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Photovoltaic plant with storage for caravans: guide on how to choose the battery

Published : 07/10/2018 10:50:39
Categories : Photovoltaic Panels , SOLAR CHARGERS

In planning a photovoltaic plant with storage for caravans, the choice of the rigt kind of battery is of great importance. In this article we will show you an overview of batteries available on the market and their peculiarities and differences.

Nowadays, market mainly offers 3 kinds of batteries: lead AGM batteries, lead gel batteries and batteries made of lithium or of free acid.

AGM technologies and gel ones are very similar, however these two kinds of batteries show some differences not only by price level, but also by different performances.

Lead AGM batteries are made of a glassy absorbent material. We are talking about a class of accumulators with lead-acid and valve regulated where the electrolyte is contained inside a thin fiberglass matrix, in contrast with what happens in electrolyte-liquid batteries.

As already mentioned these are batteries sealed with a valve system.

Cells of these batteries are very compact and guarantee a better resistance to vibrations if compared to standard ones. Furthermore, AGM batteries self-unloading is very low (less than 3% per month) but, above all, they are being constructed in order to not to be opened, in fact AGM battery is loss-proofing and doesn't require maintenance.

Concerning services utilisation, AGM technology is more appropriate because it is more adequate to support various rounds of deep electric shocks. These accumulators, in fact, are completely made by calcium lead cells in which a soaked in acid material is wrapped. This technology allows the electrolyte to evaporate during the charge-discharge phase, at the end of which, produced bubbles mix themself up with acid, letting the battery to mantain an efficient return. These factors make the batteries appropriate to caravans because they can be placed in every position: horizontal, vertical or even overturned.

AGM accumulators are perfect both for cyclical use (the ones which require a lot of charge-discharge rounds) buffer use (that means as a power reserve).

Among the most frequent utilisations we can find:

  • electric engines

  • electric bycicles

  • photovoltaic plants

  • electric vehicles.

Furthermore, they properly work at low temperature and, thanks to the internal resistance, they can be quickly charged and discharged. As far as the depth of electric shocks is cencerned, AGM batteries are theoretically dischargeable up to 80%, however, if daily discharged at this levels, they will be short-lived. For a longer longevity, (up tp 1200 cycles) we recommend to mantain a discharge percentage of 30%-40%. this batteries are usually recommended for installations on caravans due to their price and quality performances.

Gel batteries are considered to be state of-the-art solutions in the field of energy accumulators. In this case, the electrolyte doesn't show itself in a liquid form but in the form of a gel that arises from an addition of silicium in predetermined concentration with the advantage of significantly reduce gas and volatility of the cell. These batteries, as AGM ones, are hermetic batteries. They are internally composed by a plate of lead, calcium and jelly acid. This jelly mixture allows the accumulator to benefit of considerable advantages. For example, the battery itself does not provide charging stoppers anymore, but on the contrary is completely sealed limiting in this way the leakage of gas and liquids thanks to particular pressure valves that cause internal chemical recombination (VRLA).

Advantages of gel batteries:

  • they do not cause spillages in acid

  • they do not require any maintenance

  • they can live 3 times more than a common battery.

However, they are less apropriate for caravans because they are more sensitive and they can get easily damaged in time if discharged too rapidly due to the difference of temperature that is generated in the internal part of them. In this case it may be that some gas plates over the cells will be formed, causing gel solidification with consequences on elettric performance and the life expectation of your battery. Due to these characteristics, we recommend to use them for a buffer utilisation and not for a cyclical one, even if they can provide high performances. Furthermore, gel batteris are appropriate to absorb a higher quantity of energy because they can resist at deeper energy shocks. Concerning the price, gel batteries are a bit more expensive then AGM, mantaining a great ratio between money and efficiency.

Lithium batteries differ a lot, made with a more recent technology. They are commonly employed in consumer electronics, for example in laptops, smartphones and electric cars. Compared to lead batteries, lithium batteries provide a deeper power surge that can get to 80-90%. In this way, they allow to optimise solar energy at best.

Another important data to highlight in terms of performances is that lithium batteries live more than the other two kinds of batteries above mentioned. In fact, if you use a 80% depth of discharge and a work efficiency of 92%, life span of ion, lithium batteries can resist up to 7000 charge-discharge cycles (at the expense of the 1200 cycles of AGM an the 1500 of gels ones). In practice, assuming a daily cycle, you can use them up to 15-20 years, such as batteries for photovoltaic panels. Others pluses for this technology are the faster charge speed (0,5C instead of 0,2 C) and the lower weight compared to lead batteries. An important flaw, however, is represented by the economical point of view. The price is significantly higher, with a unit cost that almost doubles the one of other batteries, in fact you can estimate an indicative cost of 700 €/kWh, but, for their higher depth of discharge, for a catchment of 3,0 kW of power, it will be sufficient an accumulation from 2kW to 5kW.

Free acid batteries are mainly used for the vehicles start-up. Traditional WET free acid batteries are acid-lead accumulators. This typology of batteries is one of the most classical and economical battery on sale. The electrolyte consists in sulphuric acid and it is present inside the various cells and, for this reason, it needs a perfect installation on the plan. This particular installation makes it possible that this typology of accumulators handle with operating temperatures even above average, but during the installation it is necessary to take note of the fact that free acid batteries emit gas externally, and, for this reason, you must install them outside the passenger compartment. Compared to gel and AGM accumulators, they offer an average resistance at cycles, at deep discharge and at self-unloading, furthermore they require a minimum degree of maintenance. The weight of this battery is even inferior to AGM one because plates are thinner even if they are more numerous.

Capacity cues are good. This is an aspect that makes them appropriate to be start-up batteries together with resistance to vibrations and sensitivity at charge, while at price level, a free acid battery is averagely offered at a bit more than the half of one as the same power in gel and more or less 30% less than an AGM of an equal rank. A last but not least parameter to pay attention to is the one about maximum recharging tension and maximum recharging power: this kind of accumulators supports, in fact, higher maximum tensions (up to 15,8 V) and maximum power measured in 1/10 of the total capacity in Ah.

Considering what has already been mentioned, even if lithium batteries have various advantages and very high performances, AGM batteries are still more appropriate for a stand-alone photovoltaic plant for caravans, and guarantee perfect performance levels and lower costs. Obviously, the acquirer will be the one who will value advantages and disadvantages according to his needs.

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