Before buying batteries for you solar power station it is important to find out which batteries and what capacity you need or if you need batteries at all. Below we will review different types of batteries, their features, advantages and disadvantages.
Sizing battery capacity
Batteries are an integral part of any off-grid solar power station. They are designed to store the energy produced by solar panels during daylight hours. Their capacity is defined based on your daily energy needs but in some cases you can do without them. For example, if you don’t need to store power because all the electricity is typically consumed during daylight hours or transmitted to the public grid.
Otherwise, it is necessary to define the amount of power you need to store. It is better to have a storage equal to 2-3 days energy use in case of long periods of limited sun (for more sunny regions it will be enough to ensure 1 day of autonomy). For this, you should work out the power consumption of all your household appliances. Power consumption consists of the capacity of each separate appliance multiplied by its average operation time. However, it will be much easier just to check your electricity meter to find out the power you use during the day.
Please note that the resulting value will be in kWh while battery capacity is indicated mostly in Ah. To convert ampere-hours into kilowatt-hours you need to multiply the figure specified on the battery (for instance 200Ah) by its voltage (typically 12V, 24V or 48V) and divide the resulting value by 1,000.
It is the most common type of batteries, used mostly in cars. Its operation principle is simple and based on interaction of lead and sulfuric acid solution. Continuous cyclic electrochemical reaction takes place in a sealed system – under sulfuric acid exposure the lead is oxidizing to lead sulfate resulting in electric potential on battery terminals. During the reverse reaction lead sulfate exposed to electrical current is deoxidizing to lead and sulfuric acid. Unfortunately, this process has the following weaknesses:
- For proper functioning it needs steady operating temperature – in low temperature battery efficiency falls to zero;
- Battery capacity drops sharply in case of constant low charge. Therefore, the battery should not discharge to below 30% of its capacity to avoid the so-called memory effect;
- Small number of discharge/charge cycles and, as a result, short lifespan of the battery.
All the above effects are related to the sponge structure of lead sulfate. Being white and flocky, it flakes off from the plates and falls down to the bottom. With the lapse of time this residue starts hardening and crystallizing, which prevents the sulfate from transforming back to lead and battery capacity drops. In subzero temperature water is freezing, which is not good for the recovery of sulfate lead as well.
Charging process is also complicated and consists of three stages.
- For charging you should use the current equal to one tenth of battery capacity, indicated in ampere-hours. For example, if the capacity is 200Ah, you should use 20A current. This is required to give the system enough time to absorb the current. Excessive power will transform into heat and can damage lead plates; therefore only lower current should be used.
- The second stage begins when terminal voltage reaches 14.4V (rated battery voltage is 12V). Current starts slipping and prevents voltage from growing. Any further increase in voltage will result in battery gassing, which is dangerous, as it will lead to water loss in the sealed system;
- When the current is close to zero, terminal voltage drops to 13.7V and should be kept at this level to compensate internal losses.
The overall charging process takes about 5-10 hours. If you skip the third and fourth stages, crystals of lead sulfate will grow leading to gradual degradation and destruction of the battery. If you do everything right, i.e. keep lead battery warm, charge and discharge it properly, it can survive for up to 300 full discharge/charge cycles and up to 600 cycles being 50% discharged or around 2 years.
In fact, lead battery is not the best option for solar panel. GEL and AGM technologies ensure lower lead sulfate flaking and extend battery life significantly to 10-15 years. Therefore, they are better suited for solar power stations.
AGM battery (Absorbent Glass Mat)
The space between electrodes of this lead-acid battery is filled with a fiberglass-based porous mat. The mat acts as a sponge soaked in acid.
Chemical reaction can generate gases, mostly hydrogen and oxygen. These gases take part in cyclic reactions during battery recharge process, evolving and returning to battery acid. This process is called gas recombination. Such cycling process cannot be 100% efficient but in AGM batteries gas recombination can achieve 95-99% efficiency while slight amount of free gas does not affect acid quality, which enables the system to function well for many years. Besides, the battery has a special valve activated by overpressure for relief of excessive gas. It also protects the battery in case of emergency such as temperature surge, etc.
One of the key features of AMG batteries is their low internal resistance. It helps to speed-up their charging, which can be useful in winter and cloudy weather for battery recovery after deep discharge as well as for extension of its life.
AMG batteries do not require additional venting or any special maintenance. With 20% discharge depth they can last for up to 10-15 years. But constant 60-70% discharge will significantly cut battery life. In any case, they are better than usual lead-acid batteries as they can work longer in deep discharge mode and last for over 600 deep discharge cycles. Besides, their self-discharge current is extremely low and shelf life of the charged battery is quite long. For example after 12 months on shelf battery charge drops only by 20% from its initial level.
Gel battery and GEL technology (Gel Electrolyte)
Electrolyte in these batteries is a gel-like substance. This is achieved through adding silicon dioxide based substance to the acid, which makes the resulting mass gel-like.
Gel batteries better recover after deep discharge even in case of late charging and can withstand over 1000 deep discharge cycles without any loss of capacity. Since their electrolyte is thick, it is not prone to decomposition into water and acid and can stand low temperature without serious drop in capacity.
It should be noted that price for gel batteries is a bit higher than for AMG batteries. But their lifespan is 20-30% longer. Besides, better performance makes them well suited for off-grid solar systems, when batteries are constantly running in deep discharge mode and ambient temperature is often below zero. As a result, they have lower maintenance cost and shorter payback period.
It is a state-of-the-art battery. Lithium-based batteries outperform lead batteries in every important parameter. Their key features include:
- High capacity;
- They are absolutely tolerant to constant deep discharge mode;
- They last for over 4,000 deep discharge/charge cycles;
- High charging rate – five times higher than for lead-acid batteries (around 2 hours);
- DC voltage and high direct current can be used for charging without any stages;
- High capacity per time unit, which can be useful for systems with a variable and high load;
- 95% efficiency;
- Light-weight, weigh 4 times less than lead batteries.
Through their tolerance to deep discharge, lithium batteries enable using three times lower capacity in comparison with lead batteries.
As a result, despite the higher cost per unit of capacity, total price of lithium-ion battery within a system will be lower than that of a lead-acid battery.
Lithium batteries are essential for off-grid systems running with regular charge/discharge process, such as household power stations equipped with few solar panels, solar power plants operating in limited-sun conditions (in winter or cloudy weather).
Besides, smaller weight makes lithium-based batteries convenient for portable and mobile applications, when going for a hunt or a picnic (you can install them in your car or boat), as well as for emergency services.