What size dc to dc charger do i need? a step by step guide
Electrics Planning

What Size DC to DC Charger Do I Need? A Step-By-Step Guide

Are you currently working on your camper and wondering what size DC to DC charger you need? Look no further, as we have some insights that might be helpful for you. 

In this blog post, we will discuss the important factors to consider when selecting a DC to DC charger for your camper to efficiently charge your campervan battery.

Choosing a charger that fits within the maximum current limit is a safe way to ensure your battery and alternator remain healthy. However, including the practical elements like usage and lifestyle will ensure optimum output from your DC to DC battery charger.

By the end of this post, you’ll have a better understanding of what size DC to DC charger is best suited for your needs.

Let’s get started!

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Portrait photograph of Jolly sipping a coffee

Trust Me, I’m An Electrician

Jolly is a self-confessed electrical geek with over 17 years of experience as a qualified electrician and electrical engineer.

His focus now is campervan conversion electrics which started in 2019 with the birth of Vandercamp. Jolly is either building his own campervan or providing electrical help and guidance to others.

The Renogy 40A DC to DC charger is installed on his current campervan powering a Renogy 200Ah Lithium Leisure battery. 

More About Us


Here are five, easy to use, calculators to help you determine what size DC to DC charger you need. They do the hard work for you. So, find the calculators throughout the guide or jump to which one you need.

What is a DC to DC Battery Charger?

A DC to DC battery charger (also known as a battery to battery or B2B charger) is a clever device that sits between the starter (vehicle) battery and the leisure (house) battery. The starter battery supplies power to the engine and vehicle electrics, whereas the leisure battery powers the campervan electrics (lights, sockets, fridge, heater, water pump etc.).

What is a DC DC battery charger? The alternator powers the starter battery that powers the dc dc charger. The charger powers the leisure battery

While the engine is running, the alternator charges the starter battery. The same charge current is converted by the DC-DC charger to ensure the leisure battery receives optimal charging.

DC-DC chargers are clever because they adjust their charge voltage based on the type of battery (Lithium, AGM, GEL) it’s charging. 

Additionally, when the alternator is not providing any charge current, the DC-DC charger will separate the leisure battery from the starter battery to prevent the starter battery from draining.

Further Reading | Discover 4 Other Ways Of Charging A Campervan Leisure Battery

Is A Bigger DC to DC Charger Better?

To clarify, by ‘bigger’, we mean the output charge current. So, is a higher output charge current better than a lower output charge current?

The obvious answer is yes because a higher charge current will charge the leisure battery faster. So why would you need a lower charge current?

Why bigger is better

  • Faster battery charging (More charge current flow into the battery)
  • For powering a load as well as charging the battery (e.g. fridge)
  • Large battery banks
  • Higher solar input capabilities for DC-DC + Solar chargers

Why bigger is not better

  • They’re bigger, so more space is required to install them.
  • More expensive (Charger, wiring and fuses)
  • Bigger input and output cables are more difficult to route and install.
  • Charge current is limited by the maximum charge capacity of the leisure battery
  • More stress on the alternator (compromise charging and alternator lifespan)
  • Heavier.

Determine the maximum DC to DC charge current for your camper (3 things to consider)

To determine what size DC to DC charger you need, you need to know these three things from your campervan electrical system:

1. Alternator and starter battery voltage (V)

2. Alternator output current (A)

3. Leisure battery maximum charge current (Ah)

By understanding these figures, you can determine the maximum DC to DC charge current that can be used in your campervan’s electrical system. Then we can move onto the other, more practical, factors to consider.

Once you know these figures, we show you how to determine the maximum DC-DC charge current for your campervan by using our simple DC-DC charger size calculator below.

Alternator And Starter Battery Voltage

Campervans typically use 12 or 24 volt DC electrical systems.

When setting up your campervan electrical system, check whether your vehicle has a 12-volt or 24-volt alternator and starter battery. This will help you choose the appropriate DC to DC battery charger.

