After having approached installation of photovoltaic panels, it seems logical to us to cover the subject of batteries, essential for the realization of an autonomous solar installation or to increase its solar self-consumption. Indeed, if you have opted for an off-grid installation that is therefore not connected to the electricity grid, the electricity produced by your solar panels will have to be stored in batteries so that it can then be used as needed. In self-consumption, you may also want to store part of your energy produced, rather than reselling it directly on the network. To do this, different choices of batteries are available to you.
Let’s take stock of stationary solar battery technologies in 2022.
Reminders on how a battery works
The battery is an electro-chemical device used to store and then supply electricity. It consists of two electrodes: the anode and the cathode which both bathe in liquid, a paste or a gel called electrolyte which is a medium allowing the transfer of electrons, therefore electricity.
The electrodes react with the electrolyte to convert chemical energy into electrical energy. Indeed, the electrodes are composed of different materials which causes an electrical voltage since when the circuit is closed, the anode tends to yield electrons while the cathode tends to attract them. The battery gradually discharges during this reaction.
During charging, a reversal of the chemical process takes place, allowing the restructuring of the electrodes. During battery discharge and charge cycles, the battery tends to wear out gradually, losing some of its capacity over time. This is called discharge sensitivity. Thus, depending on the type of battery used, the sensitivity to discharge may vary, sometimes increasing or decreasing the life of the battery considerably. It is therefore essential to choose the right type of battery according to the desired use.
What are the different types of batteries?
There are three main types of battery:
- The lead acid batteries are the best known, the oldest and the least expensive to buy. In this category, for example, we find lead/acid batteries used to start cars. Overall lead batteries have a low energy density allowing them little energy storage according to their weight. They will therefore mainly be heavy batteries to handle, but this criterion is not very important in stationary storage. On the other hand, these batteries are rather suitable in situations where a large amount of energy will be needed to be supplied very quickly.
- The lithium batteries, whose technology is the latest, have a longer lifespan but are much more sensitive to maintenance. Their purchase price is higher, but is beginning to become more democratic. Their energy density is high, which makes them rather light batteries to handle. However, due to their electro-chemical sensitivity, the use of lithium batteries requires a Battery Management System or BMS, ie a circuit that allows the voltage to be rebalanced between each cell of the battery. This BMS can be integrated or have to be added depending on the brand chosen. These are therefore batteries that can be more complex and expensive to install, but which represent a guarantee of safety and performance for a long-term installation.
- The nickel batteries are the third technology available. These are robust batteries capable of easily lasting twenty years if they are maintained regularly. On the other hand, these are batteries that can be reconditioned to increase their lifespan. However, nickel batteries can cause problems with the environment and during recycling. The most polluting were Nickel Cadmium batteries, fortunately prohibited today.
Which battery to use for my solar installation: our comparison?
Despite the growth of lithium batteries, lead-acid battery technology retains a market share dedicated to solar. In effect; their advantageous price makes them interesting batteries for small installations. Especially if they are to be used temporarily or for short periods. These are batteries that have an average lifespan of about ten years in the best case and provided that the recommended depth of discharge is respected.
Some lead acid batteries are optimized for deeper discharges. This is particularly the case for AGM or opzs batteries that can have very interesting characteristics for your solar installation, while remaining within a reasonable budget.
The following tables are taken from the video:
Comparison of lead batteries
| Battery Type | Lead/Acid | AGM | AGM Supercycle | Gel | OPzS | opzv |
|---|---|---|---|---|---|---|
| Average purchase price | Approx 100 euros per kWh | Approx 190 euros per kWh | Approx 250 euros per kWh | Approx 200 euros per kWh | Approx 340 euros per kWh | Approx 410 euros per kWh |
| Depth of discharge | 80% | 100% | 90% | 80% | 80% | |
| Possible number of cycles depending on the depth of discharge | 30% = 800 cycles 50% = 500 cycles 70% = 300 cycles |
30% = 1500 cycles 50% = 600 cycles 70% = 400 cycles |
40% = 1000 cycles 60% = 700 cycles 100% = 300 cycles |
30% = 1800 cycles 50% = 750 cycles 70% = 500 cycles |
30% = 4400 cycles 50% = 2500 cycles 70% = 1350 cycles |
30% = 4600 cycles 50% = 2600 cycles 70% = 1400 cycles |
| discharge speed | Spring | Acceptable | Spring | Fast | Fast | |
| Energy density | Weak (heavy battery) | Weak (heavy battery) | Lighter than standard AGM batteries | Weak (heavy battery) | Weak (heavy battery) | Weak (heavy battery) |
| Memory effect | Low | No memory effect | Low | Low | Low | |
| Performance | 80% | 95% | 90% | 85% | 85% | |
| BMS required | not | not | not | not | not | |
| sealing | not | Yes | Yes | Yes | not | Yes |
| Care instructions | Sometimes requires adding fluid every 6 months | Requires virtually no maintenance | Very little maintenance required | Sometimes requires the addition of liquid | Requires virtually no maintenance | |
In autonomy, or for larger installations, being able to store the energy produced efficiently becomes essential. Lithium batteries offer many advantages. Less cumbersome, equipped with better sealing but also with a longer lifespan, the practical aspects quickly make you forget their purchase price which remains higher. Their lifespan is also about ten years, but their depth of discharge is more flexible, which gives them better performance!!
