What is a solar battery?

Solar batteries are supply batteries designed for the continuous supply of small amounts of electricity. They are charged by the photovoltaic system on the roof.

Differentiation from car and starter batteries

Solar batteries differ fundamentally in their design from the starter batteries used in vehicles, which have to provide very high currents for a short period of time. To ensure this, lead-acid batteries have large plate surfaces. The cells also integrate many thin electrodes connected in parallel. Constant discharging of more than 50 percent of the nominal capacity leads to premature wear. Car batteries are therefore not suitable as a second battery in a caravan. Solar batteries tolerate the continuous supply of small amounts of electricity and deeper discharges better; the manufacturers optimize the storage units for this type of application. The usable capacity is higher. So-called deep-cycle batteries can be discharged by up to 70 percent without shortening their service life. To protect against damaging deep discharge, the solar charge controller integrates deep discharge protection.

Requirements for solar batteries

Solar batteries serve as intermediate storage for electrical energy, they constantly supply electricity to the connected consumers. Important properties are therefore

• Sufficiently high charge absorption
• high cycle stability
• low self-discharge
• low-maintenance operation

The advantages of the different battery technologies for solar batteries

Brand manufacturers such as Varta, Exide and Ective offer storage batteries as wet batteries, gel batteries and AGM batteries. Extremely powerful and long-lasting lithium solar batteries are available for the most demanding requirements. Differences include the usable storage capacity (Ah) and the cycle stability.

Wet battery, EFB & SMF

• Properties: for occasional use
• Cycles at 50 % DOD*: approx. 400
• Energy density: +
• Price range: very affordable

Gel (VRLA)

• Properties: minimal self-discharge, ideal for seasonal use
• Cycles at 50 % DOD*: 400-800
• Energy density: ++
• Price range: favorable

AGM (VRLA)

• Features: also delivers higher currents for demanding applications
• Cycles at 50 % DOD*: 400-1,000
• Energy density: +++
• Price range: medium

LiFePO4

• Features: the ultimate in maximum performance and longevity
• Cycles at 50% DOD*: up to 6,500
• Energy density: ++++
• Price range: expensive

* 50 % DOD (depth of discharge): The number of cycles indicates how many discharges up to 50 percent of capacity are possible with subsequent full charging. In the product descriptions you will find further information on the cycles at 30 % DOD, 50 % DOD, 80 % and 100 % DOD.

Wet batteries, EFB and SMF batteries for storing solar energy

Lead-acid batteries with liquid electrolytes have been continuously developed and are now optimized for various applications. Wet batteries designed as supply batteries with particularly thick lead plates reliably supply energy even at discharge depths of up to 75 percent. There are dual-purpose batteries that can be used as starter and supply batteries.

EFB batteries (enhanced flooded batteries) are a further development of classic lead-acid batteries. They integrate a polyfleece material. Models with deep-cycle technology tolerate high cyclic applications better. Thanks to reliable performance with a low state of charge, they are well suited to the use of renewable energies.

There are also maintenance-free SMF (sealed maintenance free) solar batteries with calcium technology, which are very resistant due to the special plate construction. They are a favorable choice for operation with solar, inverters and as a second battery in the vehicle.

VRLA batteries as storage for solar power

VRLA batteries (valve-regulated lead-acid) have a sealed housing and integrate a pressure relief valve. They do not emit gas and are suitable for use in enclosed spaces, such as in motorhomes or boats. Water decomposition is reversed internally, eliminating the need to refill with distilled water. VRLA batteries cannot leak, can be used in an inclined position and are safe. In the event of severe overcharging, the pressure relief valve triggers and the storage tank is defective.

Lead-gel battery (VRLA)

In the lead gel accumulator, silica binds the electrolyte, which is no longer liquid but gel-like. A major advantage is the very low self-discharge, which is why gel batteries are ideal for seasonal and occasional use. The internal resistance is relatively high, which is why a solar battery with gel is not suitable for the short-term delivery of high currents. In the solar sector, models such as the Exide Equipment Gel and other lead-gel batteries are a popular choice.

Lead-acid AGM battery (VRLA): Storage with fleece for solar

In batteries with AGM technology (absorbent glass mat), glass fiber fleece binds the electrolyte. Due to the low internal resistance, the solar batteries can also provide high currents. There are numerous dual-purpose variants to choose from, which you can use to start the vehicle's engine and provide a continuous supply of power to small consumers. Thanks to the powerful output, you can also connect an inverter.

Long-life deep-cycle batteries are advanced AGM batteries that can withstand up to 1,000 charging cycles at 50 percent DOD. These solar storage batteries can withstand discharges of up to 80 percent better, so you can draw more current without any negative effects on the service life.

LiFePO4 (lithium iron phosphate)

LiFePO4 batteries outperform lead-acid batteries with extremely high cycle stability, maximum energy density and minimal weight. With regular battery discharge of 50 percent DOD and subsequent full charging, up to 6,500 charging cycles are possible with lithium batteries. The additional investment therefore pays off in the long term, the price-performance ratio remains simply unbeatable. They are the first choice for continuous use as solar batteries.

What capacity (Ah) does my solar battery need?

You can determine the right size of solar storage by drawing up an energy balance of the connected devices. In this way, you can calculate the daily power consumption (in Wh) that the storage system needs to provide.

For the calculation, add the electricity consumption of consumers such as light, water pump, TV, refrigerator and voltage transformer to the expected period of use.

Important: Remember that the solar batteries should only be discharged to 50 percent for optimum durability, depending on the model.

Example calculation for power consumption

The following values are examples. Information on actual consumption can be found in the manuals of the electronic devices.

Compressor refrigerator: 40 watts * 8 hours = 320 Wh Light: 10 watts * 4 hours = 40 Wh Laptop: 50 watts * 2 hours = 100 Wh Total: 460 Wh / 12 volts = 38 Ah daily power requirement

Required capacity: If the solar battery is only to be discharged to 50 percent for a maximum service life, the battery storage system must therefore have a capacity of at least 76 Ah. As the sun does not shine optimally every day, you need a bad weather reserve. For three days of independence from a power socket and sunshine, you need a solar battery with 12 V and a capacity of 230 Ah. If you invest in a LiFePO4 battery, a capacity of around 135 Ah is sufficient, as you can discharge it much deeper without damaging it.

A classic solar battery with 280 Ah is a popular choice in larger motorhomes, for example, while a solar battery with 120 Ah is usually sufficient for compact campers.

Required output of the solar modules

The solar system must be dimensioned so that it can fully recharge the battery storage system in reasonably sunny weather. The power output of the solar modules is given in watt peak (Wp) and stands for the maximum power output.

As a rough guide, a solar output of 1.25 times the battery capacity is recommended for lead batteries. For the above example of a battery for the solar system with 230 Ah, the solar power should therefore be at least 230 x 1.25 = 288 Wp. More power is better and increases your independence. It is often advisable to invest in twice as much solar power.

Summary

• If you only connect consumers to the solar battery occasionally and are rarely on the road with the motorhome, the inexpensive wet batteries with relatively few cycles are suitable.
• Gel solar batteries can withstand more cycles and require less maintenance thanks to minimal self-discharge.
• AGM batteries also deliver high currents for demanding applications and are the preferred choice as a battery for the solar system.
• If you are on the road all year round with your motorhome or boat and discharge and charge the battery daily, it is worth investing in a LiFePO4 solar battery.