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LiFePo4 batteries with a powerful, active balancer as a quality choice? Really?

Martin Debald

After the last few months the question from customers has been constantly coming up why our batteries do not have a powerful, active balancer, this is so important and many manufacturers advertise it as a quality feature, I have created a detailed explanation of how and why I do not think this is a good idea or even a quality feature. I have tried to avoid complicated, technical descriptions as much as possible and to keep the text generally understandable.

First:

The explanation written here refers to the use of prismatic cells, but can also be transferred to round cell batteries for the most part.

A balancer is a technical device that ensures that the cells are always charged at the same level. It thus counteracts the deviation of the individual cells or cell blocks, the so-called "cell drift". In most cases, this is integrated into the BMS (battery management system). But there are also external products.

But why does this happen? Each cell has minimal differences compared to other cells in production. This is completely normal and cannot be prevented, but the quality of the materials used, as well as the type of production, has a decisive influence on the cell quality, which, logically, is also noticeable in the price.

Due to these minimal differences, the cells also behave slightly differently and are therefore charged unequally during charge and discharge. To compensate for these differences, if they become too large, a balancer is used. The greater this cell drift, the stronger the balancer must be to compensate for it.

Manufacturers of high-quality batteries put a relatively high effort into reducing cell drift as much as possible. This includes the fact that only cells of the same batch are installed in a battery. In addition, the cells are selected in an elaborate test procedure in such a way that only cells with as few technical deviations as possible from each other are used. In addition, the cells are charged to exactly the same energy level before assembly.

It is important to note that these tests are also carried out at different operating temperatures, as the cell chemistry reacts very strongly to temperature changes. In the case of low-cost cells, the tests to determine the performance data are usually carried out at a temperature of 21°C, in the case of high-quality cells in various temperature ranges, which also go to the limits of operation. There are often significant differences here, especially at higher operating temperatures.

This process is time-consuming, the corresponding test devices cost a lot of money, take up a lot of space and there is a great deal of time, as these tests take several hours. Of course, employees are also needed to equip the test facilities and monitor the tests.

Since these measures result in hardly any cell drift in the appropriately manufactured batteries, a powerful balancer is not normally required. Most BMS have 50-100mA (0.05-0.1A) passive balancers, which are only active during charging and are completely sufficient for a high-quality battery. There is also BMS with an active balancer, more on that in another entry.

However, if this, not exactly cheap, process is not used, or if any cells are simply mixed, such as if you buy them in the grid, you save a lot of costs, but the problems with cell drift during charge and discharge arise, which increase exponentially with the current flow. To eliminate this problem, a correspondingly strong balancer is used.

There are manufacturers who use separate, active balancers with 3 - 5A for a 100-200Ah battery.

It should be remembered that these balancers then work mainly independently and are installed in parallel by the BMS. This means, among other things, that the BMS does not record the energy "shifted" by the balancer, as well as the resulting losses (in the form of heat). Thus, the SOC display (charge level display), which is calculated by the BMS, is not correct. Depending on the quality of the cells, deviations of up to 10% of the capacity can occur, which leads to the user thinking that he still has a residual capacity available due to the SOC display, although this is no longer the case, and the BMS switches off because of the undervoltage protection.

In addition, the balancer can no longer do anything at high currents, e.g. when operating an inverter or a high-performance charger, such as the TBBPOWER CM2.0 or 3.0 (120-180A charging current) or at cell voltages that are below its switch-on threshold of usually 3.4V. So if a battery is used a lot with high currents and or never nearly fully charged, the cell drift becomes greater and larger and can only be compensated for when the battery is charged (usually a SOC above 80% corresponds to the corresponding voltage value). However, if the battery is not charged to this range, which is quite common in many applications, e.g. in "inhabited" motorhomes and self-sufficient operation, the balancer never becomes active and the cell drift thus increases and it can happen that the BMS switches off the battery due to too high a cell voltage difference. This can happen both during unloading and charging and can usually not be avoided.

This problem does not occur with the high-quality batteries I use, including the TBBPOWER brand. The cells all work with almost exactly the same parameters over the entire power and temperature range. Tests carried out by me with a TBBPOWER M12-200 battery (12.8V 200Ah, 2560Wh) in real use in my motorhome have shown that the integrated balancer was only active for 3 hours with a measured BMS runtime of 1550h. During this time, however, the battery was loaded several times to its performance limits and discharged until safety shutdown.

Here are some figures on the cell costs (source Aliexpress, Wish etc without special discounts):

4 pieces LifePo4 prism cell 3.2V 150Ah CHF 70-80 per piece = 280 – 320CHF per block

4 pieces LiFePo4 prism cell with the same data, CHF 572 per block, not individually.

This results in a relatively large price difference. This results from the elaborate selection procedures described above.Of course, the total price of the battery increases accordingly.

Unfortunately, many end customers do not understand this and orient themselves to the advertising messages of certain manufacturers.

Again, it should be noted that no premium manufacturer advertises with such a statement. Even medium-priced batteries, where price/performance fits, use standard BMS and do not require external balancers.

My experience so far shows that most batteries that are advertised as "Active balancer with 3-5A" come from smaller "manufacturers" and are often assembled by themselves and in small series, which is often also advertised as a separate quality feature.

My conclusion: A powerful, active balancer is more of an indication of cheap batteries or cells and by no means a quality feature.

 

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becaTec Martin Debald

becaTec Martin Debald. Created with Wix.com

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