The second battery in the mobile home


Why a second battery?

As soon as electrical consumers are operated in the mobile home, a second battery (also known as a supply battery or auxiliary battery) is recommended and, in most cases, even necessary. Such consumers are: a refrigerator, lighting in the living room, a music system, even an inverter for the 220V power supply. The second battery supplies the electrical devices (“consumers”) with energy when the van’s engine is not running. If all consumers were to draw their power from the starter battery, it would be empty after a short time, and the engine could no longer be started. A second battery makes sense if electrical consumers are to be operated when the engine is switched off.

Starter battery vs. consumer battery

The starter battery of a car or mobile home is intended for exactly what the name suggests: starting the engine. Starter batteries are designed for exactly this task: they deliver a very high current for a short time. In addition, they are rarely used when the engine is not running and are almost always fully charged.

The requirements for a consumer battery are different: as a rule, they do not have to deliver very high currents but rather smaller currents over a longer period. In addition, a consumer battery must also withstand deeper discharges than the starter battery since the battery may not be fully charged again for several days.

Different battery types for the mobile home

The RV battery types that come into question as consumer batteries can be divided into three types:

Lead-gel batteries

The gel battery (VLRA or gel batteries) is based on lead-acid technology. The electrolyte of the gel battery is thickened with silicic acid, and the battery housing is sealed and requires no maintenance.

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Gel batteries have a comparatively high internal resistance – this means that the charging current is lower, and the time it takes to charge fully is longer than other battery types. This is fine for motorhomes that either drive a lot or are often on shore power. However, suppose the charging current is only available for a limited period (e.g., via the solar system, in which the charging current is only very strong for a relatively short time when the sun is at its zenith). In that case, the gel battery may not be able to absorb the energy fully or cannot be fully loaded.

AGM batteries

AGM batteries (absorber glass mat batteries) are also lead-acid batteries. The special feature is that the battery acid is bound in a fleece and therefore does not slosh around freely, as with a starter battery. AGM batteries are also maintenance-free. This means that the case cannot be opened here either. In contrast to gel batteries, AGB has a lower internal resistance and can absorb higher amounts of energy in a shorter time / tolerate higher charging currents. The same applies to energy consumption: if, for example, an inverter is connected to the battery and a strong consumer is operated via it (e.g., an induction hotplate), an AGM battery can provide this power more easily than a gel battery.

Another advantage of AGM batteries is that they can be placed anywhere. This means that AGM batteries can also be installed lying on their side or, theoretically, even upside down.

LiFePO4 batteries 

The LiFePO4 battery is a kind of lithium-ion battery. It is a lithium ion concentration difference battery, with lithium ion in the two electrodes back and forth between decompression. When charging, Li+ is removed from the positive connection and embedded in the negative grid; when discharging, Li+ is removed from the negative electrode and embedded in the positive electrode. Under normal charge and discharge conditions, it generally only causes changes in level spacing and does not destroy the crystal structure. Because the lithium ions have a relatively stable space and position in the positive and negative electrodes, the battery charging and discharging is better reversible, ensuring battery life and work safety.

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In terms of weight, the energy density of lead-acid batteries is generally 50~70wh/g, and the energy density of LiFePO4 batteries is generally 200~260wh/g. The weight energy density of Lifepo4 batteries is 3 to 5 times that of lead-acid batteries, which means that under the same weight, the capacity of LiFePO4 batteries is 3 to 5 times that of lead-acid batteries.

In terms of volume, the volumetric energy density of LiFePO4 batteries is typically around 1.5 times that of lead-acid batteries, so for the same capacity, LiFePO4 batteries are around 30% smaller than lead-acid batteries.

Charging the second battery

The secondary battery can be charged from different sources. Often several or all of these charging options are combined.

Charge while driving

The body battery is charged via the alternator in almost every mobile home. During longer journeys, the second battery is charged very reliably. The installations differ depending on whether a simple isolating relay or a high-quality PWM controller is used.

Charging via a solar system

Charging via a solar panels system is particularly interesting for those who like to stand in a parking space for 5 days or more and do not want to do without electrical energy. The installation effort is higher than the other options, but the solar system is the energy supplier with the highest independence factor. Shore power is unlikely to be available on the beach in Portugal – and leaving the engine running for hours to recharge the battery isn’t an option either.

Charging via a shore power connection

A shore power connection is standard on board many mobile homes. The blue plugs are the common tool to connect the motorhome and charge the battery on official campsites with electricity. We deliberately avoided using shore power during our expansion, as we can cover our energy requirements with an alternator and solar system. There are only rare opportunities to use shore power.

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