The best DC to DC battery chargers for campervans are all 12 volts. 24 volt chargers are available, but the selection is limited.

How To

How To – Test The Alternator And Starter Battery Voltage

  1. Start the engine
  2. Use a voltmeter set to read DC voltage.
  3. Test the starter battery voltage by putting the negative (black) probe onto the ground terminal (Black/-) and the positive (red) probe onto the positive terminal (red/+).
  4. Check the voltage reading on the voltmeter

12v systems = The charge voltage should be slightly higher than 12 volts. Approximately 13.5v-14.5v.

24v systems = The charge voltage should be slightly higher than 24 volts. Approximately 28.8v-29.4v

Alternator Output Current

The output current of an alternator is measured in amp hours (Ah). A percentage of this current charges the starter battery, which powers the vehicle’s electrics.

Alternators come in many sizes, depending on the engine and vehicle size. As a general guide;

  • Standard vans have alternators rated at 70-120Ah
  • Larger vans and commercial vehicles have alternators rated at 120-180Ah

It’s important to ensure a DC to DC charger does not exceed 50% of the alternator’s output current (Ah). This will prevent the alternator from overloading (which could reduce its lifespan and burn it out).

For Example, if the alternator is rated at 90Ah, the DC-DC charger should not exceed 45 amps.

Additionally, during start-up, a DC-DC battery charger can draw up to 50% more current from the alternator than the rated current. 

For Example, a 40 amp Renogy DC-DC charger could draw up to 60 amps of power during start-up.

Therefore, without this 50% rule, it would be easy to overload an alternator.

How To

How to – Find Your Alternators Output Current

Determining the output current of your alternator can be tricky. Luckily, alternators typically have a stamp with a part number and amperage. However, alternators are mounted to the engine, so they aren’t easily accessible and are usually covered in oil and engine guck. 

If the amperage isn’t legible, a search on Google with the part number will give you the amperage of the alternator. Even though it’s tricky, this is the best method because it’s the most accurate.

The easiest and cleanest way is to search for your vehicle’s alternator using an online parts specialist like Euro Car Parts. Just pop in your registration to see the parts for your vehicle. However, vehicles have different models with different size alternators, which is less accurate than the previous method. If you use this method, always opt for the lowest Ah-rated alternator to be safe. Also, it could be a gamble if the previous owner has installed an aftermarket alternator.

Alternatively, use a mechanic who will easily identify what alternator you have.

Leisure Battery Maximum Charge Current

Find the maximum charge current a DC-DC charger can deliver to your leisure battery.

There are different types of leisure batteries; open & closed-cell lead-acid, GEL, AGM and Lithium, each with their own charging characteristics.

A battery’s type and capacity (Ah) will determine the maximum charge current it will allow. Exceeding the maximum charge current will damage the battery. 

As a general rule:

Battery TypeMax. Charge CurrentE.g. 100Ah Battery
Sealed Lead-Acid30%30Ah

A battery’s maximum charge current can also be found in its specification (online or in the manual). 

A chart showing the maximum continuous charge current for the Renogy 200 amp hour lithium battery
Renogy 200Ah lithium battery maximum charge current taken from the user manual

How To Determine The Maximum Charge Current

Now you know the alternator and battery voltage, alternator charge current and maximum battery charge current, use Calculator #1 below to determine the maximum size DC to DC battery charger for your campervan.

Calculator #1

Maximum DC To DC Charge Current

Other things to consider

Many factors will affect what size DC to DC charger is best. Of course, campervans have varying requirements and needs, so here are other factors to consider.

Usable battery capacity

To keep a battery healthy, you need to know its usable capacity (the amount of stored power that can be accessed). 

