The right onboard battery for your mobile home

Laptops, smartphones, lights, and monitors or televisions all need one thing: electricity. Whether you live in a motorhome like us or go on holiday with it, we all need the same thing: electricity, 12 volts and sometimes 220 volts. If there is no shore power available, we get it from the onboard batteries or supply batteries. In this article, I would like to show you the different types of batteries available on the market, how they differ, and which is best for you.

Why an onboard battery or mobile home battery at all?

Almost every mobile home has at least two batteries on board. One is in the engine compartment, the starter battery. This supplies the starter and light as well as the radio and other consumers at the front of the camper. When starting, a high current is drawn from the battery for a very short time. As soon as the engine is running, everything is supplied by the alternator, and no more current is drawn. This battery (the batteries in the mobile home are nothing else) is made to deliver hundreds of amps for a few seconds. These batteries have a flat cycle. They should only be discharged to 10-20 percent of their capacity.

To supply the various consumers such as light, water pump, television, and satellite system with electricity when there is no power connection nearby, we need a mobile home battery that supplies electricity over a longer period and sometimes does not have to be fully charged. And that’s exactly why there are extra supply batteries in the mobile home. These lithium RV batteries are designed to deliver a few amps for many hours. These batteries have a deep cycle. They can be discharged between 50 and 80 percent of their capacity.

The most important: are cycle stability and charging cycle

Accumulators are recharged again and again. Manufacturers often give the number of charging cycles as a value. A charge cycle can be:

A battery is half-discharged twice and fully charged each time again
A battery is 20% discharged and recharged, then 30% discharged and recharged, and then 50% discharged and recharged
The battery is discharged to 100% and fully charged again

The number of charging cycles the battery survives “relatively undamaged” is called the number of cycles. If a manufacturer states 800 charge cycles, the battery still has 80% of its capacity after 800 charge cycles. So it still works afterward, but the storage capacity continues to decrease.

With onboard batteries, it is important that you can charge them as often as possible. Because everyone wants them to last as long as possible, for this reason, deep-cycle batteries are used as onboard batteries as far as possible. This property, along with cycle depth, is critical and important. Always keep this information in mind when shopping for batteries.

There are different types of batteries

Batteries in mobile homes, caravans, cars, and boats are still almost exclusively lead-acid batteries. These consist of a positive and a negative lead plate and an electrolyte liquid between – diluted sulfuric acid (H2SO4). The marketing departments use all sorts of terms to describe batteries. Supply batteries, construction batteries, and solar batteries are just a few. Don’t let that fool you. These are available in 3 variants:

Wet batteries – as described above. A positive and negative lead plate and diluted sulfuric acid
Gel batteries – here, the lead plates are thicker, and the electrolyte is bound in a gel.
AGM batteries – with these, the electrolyte is bound in a glass fleece.

Recently, however, lithium technology has also found its way into mobile homes. With it, new technology is increasingly coming onto the market, therefore becoming cheaper. You know these batteries from your smartphone, model making, or other electronic devices with a battery. In the following, I will explain the individual battery types and list the advantages and disadvantages.

Wet batteries

This type is constructed almost like a starter battery. The thickness of the lead plates and the alloy on which the lead is applied are different. Both of these can significantly improve cycle stability.

If the batteries are discharged too much, sulphation occurs. The lead sulfate crystals combine to form chunks on the surface. The free surface becomes smaller and smaller, and the performance decreases because the electrodes conduct less well. These chunks can also fall to the ground and lead to siltation there. If the mud rises to the electrodes, a short circuit occurs. This also occurs when the batteries remain unused for a long time. This short circuit will destroy the battery fairly quickly.

Wet batteries should not be discharged more than halfway if you continue to discharge them. The battery ages much faster and dies earlier.

If you don’t use your mobile home in winter, for example, then connect it to the electricity. Modern chargers then switch to trickle charging and thus care for the battery! If that doesn’t work, fully charge the battery and disconnect all consumers.

Advantages:

Cheapest battery technology
Relatively simple charging technology is sufficient

Disadvantages:

High-maintenance, the acid level should be checked regularly, and water topped up.
Outgassin – during operation, gases are produced in the battery, which is discharged through a valve. This gas must be piped out of the mobile home, not leakproof – acid could leak, and the battery must be stored in an acid-proof container.
Depth of discharge 50% – 60%, so should not be discharged more than half. The battery must therefore have twice the capacity you need.

Maintenance-free wet batteries

A modification of the wet battery is the VRLA accumulators. Translated, this means “valve-regulated lead-acid battery.” These supply batteries are welded. Unlike normal starter batteries, they do not have small caps at the top for turning off and topping up with acid. They only have a pressure relief valve. This is also one of the reasons why I don’t use starter batteries. In these RV batteries, the electrolyte is thickened and, therefore, cannot easily flow out. We use this in our mobile home.

Advantages:

Cheapest battery technology
Relatively simple charging technology is sufficient

Disadvantages:

Outgassing – during operation, gases can form in the battery, which is discharged through a valve. This gas must be piped out of the mobile home.
Depth of discharge 50% – 60%, so should not be discharged more than half. The battery must therefore have twice the capacity you need.

Lead-gel batteries

As I wrote above, the electrolyte in these batteries is bound in a gel. Furthermore, these batteries have thicker plates. As a result, they can deliver electricity much longer but also charge longer. You also have a better depth of discharge.

The gel often prevents the gases produced in the battery from escaping directly. Only a small amount escapes through a valve. Since this is oxyhydrogen, there should be no ignition sources in the vicinity of gel batteries.

