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People often ask whether the high voltage of household storage systems is a deterministic trend and what is the logic behind it. Household storage high voltage replacing low voltage is a deterministic trend, just like the upgrade of electric vehicles from 400V to 800V platform. In other words, household storage high voltage will at least take up a considerable market share.
The household storage high voltage market originated from the German-speaking area and the Nordic market, and spread to other developed countries such as Europe, Japan, Australia, and the United States. The speed of taking up low voltage depends on the promotion efforts of brands and channel dealers.
First of all, it is clear that low-voltage batteries have nothing to do with low-voltage cells. Household storage lithium batteries are now basically lithium iron phosphate cells, with a nominal voltage of 3.2V, and high voltage is achieved through multiple strings. Most players are like this, such as BYD HVM HVS series.
Some manufacturers' high-voltage batteries are low-voltage battery packs that have DC/DC boost. Some people jokingly call them "pseudo-high voltage", but in fact, the DC/DC technology content is not low. For example, Tesla Powerwall, Huawei, and Shouhang are representatives. Whether they are products in the transitional stage or final products, there are different opinions at present.
In short, "pseudo-high voltage" may be "real high voltage", so wait and see. The high voltage in this article is temporarily limited to the series high voltage solution.
Energy storage batteries can be divided into low-voltage energy storage batteries and high-voltage energy storage batteries according to the different connection methods in the circuit. This high voltage and low voltage do not refer to the numerical value of the voltage of the two, but to the circuit connection method of the battery in the entire system.
Low-voltage energy storage batteries usually have a voltage between 48-60V. When used, batteries cannot be connected in series to boost voltage (that is, no matter how many batteries are connected, the voltage remains unchanged). The battery belongs to the low-voltage part of the entire system, so it is called a low-voltage energy storage system.
The voltage of high-voltage energy storage battery cells is usually between 80-100V. Some manufacturers will make it lower and some will be higher. When in use, high-voltage batteries are connected in series to increase the voltage, and the overall voltage can eventually rise to about 400-600V (household storage). The battery is a high-voltage part in the entire system, so it is called a high-voltage energy storage system.
For example, let's sort out some basic logic:
If for a 5kWh battery, the same 2.5kW power discharge: kWh=V*Ah
Discharge rate C=discharge current/rated capacity
If the low voltage is 51.2V, the capacity is 5kwh/51.2V=100Ah, and the corresponding current is 50A when discharged at 2.5kw, and the discharge rate=50A/100Ah=0.5C
If the high voltage is 250V, the capacity is 5kwh/250V=20Ah, and the corresponding current is 10A when discharged at 2.5kw, and the discharge rate=10A/20Ah=0.5C
So for the same power discharge, no matter how much the voltage is, as long as the power is the same, the corresponding battery discharge rate is the same, then the loss of the battery itself can also be understood as the same.
In addition, for batteries with the same power output, the higher the voltage, the smaller the current, and the smaller the current, the smaller the wire diameter (less cable investment).
However, high voltage requires high insulation of the line, otherwise it is easy to produce arc or breakdown, and the safety is reduced; the high-voltage system has a large number of cells in series, so the consistency requirements are high, and the BMS R&D requirements are high and the cost is high (the component selection must have a high withstand voltage).
From the perspective of household storage systems, high-voltage household storage systems with the same capacity have higher system efficiency, simplified hybrid inverter circuit topology, reduced size and weight, and reduced failure rate.
Conversely, for batteries with the same power output, low voltage means high current, large wire diameter, large copper loss, and large operating temperature limit. Charging and discharging are slow, so low voltage is not suitable for users with high energy requirements.
The following focuses on some key points:
Due to the higher discharge rate, high-voltage batteries can have a higher load capacity than low-voltage batteries.
With a higher discharge rate, high-voltage batteries charge and discharge faster, providing the necessary power to meet the rapid surge in power demand, and can drive higher power loads.
