A Comprehensive Guide to Battery to Battery Chargers: Everything You Need to Know

Beach Van

A Comprehensive Guide to Battery to Battery Chargers: Everything You Need to Know

Battery to battery chargers, also commonly known as DC to DC chargers, are fitted between your engine battery and leisure battery system. They take the voltage produced by the alternator of the vehicle and turn this voltage into a preferred charge voltage and current better suited to your leisure batteries. These devices are critical to the longevity of a system. Batteries with a more complex make up cannot just be blasted with 14.4V straight from a alternator, they require a delicate three stage charge process.

Understanding battery to battery chargers:

Battery to battery chargers are electrical devices designed to convert the direct current (DC) power from a vehicle’s starting battery into a suitable charging voltage for auxiliary batteries. Unlike conventional battery chargers, which typically rely on alternating current (AC) sources, DC to DC chargers are specifically designed for mobile applications and can efficiently charge the auxiliary battery system while the vehicle is running.

1. Voltage Conversion: The charger steps down the higher voltage output from the alternator to a suitable charging voltage for the auxiliary battery system. This voltage conversion ensures that the battery receives the optimal charging voltage without overcharging or damaging the cells.

2. Multi-Stage Charging: Battery to battery chargers often incorporate multi-stage charging algorithms. These algorithms monitor the auxiliary battery’s state of charge (SOC) and adjust the charging voltage and current accordingly. The charger begins with a bulk charging stage, delivering a higher current to rapidly recharge the battery. Then, it switches to a absorption and float stage to maintain the battery at full capacity without overcharging.

3. Charge Isolation: Battery to battery chargers provide charge isolation between the vehicle’s starting battery and the auxiliary battery system. This ensures that the auxiliary battery is charged independently and prevents any power drain from the auxiliary battery to the starting battery, which could potentially leave the vehicle stranded.

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How They Work

As mentioned above in the multi-stage charging section, battery-to-battery chargers utilise a three-stage charging process. The three stages are:

1. Bulk Stage: During this initial stage, the charger delivers a constant current to the leisure batteries. It charges the batteries rapidly until they reach a predetermined voltage, typically around 80%.

2. Absorption Stage: Once the leisure batteries reach the predetermined voltage, the charger switches to the absorption stage. Here, the charger supplies a constant voltage, allowing the batteries to absorb the remaining charge gradually. This stage prevents overcharging and maintains a healthy lifespan for the batteries.

3. Float Stage: After the absorption stage, the charger enters the float stage. The charger continuously monitors the leisure batteries voltage and maintains it at a safe level, preventing overcharging. This stage is essential for long-term battery health, as it keeps the batteries ready for use without any risk of damage.


The Big Question: Isolated or Non-Isolated?

An isolated battery to battery charger and a non-isolated battery to battery charger differ in terms of their electrical isolation and how they handle the transfer of power between batteries.

Here’s a breakdown of each type:

Isolated Battery to Battery Charger:

Electrical Isolation: An isolated charger physically separates the input and output circuits using a transformer. This isolation provides a complete separation between the input and output voltages, ensuring there is no direct electrical connection between the two batteries.

Separation: Isolated chargers are incredibly useful for breaking up ground loops, thus separating parts of a circuit that are sensitive to noise. Electrical noise occurs when electrical signals produce undesirable effects in the electronic circuits of the control system. These electrical signals can affect sensitive equipment such as can-bus systems.

Safety: As touched on above, isolated chargers offer the safety of the the vehicle electrical system and the leisure system being isolated away from each other. Reducing the risk of interference or even damage to sensitive equipment in your vehicle.

Non-Isolated Battery to Battery Charger:

Electrical Connection: A non-isolated charger does not have physical separation between the input and output circuits. The input and output batteries are connected directly through the unit.

Benefits: Non-isolated chargers are typically simpler, smaller, and less expensive compared to isolated chargers. They are suitable for applications where the input and output batteries have the same voltage.

Note: These should only be installed if you can confirm there is a common ‘ground’ between your two battery systems. For example, the metal work of the vehicle could be sufficient to be a common ground between the batteries and therefore the charger can work efficiently. If your campervan or motorhome does not have a metallic body, you should opt for an isolated model. That way, each side of the charger has its own respective ground from each respective battery.

