Why a DC-DC Charger is a Must-Have for Deep Cycle Batteries


A question I get asked quite often, especially by those who are new to kitting out a vehicle for long distance travel, is what is a DCDC charger and why is it important and is a dual battery system necessary. In this post I will explain it all without using any of the technical jargon. Also, I will not be going into new technologies such as Lithium, as I will discuss this in a separate post. 


Long-distance travel often involves extended periods without access to external power sources. With a dual battery system, you can power your 12V accessories for extended periods without worrying about depleting the primary starting battery. 

12V accessories, such as refrigerators, lighting, communication devices, and camping equipment, require a reliable and continuous power source. A dedicated second battery ensures that the power supply is separate from the vehicle's primary starting battery thus preventing draining of the starting battery and ensures that essential vehicle functions are not compromised.



The vehicle's main battery (as supplied by the vehicle manufacturer) is always a quick discharge cranking battery whilst the second battery which powers all the accessories is a slow discharge battery commonly known as  deep cycle battery.

Cranking Battery: A cranking battery, also known as a starting battery, is designed to deliver a high burst of power (amps) for short periods, typically to start an engine. It features thinner lead plates with larger surface areas, which allow for quick and powerful energy releases during engine cranking. 

Deep Cycle Battery: A deep cycle battery is specifically designed to provide a steady amount of power over an extended period. It is built with thicker lead plates and has a larger capacity for storing energy. These batteries are constructed to deliver a slow discharge over a long period of use and withstand repeated deep discharges and recharges without significantly impacting their performance or lifespan. Cranking batteries are not designed to handle deep discharges and recharges as a deep cycle battery can.

It's important to note that while some batteries may be labeled as dual-purpose or hybrid batteries, designed to offer a balance between deep cycling and cranking capabilities, they might not perform as efficiently as specialized batteries in their respective applications. It's recommended to select a battery specifically designed for the intended use to ensure optimal performance and longevity.



So now that you understand the difference between the two types of batteries and how they suite the two different purposes, the next part to understand is how do we keep these two batteries optimally charged. If I was to describe this by way of an analogy think of the cranking battery as a sprinter and the deep cycle as a long distance marathon runner. Each needs two different types of training and each has to sustain and discharge their energy in different ways. 

In many dual battery systems I have come across it comprises of a simple isolator or a voltage-sensitive relay (VSR). What this effectively does is distribute the charge coming from the vehicle's charging system (powered by the alternator) between the two batteries. The vehicle's charging system's primary role is to charge the starting battery. Once this happens the system goes into a maintenance mode keeping the battery "topped up" because continued charging will lead to over-charging resulting in battery damage. The down side to this is (due to the different charging requirements of the two batteries as explained above) the deep cycle will not get fully charged. In other words whilst both batteries may show 12v when tested with a volt meter (or a voltage meter on your vehicle) this does not mean the deep cycle has a full compliment of amps. For example a deep cycle battery rated at 120Amh may not necessarily have the full 120. The Amps in a battery is as equally (if not more) important than the voltage of a battery. 

 My (crude) sketch illustrates how as soon as the charging battery is full and the charging system goes into a maintenance mode, the deep cycle still remains not fully charged in terms of amps. Think of it as a glass half full. Still has water in it but not enough to quench the thirst.



A DC-DC charger, also known as a battery-to-battery charger or a smart charger, is commonly used to keep a deep cycle battery optimally charged. 

As explained above, due to the design nature of deep cycle batteries they need to be charged quite differently to a starting battery. The charging is done  in multiple stages, including bulk charging, absorption charging, and float charging. A DC-DC charger is designed to provide the necessary voltage and current levels at each stage to ensure a proper and controlled charging process. This process will continue even after the charging system of the vehicle goes into "top up" (or maintenance) mode. 

Deep cycle batteries require precise voltage regulation during the charging process. Overcharging or undercharging can lead to reduced battery life or performance. A DC-DC charger monitors the battery voltage and adjusts the charging voltage accordingly to prevent overcharging or undercharging

 Furthermore since deep cycle batteries are often used in applications where the input voltage source can fluctuate, such as in off-grid or renewable energy systems (solar panels or blankets), a DC-DC charger can handle input voltage variations and provide a stable charging voltage to the deep cycle battery, compensating for fluctuations in the input power source.

I hope this has been of use to you. If you have any questions (or comments) please write to me

This article is in a series of other articles which can be found in the EDUCATIONAL section of the website. These are:

-Difference between AGM and Led Acid batteries.

-Understanding Usable Power Capacity of a Deep Cycle Battery 

-How to chose a deep cycle battery for your consumption.

-How to maintain batteries when parked for a prolonged period of time.



Duncan Udawatta



  • Hi Alwyn

    Please check email. I have replied to your question.


  • Hi Duncan
    Can I use a 100 Ah 12 V lithium ion battery as a second battery ( in place of my deep cycle) – with a DC -DC charger.
    2nd battery is under bonnet. 2.4 D Hilux .

    Alwyn Visser

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