It’s important to note that even if a battery is rated at 100Ah, you may not be able to access all that power. The usable capacity of a battery depends on its type. Typically,

Lead-Acid = 50% usable capacity

AGM = 80% usable capacity

Lithium = 95% usable capacity

For Example, a 100Ah lead-acid battery’s usable capacity is 50Ah (50% of 100Ah = 50A). Whereas, a 100Ah lithium battery’s usable capacity is 95Ah (95% of 100Ah = 95A)

Discharging a leisure battery past its usable capacity will significantly affect its health and lifespan. 

Calculator #2

Usable Battery Capacity

Battery Recharge Time

Ideally, we want our leisure battery fully charged when we reach our destination, right?

So, providing we know the maximum DC-DC charge current for our campervan electrical system and the usable capacity of the leisure battery, we can work out the recharge time of the leisure battery and how long you need to drive until the battery is fully charged.

Usable Capacity ÷ Max Charge Current = Battery Recharge Time

Calculator #3

Battery Recharge Time

NOTE: These results are basic recharge time figures. Other factors will effect the recharge time so please take these results as a guide.

For Example, A 200Ah lithium battery has a usable capacity of 190Ah (95%). The maximum charge current of the electrical system is 75 amps. Therefore, it will take 2.5 hours to recharge the battery from empty (190Ah / 75A = 2.5 Hrs)

This example shows it will take approximately 2.5 hours to fully recharge a 200Ah leisure battery using a 75 amp charge current. However, a 75A DC-DC charger doesn’t exist, so you would need a 70 amp charger or less. As a result, the recharge time would increase to 2.7 hours (190Ah / 70A = 2.7 Hrs).

Powering a load while driving

The joy of having a DC to DC charger is you can charge your stuff on the move; when you reach your destination, the beer’s cold and everything’s charged.

However, using power while driving will reduce the charge current to the leisure battery.

For Example, while driving, you may need to power a fridge and charge a phone/camera – a total power consumption of 10 amps. If your charger is rated at 40A, the remaining charge current to the battery will be 30A. Less charge current will mean you’ll need longer to charge your battery.

Alternatively, if you require 40A of charge current to the battery and have a driving load of 10A, you will need a DC-DC charger rated at 50A (40+10 =50).

Note: These figures are an example. The charge current you need may be restricted by the alternator’s output current and the battery’s maximum charge current.

How To

How to – Calculate Your Power Consumption While Driving

1) Determine what you may need to power whilst driving.

Example – Fridge, phone x2, camera battery

2) For each appliance, determine its watt/amp rating (sticker/stamp on the appliance or adaptor plug socket).

Example – Fridge 150w, phones 3A, camera battery 1.3A

3) If it’s stated in Watts, use this formula and calculator to convert Watts to Amps. Watts/Voltage = Amps

Example – Fridge – 150 watts/12 volts=12.5 amps

4) Add up all the appliance’s Amp ratings. This is your total amount of consumption per hour (Ah). However, this is the worst-case scenario because you’ll unlikely need to charge all appliances simultaneously. Additionally, appliances don’t use 100% of their energy (e.g. fridges turn on/off to keep the temperature constant, and phone chargers only use energy while the phone is charging)

Example – Total= 12.5 + 3 + 1.3 = 16.8Ah.

5) Deduct your total Ah usage from the DC-DC charger output amp rating.

Example – 16.8Ah -40A = 23.2Amps

Therefore, 23.2 amps is the lowest (worst case) charge current the leisure battery will receive from the charger.

Calculator #4

Powering A Load While Driving

If the appliance is rated in Watts (W). Use this calculator to convert Watts to Amps.

Factoring in the power consumption while you drive will ensure you have optimum battery charging.


The Renogy DC-DC battery charger is the best value charger for any campervan conversion. It’s much cheaper than its rivals without compromising on quality and efficiency. Check out our full review and comparison table.

However, if you don’t have a strict budget, there is a wide range of campervan DC-DC battery chargers. Our post on the best DC to DC battery chargers for campers compares the top 5 chargers, including their cost, so you’ll be sure to find one that suits your budget.