The choice between isolated and non-isolated chargers depends on many factors. Some applications require isolation for safety reasons, and others may benefit from a floating output by breaking up ground loops or shifting of reference voltages. However, where isolation is not required, a non-isolated converter may provide a decrease in cost, size, and/or an increase in efficiency.

What if my engine and leisure batteries voltage differ?

Certain battery to battery chargers on the market are designed to charge one battery from another battery with different voltages. These chargers typically convert the voltage from the engine battery to match the voltage requirements of the leisure batteries. This allows you to charge a battery with a higher or lower voltage from another battery without the need for a direct connection or voltage conversion from an external power source.


When using a battery to battery charger, it is essential to ensure that the charger is compatible with the specific voltage requirements of the batteries you want to charge. Additionally, make sure to follow the manufacturer’s instructions and guidelines for safe and proper usage.

Temperature Regulating Your Battery to Battery Charger

1. Temperature Regulation: Battery to battery chargers, like any electronic device, generate heat during operation. Excessive heat can not only damage the charger but also reduce the charging capabilities of the charger itself. With heat comes resistance and with resistance comes a lower charging current as the charger is having to work harder.

2. Preventing Overheating: In a campervan, the charger may be located in a confined space or near other heat-generating components like batteries or inverters. In such conditions, the charger may be prone to overheating, which can lead to reduced performance or even premature failure. Cooling mechanisms such as fans or heat sinks help dissipate the heat and keep the charger operating within its optimal temperature range. The surface in which you mount your battery to battery charger plays a huge part in how heat is dissipated away from the charger, therefore think about mounting the charger on a non combustible material such as an aluminium sheet.

3. Efficiency Optimisation: Battery to battery chargers are designed to convert power from one voltage level to another, typically from the campervan’s engine battery to auxiliary batteries. Cooling the charger can help maintain its efficiency by preventing heat buildup, which can cause energy losses. When the charger operates at a lower temperature, it can deliver power more effectively and maximise the charging efficiency.

4. Safety Considerations: Overheating can pose safety risks in a campervan. High temperatures can increase the likelihood of electrical malfunctions, short circuits, or even fires. By cooling the battery to battery charger, you mitigate these risks and promote a safer environment within the campervan. By installing the charger on a non-combustible surface you will help to dissipate the heat and mitigate any potential hazards.


Sizing your battery to battery charger

To size a battery charger for your campervan, you’ll need to consider a few key factors. Here are the steps to help you determine the appropriate battery charger size:

1. Determine your battery type and capacity: Identify the type of battery used in your campervan (lead-acid, AGM, lithium-ion etc) and find out its capacity, usually measured in amp hours (Ah). The battery capacity is an important consideration when sizing the charger.

2. Calculate the charging current: Determine the desired charging current for your battery. Here you must consult the battery’s datasheet. Within the data sheet you will be given the parameters in which the battery can be charged at. This is a vital bit of information when selecting your battery to battery charger.

3. Consider charging efficiency: Take into account the charging efficiency of the battery charger. Not all energy drawn from the charger is converted into stored energy in the battery due to losses in the charging process. Typically, a charging efficiency of around 80-90% is considered reasonable.

4. Account for the size of the vehicles alternator: Here you must determine the size of the vehicles alternator. This can be found on the data sheet of the vehicle. Although this will give you the output of the alternator this is by no means the full amount of current you have to play with. You must take into account other loads that are put onto the alternator, such as powered air conditioning, heated seats, lights, heaters and entertainment systems. We at TBE use equations to calculate the loads on the alternator and therefore match you more accurately to a battery to battery charger. To find out more send us a message and we’ll be more than happy to help!

5. Select an appropriate charger: Once you have calculated the charging current required, you can look for a battery charger that meets your specifications. Choose a charger that can provide the desired charging current and is compatible with your battery type.

6. Additional features: Consider any additional features you may need in a battery charger, such as multi-stage charging, built-in safety features, compatibility with solar panels or alternators, and user-friendly interfaces.

7. Seek expert advice if unsure: We at Tiny Build Electrics can help you with choosing the correct battery to battery charger. Once we’ve matched your requirements with the correct charger we can also supply the item.

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Tom Alderdice

Hey, I’m Tom – founder of Tiny Build Electrics

My mission is to help sustainable-minded folks develop their electrical knowledge, giving them the confidence to do their own tiny build electrics.

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