Comparison of Lithium batteries
| Battery Type | Lithium Iron Phosphate LiFePO4 or LiFe |
Lithium Titanium Oxide LTO |
|---|---|---|
| Average purchase price | Approx 480 euros per kWh | Approx 600 euros per kWh |
| Depth of discharge | 95% | 95% |
| Possible number of cycles depending on the depth of discharge | 30% = 10000 cycles 50% = 4500 cycles 70% = 3000 cycles |
30% = 20000 cycles 50% = 9000 cycles 70% = 6000 cycles |
| discharge speed | Fast | Fast |
| Energy density | Strong (light battery) | Strong (light battery) |
| Memory effect | Very weak | Very weak |
| Performance | 95% | 97% |
| BMS required | Yes sometimes integrated |
Yes |
| sealing | Yes | Yes |
| Care instructions | Requires virtually no maintenance | Requires virtually no maintenance |
Finally, nickel batteries are suitable for installations carried out under complex conditions. They support the discharge
deep, over-charging and require almost no maintenance in addition to being waterproof. On the other hand, these are batteries that will be enough
expensive to buy. However, they will be particularly useful for carrying out installations on an isolated site or in climatic conditions.
.
Nickel battery comparison
| Battery Type | NiCd (cadmium = polluting) | NiMH | NiFe (Nickel/Iron) |
|---|---|---|---|
| Average purchase price | Approx 400 euros per kWh | Approx 700 euros per kWh | Approx 500 euros per kWh |
| Depth of discharge | 85% | 85% | 85% |
| Possible number of cycles depending on the depth of discharge | 70% = 2000 cycles | 70% = 2000 cycles | 70% = 2400 cycles |
| discharge speed | Fast | Fast | Quite fast |
| Energy density | Strong (very light battery) | Strong (very light battery) | Strong (light battery) |
| Memory effect | Important | Low | Low |
| Performance | 90% | 90% | 80% |
| BMS required | optional | optional | not |
| sealing | Yes | Yes | Yes |
| Care instructions | Requires virtually no maintenance | Requires virtually no maintenance | Requires virtually no maintenance |
In parallel with the criteria directly linked to the battery technology to be used, other elements must also be taken into account. Storage capacity is an important point, since your installation must be able to store all the energy you produce. The duration of the warranty period is also an interesting point to study insofar as it is quite a variable parameter according to the different suppliers!! Finally, care must be taken to ensure the compatibility of the batteries chosen with the rest of the solar installation.
How are end-of-life batteries recycled?
The following video provides an overview of the battery recycling process in France:
It is a recycling in several stages involving chemical processes aimed, among other things, at recovering purified metal salts that can be used to manufacture new batteries. In view of the complexity of recycling, it is easy to understand the interest of using batteries that can have a substantial lifespan, thus reducing the need to change them and therefore to recycle them too often.
To optimize the life of your batteries, some advice may be useful. Storing the batteries indoors and in a temperate place optimizes their capacities. In fact, batteries do not tolerate heat or very cold temperatures well, and are not all equal in the face of humidity. For example, the optimum operating temperature for a lead-acid battery is 19°C. On the other hand, during installation, the batteries should be arranged so as to leave a space between them. In operation your batteries will produce heat which can quickly damage the device if nothing is done so that it can be evacuated correctly. Finally, it is important to respect the indications given by the supplier, in particular with regard to the optimal depth of discharge.
Virtual batteries? The battery of Start-Ups!
The solar energy sector is constantly evolving, and new technical solutions appear regularly. So if you are looking to perform an installation, it is possible that you have come across the terms " virtual battery" , or " thermal batterye" during your search.
La virtual battery is an “on-grid” energy storage solution. The excess energy produced is injected into your supplier's network, which must be compatible, and can then be “taken back” in the form of an authorized energy credit when needed. However, this solution necessarily requires a self-consumption installation connected to the network. It also has a cost from your energy supplier. Some countries are starting to tax this practice like Belgium with the Prosumer tax.
Finally, the thermal battery consists of storing energy not in the form of electricity, but directly in the form of heat, for example in a hot water tank. This option drastically reduces the profitability of your solar installation since an electric kWh is much more expensive than a thermal kWh.