However, they can be deducted up to 80% as shown in the table below:
 

The graph above illustrates the performance of the AGM battery in a moderate climate (average temperature of 25°C): if the DoD is 80%, the battery would provide only 500 cycles or about 1.5 years of lifespan (if cycled daily), while with 50% you can get 1000 cycles or about 3 years of lifespan.

This cycling area is also illustrated in the table below:

The yellow zone above 50% is not excessively harmful for batteries, but constant charging cycles at this level will reduce their lifespan. Clearly, the red zone above 80% should be avoided unless absolutely necessary, for example in an emergency. Therefore, 50% represents a good balance between capacity and cycle life, also considering the replacement cost.
 

So why shouldn't we discharge more than 50% for lead-acid batteries? This is because a discharge greater than 50% would reduce the battery's lifespan. How and why?

The internal resistance of the lead-acid battery increases as the discharge increases, as shown below. Note that the open circuit voltage (OCV) decreases with continuous discharge and, in the graph below, you can see that the internal resistance increases with continuous discharge. The internal resistance essentially indicates the difficulty of inputting and outputting electricity through the electrochemical reaction inside the battery. Why does the internal resistance increase?

Below are the reasons:
 

The internal resistance increases due to the decrease in specific gravity and the depletion of the electrolyte as it becomes more diluted.
 

Studies have shown that internal resistance increases significantly during cell recharging after an excessive discharge. This process, combined with increased gas production, causes a rise in temperature inside the electrode being recharged. This greatly affects aging, particularly capacity loss, as there is a simultaneous high temperature increase and gas flow. In practice, it compromises the battery's ability to accept a charge.
 

There are also other reasons:

• Excessive discharge plays an important role in aging because it increases grid corrosion, sulfation, and loss of active mass.
• During over-discharge, the basic reaction proceeds to a lesser extent and is replaced by other reactions (such as increased gassing).
• In the positive electrode, capacity loss is caused by mechanical stress
• In the negative electrode, the main cause of aging is the irreversible oxidation of the expanders

For more information, you can consult the scientific article " Internal resistance and temperature variation during the over-discharge of a lead-acid battery " by Balázs Broda and Gyorgy Inzelt.
 

Now the question arises: “How can we ensure that the capacity does not drop below 50%?”
 

For this, we need to set the Low Voltage Disconnect (LVD) parameter in the inverter to a voltage value that indicates 50% DoD. Should we then refer to Table 1 and set the LVD to 12.06 V?
 

The answer is a resounding no, because the voltage in Table I is the resting voltage and not the under-load or discharge voltage, which is what the inverter would see. In fact, the battery has three different voltages: resting voltage, charging voltage, and discharging voltage, depending on whether it is idle, charging, or discharging.
 

The battery voltage varies at the same state of charge depending on whether the battery is charging or discharging and depending on the current flow in relation to the size of the battery. The following graph gives an idea of the state of charge for different battery conditions in lead-acid batteries with flooded cells:
 

To understand the difference between resting voltage and voltage under load, let's take a look at the test conducted by Rod "RC" Collins and his inference. In that test, an AGM chart like the one shown below was compared with the actual voltage under load.
 

At the end of the test, we notice that the actual voltage under load is quite different from the resting voltage in the real world:

In that test, after letting the battery rest for 16 hours, the open circuit voltage at rest returned to 12.32 V. Looking at the graph above, 12.32 V falls within the range between 70% and 80% of the resting voltage, which would lead us to suppose that the battery is at about 70% SoC and that we can still continue to discharge it. However, in reality, the battery is already at 49% SoC. So, if we continue to discharge it, it will drop further below 50% DoD and the battery will be damaged.
 

Experienced users do not consume more than 50% of the available energy in a battery before recharging it. This means they never let the resting voltage drop below 12.3 V. If the resting voltage reaches 12.1 V, we know that the battery has been fully discharged for one cycle and that a battery is only able to sustain a limited number of cycles (from a minimum of 20 in a car battery to 180 in a golf cart battery, with a typical battery sometimes able to sustain no more than 30 cycles).

