When it comes to on-board power supply, lithium batteries are the hot topic. On the one hand, there are glowing praises of their advantages. On the other hand, there are still major reservations. The problems experienced by fire brigades when extinguishing electric cars that have caught fire are not exactly helping to clarify the situation.
Let's start with a basic all-clear: lithium-iron-based batteries are no more dangerous than other types - provided they are handled correctly. In addition to safety, the price is considered by many to be the knock-out criterion. For a long time, 800 to 2000 euros for 100 ampere hours or, better still, 1200 watt hours was the norm. This meant that the lightweight storage batteries were at least three times as expensive as a high-quality AGM battery with the same nominal capacity. Goods.
The difference between lead and lithium
The boom in motorhomes and balcony power stations has caused the market to take off. While brand manufacturers such as Mastervolt or Victron still charge prices of around 700 to 1800 euros depending on the model, there are also significantly cheaper suppliers. In fact, the market is literally flooded with products. Anyone buying from Amazon or eBay usually pays between 250 and 500 euros per 1200 watt hours. Some batteries are even available for 170 euros. However, the devil is in the detail, as the batteries may have the same capacity, but unlike a lead-acid battery, a lithium battery does not just consist of cells connected in series.
The most important difference: with lead, you simply bought the battery here, a charger there and connected the whole thing to the existing on-board electrical system. Abuses such as deep discharge or overcharging, too high or too low a temperature reduce the service life considerably, but do not immediately lead to failure. A lithium storage system, on the other hand, requires customised monitoring electronics. Unlike lead cells, lithium cells cannot simply convert excess energy into heat - overcharging also leads to heat, but also immediately damages the structure of the battery.
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Discharge too deep
The same applies if the battery is discharged too deeply: a single complete discharge means the end of its life expectancy. This is why the batteries are equipped with a battery management system, or BMS for short. This electronic system performs two tasks: firstly, it protects the battery from deep or overcharging by interrupting the flow of current when the cut-off voltage is reached; the same happens if the permissible temperature is exceeded or if the current flow is too high.
The second task is balancing, i.e. equalising the cell voltages. Due to the production process, there are always slight deviations in the capacity and internal resistance of the cells.
Charge equalisation for batteries: passive vs. active balancers
When charging the battery, the cell with the lowest capacity is the first to reach its final voltage. In order not to damage it, the BMS stops the charging process so that the other cells with a higher capacity are not completely filled. The same problem arises when discharging. The weakest cell determines when the fuse function of the BMS intervenes and switches off the current. In total, the full capacity is not available. In addition, the voltage differences increase with the age of the cells and the current load. The balancer works against this. The simplest systems conduct part of the charging current via a resistor to the most fully charged cell, which means that it is charged more slowly than the other cells and the voltages equalise. The disadvantage of these passive balancers is that equalisation only takes place during charging and the currents are in the milliampere range. Larger differences in charge can hardly be equalised in the course of a charging process.
So-called active balancers are more efficient; they can shift the charge back and forth between the cells and continue to work even when no external charging current is flowing.
Branded products promise more consistency
However, it is not possible to recognise from the outside which system the manufacturer has fitted to the battery. As a rule, however, manufacturers explicitly label active systems. The mechanical construction of many battery bargains also remains in the dark. Whether sufficiently dimensioned cables are used and how they are connected to the cells can only be clarified by sawing open the housing. The wide range of variation in the mechanical and electrical design of the batteries can be seen in various internet videos in which users open the housing with a saw. In this respect, branded products promise more consistency.
There is "no longer any reason to rely on lead"
The quality of the cells used is often the subject of debate. The categorisation into grades A to D is carried out differently by each manufacturer. What they all have in common is that A cells fulfil the specification in full; anything that deviates from this becomes grade B or worse. What exactly leads to the devaluation remains a trade secret. Compared to lead batteries, a lithium-iron battery made from B cells is most likely still extremely powerful.
"In view of the enormous drop in prices, there is no longer any reason to rely on lead," says Kai Uhrig from Sterling Power. "Even if the cheap lithium-iron batteries don't have the best cells or management systems, they easily achieve the service life of wet batteries, and they are also superior when it comes to charging and cyclical operation. However, it is always a question of requirements. If you want to go on a long journey, you should opt for systems with communicating BMS," Uhrig continues. Michael Kögel from Philippi takes a similar view: "If several batteries are interconnected to form a bank, management systems with data exchange are advisable. This allows problems to be recognised before the system switches off and the power is suddenly completely lost."
Other arguments in favour of lithium batteries from established brand manufacturers include reliability in the event of a warranty claim or the availability of spare parts such as the BMS. In some cases, warranties of up to ten years are advertised. However, in order to be able to utilise this, the supplier must still exist.
