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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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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.
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CalculatorsHere 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.).
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
Why bigger is not better
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 VoltageCampervans 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
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 CurrentThe 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;
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 CurrentFind 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 AGM30%30Ah GEL50%50Ah Lithium-ion50%50AhA battery’s maximum charge current can also be found in its specification (online or in the manual).
Renogy 200Ah lithium battery maximum charge current taken from the user manual How To Determine The Maximum Charge CurrentNow 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
Alternator Output Current (Amps)
Maximum charge current <50% of amp rating
Leisure Battery Type
Leisure Battery Capacity (Ah)
Battery Maximum Charge CurrentAmps
Maximum Charge Current Your Electrical System Will AllowAmps
Maximum Size DC To DC Battery Charger For Your CampervanAmps
DC-DC chargers have fixed output currents. Therefore, the max. charge current is rounded down to the nearest 10.
Other things to considerMany 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 capacityTo 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 Type
*
Battery Capacity (Ah)
Usable Battery CapacityAh
Battery Recharge TimeIdeally, 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
Usable Battery Capacity (Ah)
See Calculator #2
Maximum Charge Current
See Calculator #1
Recharge Time (Hrs) From 0 - 100%Hours
To Recharge The Battery By 100%
Recharge Time (Hrs) From 50% - 100%Hours
To Recharge The Battery by 50%
Recharge Time (Hrs) From 80% to 100%Hours
To Recharge The Battery by 20%
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 drivingThe 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
Watts
Volts
AmpsA
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.
Watts
Volts
AmpsA
Journey Time (Hours)
DC to DC Charger Current (Amps)
Appliance 1
Amps 1
Ah 1
Appliance 2
Amps 2
Ah 2
Appliance 3
Amps 3
Ah 3
Appliance 4
Amps 4
Ah 4
Maximum Total Ah Power Used While DrivingAh
Charge Current To The Battery While Powering A LoadAh
Factoring in the power consumption while you drive will ensure you have optimum battery charging.
BudgetThe 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.
DimensionsThe 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.
NOTE
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;
The complete comparison table can be found here.
Future proofingDo 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 ChargerAre 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 SolarAlternatively, 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.
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.
Renogy DCC50S maximum solar input voltageStep 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
Series
Solar Panel Voc
No Of Panels
Maximum Solar VoltageVolts
Parallel
Solar Panel Voc
No Of Panels
Maximum Solar voltageVolts
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 highlightedFor 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:
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
RecapBy 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.
Thanks for the added details. I have only started my research on the LiFePO4, and need to dig a bit deeper on the brand that my solar shop carries. I am very concerned about my Arb 63 and whether the lowest voltage cut-off setting is going to limit me with a LiFePO4 battery. I am a lot weak on LiFePO4 SOC's, and am not sure whether I would be able to get the full 80% discharge. Kinda a big deal, for that kind of money. I am still a long way off from dropping the money on one of those, as I need to get panels first, But more firster is to find a basket for my roof cross racks, then I can figure what size panels I can swing. Trying to get the van correct on the first pass. For now I have my solar suitcase I can run inside, so that helps.
I definitely have a weak charging system, with a factory alt. trying to charge two group 65's in parallel to fire up my diesel. You addressed one of my concerns about taxing the alternator too much. I am clear that the LiFePO will suck the life out of any source, if given the chance. The high amp/fast recharge sure looks appealing. I will look more into the Redarc, but it sounds like you can set an amp ceiling on the unit? That would be handy.
Right now the plan is to get a line run from my dual starting batteries into the back of the van. The run will be short, just a few feet, but I am going to size the wires big, overkill for my solar suitcase, but enough to handle a LiFePO draw, if I go that way.
My only mppt controller currently is a Midnight Solar in my Cargo Trailer. It is a pretty big unit, with the fins and all. The other brand of pwm that I have are Sunsaver. Their MPPT is also large. The size of the Redarc really caught my eye. That's a lot of functionality in a small package.
Enough rambling.....
Thanks,
Craig
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