Battery to Battery Charger: How it is Important for Charging Your Vehicle’s Batteries

It would be best to have a continuous power supply to your house batteries when going on long trips.

The house batteries help light up the sockets, switchboards, heating systems, and water pumps and to activate the fridge.

You can charge these batteries through an alternator (DC to DC), solar and shore power.

In this blog post, we will read about battery-to-battery/DC-to-DC chargers and their functions and compare them with other alternatives.

Why do we need a battery to battery charger?

A vehicle’s battery gets charged with the help of its alternator, which takes energy from the engine and is programmed to know when the battery is charged and when it needs to supply power.

However, in campervans, alternators do not know the battery requirements, and that’s where you need a battery-to-battery charger, also known as a DC-to-DC charger.

With the help of a battery-to-battery charger, you can make your alternator generate more energy which then can be passed into house batteries in case you are using a campervan.

These batteries are used in campervans to supply power to different components.

The alternator can be considered as a generator due to the same working principle.

The alternator also generates electricity when water falls on the hydroelectric dam leading to the spinning of wheels, and the engine runs the wheel on the alternator, thus generating electricity.

Alternator charging

Alternator charging

Benefits of using a DC-to-DC charger

Alternator charging has several benefits and is directly related to driving time. 

Campervan Equipped With Solar Panels

Campervan Equipped With Solar Panels

Overcome issues that arise in smart alternators

Nowadays, vans have hi-tech and smart alternators, which reduce power output. It means the alternators cannot charge auxiliary batteries with their load restriction.

However, you can overcome these issues with a DC-to-DC charger, as it separates the primary battery from the vehicle’s alternator.

It converts power output from the primary battery and charges auxiliary or secondary batteries with optimal charging.

Increase the charge of the primary batteries.

The DC-to-DC charger maximizes the main battery’s charge, and it can convert 9 volts amperage into 13.5 volts, thus charging quickly and more efficiently.

If you want to travel long, once charging the battery up to 100% can take you to your destination uninterrupted.

Work with/without solar energy.

With a DC-to-DC charger, you can travel even to areas without sunlight, as you can charge the auxiliary battery with the help of this charger in case the primary battery loses its charge.

Also, the alternator can fill the gap when solar panels fail to meet demand.

Increase the life of the battery.

Charging the battery up to 100% ensures good battery health throughout its life, and an alternator charges the battery to its full potential. 

Compatible with different types of batteries

A DC-to-DC charger is compatible with various batteries; thus, you need not buy a new charger when you replace the battery in your vehicle.

Further, a DC-to-DC charger adjusts the power output depending on the battery type to avoid overcharging, which may lead to battery damage over time.

AC-to-DC charger for laptops

AC-to-DC charger for laptops

Things to consider when you buy a battery-to-battery charger

Whenever you use a new electrical system, charging it from the alternator is advisable. However, it would be best if you chose a battery-to-battery charger wisely.

Decide if you need an alternator.

Alternator charging has become relatively easy and beneficial with the combination of charge controllers and alternator chargers. The only drawback is that they will be a little more expensive.

Some people drive a lot but do not consume much electricity, so they may not feel like using solar panels and want to skip them completely.

A battery-to-battery charger and 12-volt electricity will be the budget option for off-grid electricity.

Consider issues arising from different charging options:

Alternator: With an alternator, you must ensure that the vehicle runs for a while to get meaningful power. It won’t happen in just a 15-minute drive.

Solar power: Solar power is available only when there is sunlight.

So, you will have to bear with the situation in winter or on cloudy days.

However, if there is sunlight, solar power charges the battery continuously, unlike the alternator, which charges the battery only after the engine is turned on for a while.

Shore power: In this, there is no need for driving or solar energy, but you need a power outlet to charge the battery.

Choose the size of amps for battery-to-battery charger wisely

Once you have decided that you need a DC-to-DC charger, you will have to see what should be the size of the charger.

You can conclude by doing the simple math mentioned below.

Firstly, understand that the charger’s size depends on the battery bank’s capabilities. The thumb rule is-

  • For AGM batteries: charger amp rating= battery amp hours capacity *20%
  • For lithium-ion batteries: charger amp rating= battery amp hours capacity * 50%

So, according to this formula, you can use a 20 Ampere charger for a 100-amp hour AGM battery and a 50 A charger for a 100-amp hour lithium battery.

Charging with isolator Vs. battery-to-battery/DC-to-DC charger

You can charge the vehicle from the alternator with any of the two devices, i.e., the isolator and DC-to-DC charger.

Let’s see what’s the difference between the two charging components.


Although the vehicle and the house battery are combined, the house battery remains isolated from the vehicle’s battery.

It’s because of the smart VSR isolator, which ensures that the vehicle’s battery is not drained.

The isolator only diverts the energy towards the house battery when the vehicle’s battery is at an adequate voltage.

Simple devices like isolators don’t work with new vehicles having smart alternators.

Nowadays, vehicles use smart alternators, improving gas mileage as they only charge the vehicle’s battery up to the required limit.

Thus, isolators do not charge the house batteries as much as B2B chargers if you use newer sprinters, transits and promoters. 

Also, you must know that some smart alternators allow high currents to pass through the isolator, because of which the battery’s charging rate exceeds its acceptable limit.

For AGM battery: in this battery, isolators do not give a full charge to the house battery, which may reduce the battery’s life over time as a battery needs a full charge to remain healthy.

On the other hand, solar or shore power will fulfill the remaining requirement, charging the battery up to 100%.

For lithium batteries: In this battery type, a standard isolator works but inefficiently.

You can get a lithium-ion isolator, but its cost is comparable to the B2B options, so it’s better to get B2B chargers only. 

Battery to battery charger:

Battery-to-battery charger not only passes current but also boosts the smart alternator cycling process. These chargers are like “set it and forget it” types.

After installation, they will switch on and off automatically based on the house’s and the vehicle’s battery voltage.

B2B chargers have current limiters that prevent overcharging alternators and provide multi-stage charging to house batteries, increasing their lifespan.

How to wire a DC-to-DC/battery-to-battery charger

The wiring of the DC-to-Dc charger is very simple.

Connect the positive and negative of the chargers with the vehicle’s battery and similarly with the positive and negative to the house batteries.

Don’t forget to add circuit breakers and fuses to both batteries. Circuit breakers are necessary to control the current flow and turn it off/on.

Whenever there is an electrical short, you have to change the circuit breaker and need not replace the fuse.

The breaker size should be 20% greater than the battery current.


A DC-to-Dc charger guides the alternator to charge your house batteries adequately; thus, their selection is critical. With the above information, you can pick suitable DC-to-Dc battery chargers.

Contact Cloom if you need any help regarding the battery charger cable.