A special charger for gel batteries is required for charging. They are also heavier than those with liquid electrolytes. They tolerate high temperatures much better than low ones. So not ideal for winter camping. They are also not ideal starter batteries, as they do not like large current drains. That doesn’t make them perfect for interacting with larger pure sine wave inverter, either!

Advantages:

Almost maintenance-free

encapsulated design, they are tight and do not leak,

tolerating even deeper than 70% discharge. But it should be reloaded quickly.

Disadvantages:

special charging technology necessary to avoid outgassing

Difficult

AGM batteries

This type is an attempt to combine the above two techniques. AGM vs standard battery, in terms of cycle stability, they lie between wet and gel batteries. Don’t let yourself be discharged too deeply, either. On the other hand, they tolerate similarly high current draws as wet batteries.

In AGM batteries, the electrolyte is in a glass fleece. The battery is encapsulated like a gel battery and is leakproof. She also hardly outgasses.

This type can be charged with higher voltages than the other two. However, it would be best to have a special charger to charge the AGM RV battery. A temperature sensor is mandatory. The batteries are allergic to the heat that occurs during the charging process.

Due to the low internal resistance of the batteries, due to the construction, they hardly discharge. So they are perfect for longer downtimes when the mobile is unused.

Advantages:

Less sensitive to deep discharge than wet batteries
Maintenance free
Low self-discharge

Disadvantages:

Special charging technology required

Lithium batteries

These batteries are based on different technology, and you are probably familiar with them from your smartphone. It’s not the same, but similar. The technology is called LiFePo4.

These batteries are superior to the lead-acid versions in every area. They are smaller, lighter, store more electricity, can easily be deeply discharged, recharge quickly, and are maintenance-free.

My recommendation should go to LiFePo4 batteries for your motorhome. However, they have one disadvantage: they are extremely expensive. While you can buy a 120 Ah wet battery for around 120 dollars, a 90 Ah LFP battery costs 630 dollars.

There are only a few suppliers for these batteries and charge controllers. But in 1-2 years, it will certainly look very different.

The capacity of the batteries

The different battery types have different depths of discharge:

Wet batteries: 50%
Gel batteries: up to 70%
AGM batteries: up to 80%

If you look for batteries, you will see numbers like C1, C5, C10, or C20. Since the capacity of a battery depends on the current drawn, there are these parameters. The higher the current drawn, the lower the total capacity available. C1 means full discharge capacity within 1 hour, and C5 means full discharge capacity within 5 hours.

As an example, the different capacity values for an SBG 12-100 gel battery (100Ah):

C1 = 60.7Ah, 60.7A can be drawn for 1 hour until it is completely discharged.
C5 = 73Ah, 14.6A can also be drawn for 5 hours until it is completely discharged. The available battery capacity is 17% higher with the lower current draw.
C10 = 82Ah, 8.2A can be drawn for 10 hours before the battery is completely discharged. The capacity is now already 26% higher than with a one-hour discharge.
C20 = 100Ah (the nominal value of the battery). Now, 5A can be drawn for 20 hours. The capacity of the battery is thus at its highest.

How do I charge the batteries in the mobile home?

There are several ways:

Power generator – unfortunately, very loud and, therefore, extremely annoying for you and the neighbors. In addition, such a generator often only supplies a little more than 12 volts. That’s not enough to fully charge the batteries.
Shore power – almost all motorhomes charge the batteries automatically when shore power is connected.
Alternator – often does not work as desired. The alternator charges both the starter battery and the supply battery. The supplied battery only gets a little power if the starter battery is full. In addition, the cables, often too thin, reduce the voltage below 14 volts. This means that the motorhome battery never gets full. Battery boosters help here. These transform the voltage back up to 14.4 volts. They also require a constant high current from the alternator. This means that the body batteries are fully charged on longer journeys.
Solar – Ground mounted solar panels, charge controller, and cable, and the batteries are charged as soon as the sun shines. We use this path and can stand freely indefinitely, even on a few rainy days. In the article Solar module on the mobile home, I described how to do it.

End-of-charge voltage

The end-of-charge voltage is the voltage that must not be exceeded during the charging process. Otherwise, the grid will rust, which is noticeable through the “gassing” of the battery. This creates highly explosive “oxyhydrogen” and permanently damages the battery.

While wet batteries can cope with exceeding this end-of-charge voltage for a short time without being destroyed immediately, AGM and gel batteries react extremely sensitively if the end-of-charge voltage is exceeded.

The level of the end-of-charge voltage depends on the battery technology. Depending on the manufacturer, the end-of-charge voltage is 14.7V for wet batteries and 14.4V for AGM and gel batteries.

Float voltage, float charge

The float voltage is the voltage the charger should hold after full charging to counteract the battery’s self-discharge.

Float voltage is typically 13.7-13.8V. for all battery types.

End-of-discharge voltage

The end-of-discharge voltage is the voltage that must not be fallen below when discharging. If it continues to be discharged, the battery will reach the point of total discharge and be damaged.

Open circuit voltage

The open circuit voltage is the voltage that can be measured across the battery after at least 4 hours have passed since it was last charged or discharged. It is an approximate measure of the battery charge level.

Conclusion/purchase recommendation:

Never buy a starter battery. I explained why above.

A classic, deep-cycle, maintenance-free wet battery is right for you if you only have a small budget available. The same applies if you don’t have special battery requirements and your RV is moved often.