The size of campervan electrical components is sometimes overlooked, especially when choosing what size DC-DC charger you need.

If you are limited on space, factoring in the dimensions is crucial to ensure you have enough space to install it AND enough space for the cables and airflow.

Additionally, a DC-DC charger must be installed near the leisure battery to reduce voltage drop. This will ensure the battery receives the full output current from the charger. 


What Is Voltage Drop?

Voltage drop is the amount of voltage that is lost in an electrical circuit. It occurs when the voltage at the end of a cable is less than at the beginning.

Therefore, the longer the cable, the more voltage drop. 

So, if the cable from the DC-DC charger to the battery has voltage drop, the battery may not get the charge voltage it needs.

Here are the dimensions for the best campervan DC to DC battery chargers;

NameCharge Current (Amps)DimensionsView
Renogy DCC12-12-20/40/60Renogy 40A DC-DC charger20A
View Product
Renogy REGORenogy REGO 60A DC to DC battery charger 10-60354-213-102View Product
Victron Orion-Tr SmartB2B/DC-DC leisure battery charger18
View Product
Redarc BCDC 1225D, 1240D, 1250DRedarc BCDC1240D 40A DC DC Charger MPPT Solar Controller25
165x120x37View Product
CTEK D250 SECTEK D250SE DC to DC battery charger20A***192x110x65View Product
Sterling BB1230, BB1260Sterling BB1230 DC to DC charger30
View Product

The complete comparison table can be found here.

Future proofing

Do you plan to upgrade your campervan electrics in the future? Maybe to increase the battery capacity or upgrade to lithium? Future-proofing is worth considering when calculating what size DC to DC charger you need.

Whether it’s increasing the battery capacity or changing the type of battery, DC-DC chargers are compatible with all campervan leisure battery types. But will the output current be enough when you upgrade?

For Example, you may be increasing your battery capacity, so you need 30 amps of charge current now but 60 amps in the future.

However, even if you could increase the charge current in the future, you may be restricted by the maximum charge current of your leisure battery and alternator charge current. Always consider the maximum size charger your camper electrical system will allow

Luckily, two DC-DC chargers have current limiting and variable output current features, perfect for future-proofing.

Charger No 1: The Renogy DC to DC battery charger has a ‘current limiting’ feature that lets you reduce the output current by 50%. Therefore, it can run at half current now and then at full current when the battery capacity increases.

For example, The 40A charger can run at 20A (50%).

Further Reading | Discover Renogy’s DC – DC charger current limiting feature

Charger No 2: The Renogy REGO 60A DC to DC battery charger has a variable output current that can be set via the Renogy App. This feature is intended to set the maximum output current for a leisure battery.

The variable current range is 10,20,30,40,50, and 60 amps.

Both chargers are perfect if you plan to increase the battery capacity in the future and will prevent you from needing to upgrade the DC-DC charger, saving precious time and pennies. 

DC-DC + MPPT Solar Charger

Are you planning to install solar panels? Then consider a combined DC-DC charger and solar charge controller. These units are becoming increasingly popular with campervans because they’re more cost-effective, easier to install (one unit instead of 2) and save weight and space. 

Renogy DCC30S DC to DC charger with MPPT Solar
Renogy DCC30S DC to DC charger with MPPT Solar

Alternatively, you would need a separate DC to DC charger and a solar charge controller.

However, unlike solar charge controllers, the DC-DC + solar chargers have limited solar panel voltage and power inputs. Overloading the input voltage and power may cause damage to the charger. 

We’ve already discussed how to calculate what size DC-DC charger you need. However, to calculate the solar input of a combined DC-DC + solar unit, you will need to know the open circuit voltage of your solar panel array. 

Follow the steps below or jump to the calculator;

How To

How To – Calculate The Solar Power Input For A DC-DC Charger

Step 1) Open circuit voltage (Voc) of the solar panel.