In that test, the battery bank exceeds the 50% SoC threshold at about 12.1 V. This means that, with an average battery bank, you should stop discharging at a voltage between 12.15 V and 12.2 V+. I strongly recommend starting recharging at a maximum undercharge voltage of 12.2 V, because the average discharge rate will be lower. These 12.2 V represent the LVD (Low Voltage Disconnect) to set in the inverter.
 

As a concluding thought, keep in mind that most batteries fail not because they are excessively discharged, but because they are never fully recharged. Therefore, it is absolutely essential that lead-acid batteries are always fully charged after each discharge to ensure a long lifespan. The absorption phase is therefore the most important part of the charging cycle. Absorption charging is essential for the battery's well-being and can be compared to a short rest after a good meal. If it is continually deprived of this charge, the battery will eventually lose the ability to accept a full charge and performance will decrease due to sulfation. Therefore, when a battery is not fully charged, sulfate crystals accumulate on the plates, stealing energy. So, the healthiest thing for a battery is to have a 100% charge. 
 

Therefore, it becomes even more important to allow the battery time to charge to 100%, whether via solar energy or via the grid. To illustrate this point, let's let the battery discharge below 50%, say down to 70% DoD: it takes 12 hours to fully discharge it. After that, we recharge the battery, but after 7 hours a power outage occurs. During these 7 hours of charging, the battery would only be at 80% SoC. When the power outage occurs, the battery will begin to discharge further from 80% SoC, whereas ideally we would have wanted it to discharge from 100%. Therefore, the battery did not have enough time to recharge to 100%, which would accelerate the sulfation process.

The depth of discharge (DoD) of a battery indicates the percentage of the battery discharged relative to its total capacity. The depth of discharge is defined as the capacity discharged from a fully charged battery, divided by the battery's rated capacity. The depth of discharge is normally expressed as a percentage. For example, if a 100 A·h battery is discharged for 20 minutes at a current of 50 A, the depth of discharge is 50 * 20 / 60 / 100 = 16.7%.

The depth of discharge is the complement of the state of charge: as one increases, the other decreases. While the state of charge is usually expressed in percentage points (0% = discharged; 100% = full), the depth of discharge is usually expressed in units of Ah (for example, 0 is full and 50 Ah is empty) or in percentage points (100% is empty and 0% is full). The capacity of a battery can be higher than its rated capacity. Therefore, it is possible for the depth of discharge value to exceed the nominal value (for example, 55 Ah for a 50 Ah battery, or 110%).
 

In most battery technologies, such as lead-acid and AGM batteries, there is a correlation between the depth of discharge and the battery's cycle life. 

The more frequently a battery is charged and discharged, the shorter its lifespan will be. Generally, it is not recommended to fully discharge a battery, as this drastically reduces its useful life. Many battery manufacturers specify a maximum recommended DoD for optimal performance.
 

Life cycle
 

The cycle life is the number of charge/discharge cycles that a battery can sustain during its useful life and depends on the amount of battery capacity normally used. If batteries are regularly discharged to a lower percentage, they will have more useful cycles compared to when they are frequently discharged to the maximum DoD. Depending on the depth of discharge and operating temperature, a typical lead-acid battery provides from 200 to 300 discharge/charge cycles. The main reason for its relatively short cycle life is the corrosion of the positive electrode grid, depletion of the active material, and expansion of the positive plates. These changes are more frequent at higher operating temperatures. Cycling does not prevent nor reverse the trend.
 

Another factor that influences battery life

Another factor that influences battery life is its maintenance, and in particular the temperature at which it is kept. Batteries in a hot environment (over 30 °C) can overheat, reducing their lifespan. Very cold temperatures also have a negative impact on the battery, because it has to work harder and at a higher voltage to charge. To maximize the useful life of the battery, try to store it in a relatively mild environment, neither too hot nor too cold.

Different manufacturers and technologies can impact DoD performance

A good quality, properly charged and maintained deep cycle wet battery will offer the best value for money in terms of lifecycle/DoD; however, many users require a low-maintenance sealed option.
 

