system, 12V batteries - wire in series parallel

If you have a 24V system and 4x 12V batteries, you’re going to create 2x strings of 24V battery banks (i.e. wire 2x 12V batteries in series to create a 24V battery bank), and then wire these 2x strings in parallel - to create a 24V battery bank of 4x 12V batteries.

Wiring 4 batteries in series parallel

We’ve included a wiring diagram below showing how to wire 4 batteries in series parallel. In this instance, an image is probably much easier to understand than an explanation!

In summary:

  • 2 batteries are connected in series
  • Each series string of batteries is connected in parallel
  • Ensure all cables are of equal length

Wiring 4 batteries in parallel

Just to avoid any confusion, here’s a wiring diagram showing how to wire 4 batteries in parallel.

In summary:

  • Batteries wired in parallel must be connected in a certain configuration to ensure that your system draws the same current through each battery.
  • Wiring in this specific configuration means the same total amount of wire (positive and negative) is used to connect each battery. So, the resistance in each connection is the same.
  • This means that the same current draw is applied to each battery. Otherwise, one battery would be constantly overcharged and another undercharged.

If you’re still not sure how to wire your campervan leisure batteries, let Nohma do the hard work for you. We employ scientists and engineers, who, alongside our clever algorithm, have designed thousands of bespoke campervan electrical systems. Let us design and ship yours, for no extra cost over the parts themselves.

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Leisure battery wiring FAQs

Wiring batteries in series connects the positive terminal of one battery to the negative terminal of another, increasing voltage while maintaining the same capacity. In contrast, parallel wiring connects all positive terminals together and all negative terminals together, keeping voltage the same while increasing capacity.

It’s generally not recommended to mix different battery sizes or types, as this can lead to imbalanced charging and discharging, reducing overall performance and lifespan. Ideally, use batteries of the same type, age, and capacity.

Maximum discharge current is the highest current a battery can deliver, determined by its Battery Management System (BMS) rating. It’s vital to ensure your system's total current draw doesn’t exceed this rating to prevent damage to the batteries.

A 12V system is common because many campervan appliances operate at this voltage, avoiding the need for converters. However, if you're using high-power appliances or a large inverter (e.g., 3kVA), a 24V system can reduce current, allowing for smaller cables and components.

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