In contrast, low-voltage batteries charge slowly and may not be able to start certain devices that require a high instantaneous charging rate, or may experience insufficient power when there is a high-power load, which is one of the main disadvantages of low-voltage battery systems.
Traveling to Nordic homes and German-speaking user scenarios, there are many such devices, such as dishwashers, washing machines, dryers, swimming pool surfing, lawn mowers, etc., and you can understand why the power of household storage systems is moving towards 15-20KW and high voltage at the same time.
Compared with low-voltage batteries, high-voltage battery systems have higher conversion efficiency and less energy loss, mainly because the DC bus voltage on the photovoltaic side of household storage is usually between 300-500V.
In contrast, the inverter in the low-voltage household storage system needs to reduce the 300-500V input voltage. This process will cause a lot of energy loss, resulting in a decrease in the overall operating efficiency of the household storage system.
According to Wotai AlphaCloud data, the conversion efficiency of the Alpha SMILE high-voltage series has increased from 90% to 93%.
High-voltage batteries generally have a longer service life due to their relatively smooth discharge curve, which means that frequent use has less impact on their lifespan. In addition, high-voltage battery systems have low current during use and are less likely to overheat, further helping to extend their service life.
The main differentiating factor between low-voltage BMS and high-voltage BMS is the voltage range they are designed to handle.
Typically, low-voltage BMS is suitable for batteries below 192V, while high-voltage BMS is suitable for batteries above 192V.
The assembly, management and maintenance of high-voltage battery packs are relatively complex, so the architecture of high-voltage BMS requires higher measurement accuracy, connection reliability and safety requirements, and the price is high.
Low-voltage energy storage batteries have low requirements for BMS and more mature technology, so the price is also lower.
Low-voltage system batteries are mostly 100AH, and high-voltage platforms are gradually transitioning to 50AH or even 25AH.
This is because the voltage platform increases. In order to ensure that the total energy storage system capacity remains unchanged, the battery cell needs a smaller capacity, which means that more battery cells need to be connected in series. Therefore, the battery cell consistency and BMS management capabilities are required to be higher, and the technical threshold is relatively higher.
Therefore, the battery cells for high-voltage batteries are generally A-grade, but the low-voltage battery cells are out of stock in 2022, and many B-grade batteries are used as fillers and are normally practical.
The smaller the battery cell capacity, the higher the adaptability of the battery, and the more diverse the module form and the amount of power, which is more portable. The battery has a higher energy density and takes up less space.
In the Nordic and German-speaking markets that focus on aesthetics, various curved, arc-angled, flat and other structural forms are easier to display in high-voltage modules.
The main reason is that, first of all, traditional products, especially the mature 100AH battery cell manufacturing process is reliable, and the low-voltage system BMS technology is more mature and stable. The system is often safer to use and easy to install and expand, but some markets have already seen sluggish sales and declining demand.
In general, if it is a family with a large number of people, large load power, and high requirements for charging time, consider choosing high-voltage energy storage batteries, otherwise choose low-voltage energy storage batteries.
Compared with traditional low-voltage + single-phase products, the terminal price of high-voltage + high-power combination products is still doubled, and the gross profit is about 20% higher, and it is still in short supply.
From a product perspective, some people say that household storage is the air conditioner of the 1990s, but we think it is more like an emerging consumer product, and the core is that supply determines demand. For example, the changes we are currently seeing are that for markets with higher consumption levels such as the German-speaking region and Northern Europe, these markets are more willing to pay a higher price to get products with better user experience than reducing costs:
Battery: low-voltage platform (48V/51.2V) → high-voltage platform (150V+), the purpose is to increase discharge efficiency, reduce heat generation, and better match three-phase electricity (180V or above can work in three-phase electricity);
Inverter: single-phase inverter (3-5kW) → high-power hybrid inverter (10-15kW), among which the hybrid inverter has the function of grid connection, and the installation of the photovoltaic storage system can be more convenient.
Price level: Overall, the terminal price of high-voltage + high-power combination products is doubled compared to traditional low-voltage + single-phase products.