Each solar panel will have a specification label stating the open circuit voltage; it may be labelled as Voc. We will use Renogy 100 amp solar panels as an example.

For example, the VOC of the 100A Renogy solar panel is 24.3 volts.

Renogy 100 watt solar panel electrical data with open circuit voltage highlighted

Step 2) How many solar panels?

This is the number of solar panels that will be connected together as an array.

For example, 2x Renogy 100A solar panels.

Step 3) How will the solar panels be connected?

Solar arrays are connected in series, parallel or both series-parallel. If you need clarification on what they mean, read about series vs parallel in this post first.

Step 4) How to calculate the total Voc?

Series solar panels = Voc increases (add all solar panel Voc together).

For example, 2x Renogy solar panels in series = 24.3v x2 = 48.6 volts.

Parallel solar panels = Voc remains the same (same value as one solar panel)

For example, 2x Renogy solar panels in parallel = 24.3 volts.

Step 5) Use this Voc value to compare with the DC-DC + solar charger specifications.

Maximim solar input voltage for the renogy DCC50 DC to DC battery charger with solar input.
Renogy DCC50S maximum solar input voltage

Step 6) The outcome

In our example, the total Voc for the series solar panel array will exceed the ‘Max. Solar Input Voltage’ of the Renogy DCC50S charger. Therefore is NOT compatible.

Total series Voc = 48.6 volts > 25 volts Max of the charger.

However, the total Voc for the parallel solar panel array is less than the ‘Max. Solar Input Voltage’ of the Renogy DCC50S charger. Therefore, IS compatible

Total parallel Voc = 24.3 volts < 25 volts max of the charger.

Calculator #5

Maximum Solar Input Voltage



DC-DC solar chargers have a maximum solar Voc value. The solar panel total Voc must not exceed the DC-DC charger maximum solar Voc as this will damage the charger.

Renogy 100 watt solar panel electrical data with open circuit voltage highlighted
Renogy 100 watt solar panel electrical data with open circuit voltage highlighted

For Example, the Renogy DCC50S has a maximum Voc of 25 volts. 2x Renogy 100w solar panels wired in parallel have a combined VOC of 24.3 volts. The solar panel Voc is less than the chargers max Voc, so it’s ok. However, 2x Renogy 100w solar panels wired in series have a combined VOC of 48.6 volts. Exceeding the maximum Voc of the charger.

Here’s a comparison table of Voc values for the top 5 DC to DC + MPPT solar chargers:

NameMaximum Solar Voc*View
Renogy DCC30SRenogy DCC30S DC to DC charger with MPPT Solar25vView Renogy Product
Renogy DCC50SRenogy DCC50S DC to DC charger with MPPT Solar25vView Renogy Product
Redarc BCDC 1225DRedarc BCDC1225D 25A DC DC Charger MPPT Solar Controller32vView Redarc Product
Redarc BCDC 1240DRedarc BCDC1240D 40A DC DC Charger MPPT Solar Controller32vView Redarc Product
CTEK D250 SECTEK D250SE DC to DC battery charger23vView Ctek Product
Sterling BBS1230Sterling BBS1230 battery to battery solar charger31vView Sterling Product

Typically, solar panels wired in parallel are more suited to DC-DC solar chargers because the open circuit voltage remains low.

If the total solar panel Voc exceeds the maximum values of the charger, consider a separate DC-DC charger and solar charge controller.

Further Reading | The Best DC To DC Chargers With Solar input For Campers


By determining the maximum size charger your campervan’s electrical system will allow and using the FREE electrical calculators, you should have a clearer idea of how to size a DC to DC charger for your campervan. 

Still unsure what size DC to DC charger you need? No worries! Try our free DC to DC charger electrical calculator – simply fill out the form and let us do the rest. Our expertise and information in this blog post will help us determine what size DC to DC charger is best for your needs. 

Once you know what size DC to DC charger you need. The next step is to find the best DC to DC battery charger for your campervan.

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