Battery systems

In recent years there has been a significant reduction in the price of renewable energy storage systems (ESS). The storage of energy produced in photovoltaic power plants (PV) takes place mainly in batteries, which must have specific characteristics. These include a high degree of cycling, exceptional power density, the ability to regenerate from deep discharge, etc.

While in the past traction lead-acid batteries were preferred for energy storage, nowadays most PV plants are equipped with lithium-based SSE or flow redox batteries.

Comparison of the most used battery types:

Cell TypeNi-CdNi-MHLi-ionLead
Energy density (Wh/kg)45-8060-12090-12030-50
Number of cycles (at 80% depth of discharge)1500300-500>1500400-500
Projected service life5 years +3-4 years10 years+10 years+
Charge time1-2h2-4h½-4h8-16h
(at approx. 20°C)20%30%5-10%5%
Nominal cell voltage1.2V1.2V3.3V2V
Current Load Capacity20C5C25C5C
Operating temperatures (for discharging)-40~60C-20 ~ 60°C-20 ~ 60°C-20 ~ 60°C
Service Requirements30 - 60 days60 - 90 days6 months6 months
Approximate cost

Source: https://www.fg-forte.cz/cz/kategorie/lithiove--lithium-ion-baterie.aspx

Lead (gel) traction battery-based energy storage systems (SSE)

This is a time-tested energy storage technology. The lead-acid battery has a high energy efficiency (about 85%), which means that it will give out a large proportion of the "ampere hours" delivered. Lead-acid batteries can operate in many modes, and their charging and discharging characteristics are also quite favourable. It is recommended to use modern batteries with AGM technology (non-degradable - the acid is soaked into the glass fibres) or gel batteries - highly durable with a long service life (the electrolyte - acid - is solidified in the form of a gel). AGM and GEL models are completely maintenance-free, non-friable and highly resistant to shocks.

The main disadvantages of lead SSEs are their limited service life and high weight.

Li-Ion based energy storage systems (SSEs)

The biggest advantage of lithium SSEs is the higher energy density of these batteries, both in volume and mass. Another advantage of these batteries is lower self-discharge than most other batteries, the charging principle is simple, and recharging can be done at any time (state of discharge) without negatively affecting the performance of the batteries.

There are currently several variants of lithium SSE commercially available. The most common type of lithium batteries are lithium-ion (Li-Ion) cells with liquid electrolyte. These batteries are therefore relatively safe and mechanically robust. The charging voltage is 4.2 V per cell, nominal 3.6 V. The energy density is between about 150-200 Wh/kg.

The main strength of lithium-ion batteries is the fact that they can be recharged very frequently - without the memory effect that slows down and complicates the charging process. Another advantage: high energy density. Depth of Discharge (DOD) : Batteries can be deeply discharged (100% DOD) compared to lead acid batteries are more powerful because if a lead acid battery exceeds 80% depth of discharge it is damaged.

LiFe/LiFePO4-based energy storage systems (SSE)

Another widespread type is the lithium iron phosphate cell, abbreviated as LiFe/LiFePO4. The charging voltage of the cells is 3.6 V, nominal 3.2 V. LiFePO4 cells have a lower energy density (about 90-120 Wh/kg). Their advantage over Li-Ion cells is a higher current carrying capacity and some resistance to deep discharge.

Lithium-titanium-oxide (LTO)

The third type of lithium-based SSE used is lithium-titanium (LTO or Li4Ti5O12 = lithium-titanium-oxide) batteries. The cathode is the same as that of Li-Ion/Li-Pol batteries. The LTO material has a large specific surface area relative to the weight, so fast charging and discharging is possible.

Another advantage is the possible low temperature operation and very long cycling life (thousands of cycles). On the other hand, the disadvantage is the lower nominal voltage of 2.4 V. The energy density is thus lower than previous types.

Flow battery-based energy storage systems (ESSs)

According to many experts, flow batteries may be the future "pillar" of many ESSs, both at the grid level and in homes.

Basically, flow batteries consist of two tanks that are filled with electrolyte flowing through an electrochemical cell. The critical processes occur with the electrolyte, which is divided into two large tanks. Each tank has its own pump and injects the electrolytes into the regenerative fuel cell where a chemical reaction takes place through an ion exchange membrane. Energy is transferred through the membrane and stored in the electrolyte tanks.

Advantages and disadvantages of VRB technology

There are some advantages to separating the electrolyte. In the case of VRB, the electrolyte has no degradation processes, so it is possible to operate the battery for an almost unlimited number of charge and discharge cycles. The VRB can be left in a discharged state for longer periods of time without causing any damage to the battery life. According to experts, the lifetime of the battery is estimated at 30 to 50 years with a cycle count in the order of tens of thousands.
The electrolyte does not degrade, which means in practice that the capacity of the battery does not change over time. The depth of charge and discharge is 0% to 100%. VRB technology is characterized by a minimal self-discharge process.VRB flow batteries are extremely stable. The efficiency of these batteries ranges from 75% to 85%, the cell voltage depends on the electrolyte used and ranges from 1.4 V to 1.8 V.
The only disadvantage of VRBs is the low energy density in the range of 15 to 25 kWh/m3 (in comparison, Li-ion batteries have an energy density of about 300 kWh/m3). Therefore, VRBs are quite bulky and also relatively heavy. The energy density of these batteries is determined by the amount of electrolyte in the reservoirs, while the power density is affected by the chemical reactions taking place at the electrodes.

How do you know a quality battery?

A quality battery should include a BMS, or "Battery Management System", which takes care of its safety, actively balancing the cells and keeping the entire SSE in the best possible condition.
The BMS balances the energy flows between the cells and keeps them all in the same state in terms of voltage and capacity. This also protects the weaker cells from damage thanks to the stronger cells. In other words, by using the BMS, the negative effects of mismatching individual cells during the period of use are eliminated, so that the entire battery offers a constant, stable performance.
The BMS thus guarantees completely safe long-term operation with 100% utilization of the capabilities of the battery cells used. For each cell, the electronics precisely monitors the current voltage, current and temperature and corrects the charging and discharging process according to the measured values.

Main functions of the BMS:
- precise measurement of voltage, current and temperature of each cell
- protection against deep discharge and overcharging of each cell
- limitation of excessive charging and discharging currents.

Guiding principles for selecting good quality SSE

It is strongly advised not to select cheap traction lead acid batteries. When comparing the price per kWh stored (but also when comparing dimensions or weight), lithium battery storage clearly comes out better. In addition, a quality lithium-based battery (or flow battery) is a maintenance-free, clean and safe solution that is superior and more cost-effective in all respects compared to Pb batteries.
When choosing batteries, the principle that higher price = higher added value applies. You can tell a quality battery by the fact that it contains a BMS - i.e. a control system that actively balances the cells and keeps the whole SSE in the best possible condition.

TIP: When choosing a battery, look for the following parameters:
- lifetime
- capacity (usable)
- number of charge/discharge cycles
- deep discharge capability.