A sealed battery made to order in the United States or Europe will generally last longer and offer better performance than Asian products. This is due to the quality of the manufacturing process, the raw materials used, and the flexibility of use, for example, high current charging and hot environments.

Make sure to ask the right questions when your customer asks about Deep Cycle applications.

Comparison of battery chemistry

The sodium-ion battery offers numerous advantages compared to lithium iron phosphate batteries and other batteries. Compared to lithium, sodium has a relatively lower chemical activity, which makes it less prone to thermal runaway or explosions during use. This stability makes sodium-ion batteries extremely safe during fast charging and able to handle higher charging power. Sodium-ion batteries offer a wide operating temperature range, from -40 °C to 80 °C, making them suitable for applications in extreme weather conditions. The sodium reserve is 440 times greater than that of lithium; although the cost of sodium-ion batteries is higher than that of lithium batteries, it will decrease in the future and become an economical solution.

1. Introduction to the LTR Index
The LTR (Long-Term Reliability) index represents a battery's ability to maintain consistent and reliable performance over a long period, taking into account capacity degradation and overall lifespan. This index allows for the comparison of battery reliability and is essential for evaluating investment in different storage technologies, such as lithium-ion, lead-acid, and salt batteries.

2. How the LTR Index is Calculated
The LTR index is based on:
Cycle life: the number of charge/discharge cycles a battery can support before its capacity is significantly reduced.

Capacity degradation: the loss of usable capacity over time, expressed as a percentage of the initial capacity.

Energy efficiency and performance: how well the battery maintains high efficiency throughout its operational life.

3. LTR Calculation for the 10 kWh H2W Battery with Linear Degradation

For the H2W battery, there is an initial capacity of 10 kWh, which degrades to 70% of the capacity (i.e., 7 kWh) linearly over 6000 cycles.
 

Total energy generated:

Linear degradation: the average capacity of the battery gradually decreases from 10 kWh to 7 kWh.

Average capacity: (10 kWh+7 kWh)/2=8.5 kWh

(10 kWh+7 kWh)/2=8.5 kWh.

Total energy: 8.5 kWh×6000=51,000 kWh

8.5 kWh×6000=51,000 kWh.

LTR of the H2W Battery https://www.h2w.store/product-page/h2w-10kwh-con-5kw-inverter-allinone-per-la-casa-con-sistema-di-accumulo-solare

With a total installation cost of 2500 Fr.-, the cost per kWh over the lifecycle will be: LTR=2500 Fr.- / 51,000 kWh≈0.049 Fr.-/kWh
 

LTR=​≈0.05 Fr.-/kWh

4. Comparison of the LTR Index with Other Storage Technologies

Lithium Batteries:

Lifespan and Degradation: Lithium batteries have an average lifespan of 3000-5000 cycles, with degradation down to 60-80%.

Typical LTR index: Between 0.10 and 0.20 Fr.-/kWh, making the H2W battery more competitive.

Lead-Acid Batteries:

Lifespan and Degradation: These batteries have a short lifespan, between 500 and 1500 cycles, and a degrading capacity down to 50%.

Typical LTR index: Higher, between 0.15 and 0.30 Fr.-/kWh, due to the reduced lifespan and accelerated degradation.

Flow Batteries:
 

Lifespan and Degradation: Flow batteries offer a long lifespan (up to 10,000 cycles), with low degradation, but are expensive.

LTR index: About 0.05-0.15 Fr.-/kWh, variable depending on installation and maintenance costs.

5. Advantages of the H2W Salt Battery

Competitive cost: With an LTR of 0.049 Fr.-/kWh, the H2W salt battery proves to be one of the most economical solutions for residential and commercial applications.

High long-term reliability: Thanks to the stability of sodium and resistance to thermal fluctuations, the H2W battery offers high operational reliability.

Environmental sustainability: The use of sodium, a more eco-friendly and abundant material, ensures a lower environmental impact compared to lithium technologies.

6. Conclusions

The LTR index of the H2W battery demonstrates that it is a convenient, durable, and environmentally friendly energy storage solution. Compared to technologies such as lithium and lead-acid, the H2W salt battery is more advantageous for long-term applications, offering both economic savings and a lower ecological footprint.