Charging your auxiliary battery while driving |

Charging the auxiliary battery using the alternator

In addition to using solar panels or shore power, you can also charge the auxiliary battery while driving. This can be done with a DC-DC charger or a split charger/relais switch.

1. How to charge the leisure battery while driving

When you drive, your vehicle’s alternator generates power. This power is used for all electrical parts in your vehicle. You can think of the headlights or the radio. But what is particularly interesting is that the alternator also charges your starter battery. This principle can of course also work for your auxiliary battery!

2. What you should know before charging your leisure battery via the alternator

You can choose from two options for charging your auxiliary battery while driving. Before we discuss the two options, it is important to know what a ‘smart alternator‘ is, and whether your campervan has one.

What is a smart alternator?

There is no such thing as an smart alternator. When people speak of an smart alternator, they are talking about the Battery management system of your bus/campervan. This system tries to keep engine emissions as low as possible.

When this system detects that the starting battery is fully charged, the battery is disconnected. Then, when you decelerate the engine again, the system will reactivate to reuse the energy (regenerative charging).

In this blog, we do keep using the word smart alternator. But now you know that the alternator is not smart, but it is controlled by a B.M.S.

Does my vehicle have a smart alternator?

There are several ways to check if your vehicle has an intelligent alternator.

  • The most reliable way is to measure the voltage while driving. Place a multimeter on the (starter) battery and watch the voltage. If the voltage fluctuates between 13.8 and 14.7 volts, and indicates 14.7 volts during braking, you have an smart alternator;
  • Check the negative pole of your starter battery, is there a small device on it? Then you have an smart alternator. The small device is called a shunt;

3. Two ways of charging via the alternator

1. Split charger / Relay switch (Not suitable for a smart alternator)

This type of charger switches the leisure battery and the starter battery together, charging the leisure battery. This charger monitors (while driving) when the voltage of the starter battery is high enough (above 13 volts) and then pairs the leisure battery together. If you switch off the vehicle again, the voltage in your starter battery drops and the leisure battery is disconnected again.

Lead-acid batteries
For lead-acid batteries, these chargers are suitable for recharging up to about 80%. For the last 20%, they do not work as well. This is because the alternator, charges your starter battery with a voltage between 14 and 14.5 volts. So your leisure battery also charges at this voltage. This voltage is not optimal for charging a lead-acid battery and can therefore take a long time.

Lithium batteries
Charging a Lithium battery is a lot faster compared to a lead-acid battery. Because a Lithium battery has little internal resistance, it charges at a high current. This sounds very nice, but the high amperage can overload your alternator. It is therefore better to use a DC-DC charger for Lithium batteries.

Split charger |

2. DC-DC charger (suited for intelligent alternator).

These are intelligent chargers; they have different charging profiles making them suitable for any battery. The DC-DC chargers come in different sizes, ranging from 18 to 100 amps. Because you can choose which amperage to charge your battery with, it protects the alternator from overloading.

The installation is relatively simple and can be switched on when the engine starts. And can be switched off when the engine stops. If you are now going to place a lead-acid battery and in the future still want a Lithium battery. Then you can easily adjust the charging profile with this charger.

4. Additional Information

This section explains what else you should consider before purchasing a battery charger.


It is important to use the right cable sizes and fuses. This blog post goes into more detail about that. The Electrical Wiring Diagram calculates cable thickness and fuse size.

How large (amps) should my battery charger be?

The charging current should preferably not exceed 20% of the battery capacity (20A for a 100 Ah battery). The temperature of a battery will increase by more than 10°C if the charging current exceeds this percentage and shorten its life. This concerns lead-acid batteries.

Why doesn’t a split charger / relais switch work on a smart alternator?

A smart alternator tries to keep emissions as low as possible, therefore the starter battery is not always charged.

A split charger connects the starter and leisure battery in parallel between 13 and 13.8 Volts. If the starter battery is not charged, the voltage cannot be reached and the split charger will not connect the two batteries together.

5. Making a choice

To reach your own choice, there are a number of factors to consider.

Do I need ‘charging while driving’?

The answer to this question depends on a number of factors:

The weather: Do you mainly go out in summer with nice weather and do you have solar panels? Then you can choose not to install them.

Camping: Are you mainly camping and do you hook it up to shore power here? Then it is not necessary.

Full-time/part-time: If you travel full-time in a campervan, you probably need all the power you can get.

We would always recommend charging your household battery while driving, and preferably with a DC-DC charger.

6. Our choice

We drive a 2011 Mercedes Sprinter with an intelligent alternator. The choice was obvious: a DC-DC charger. Our daily (24 hour) consumption is around 40 Ah and we drive a maximum of two hours per day. Considering our consumption and the way we travel, we chose a 20A DC-DC charger.

Dc-DC charger placed neatly in a wiring diagram? You can! And to give you a hand, we have developed 3 files in which you can design your unique schematic.

Hey! By the way… any links on this page that lead to products on Amazon are affiliate links and I earn a commission if you make a purchase. Thanks in advance – We really appreciate it!

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