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battery
The battery (colloquially for battery pack) is an important part of an electric car. There are many different battery technologies and designs. There are also many myths about the battery in an electric car. On this page we want to show you how a battery is constructed and how to handle it correctly.
A battery in an electric car consists of several individual cells (accumulators). These will
mostly in Connected in series to achieve a higher operating voltage. In this photo
one sees the prismatic Form a Single cell. However, it is also available as a round cell, such as
they are known from many household appliances.
A distinction must be made between accumulators (short: accumulator) which are rechargeable
are, and the primary cells (which are colloquially called battery), the
you cannot reload.
In the electric vehicle sees a battery
(the amalgamation of all batteries) then looks like this:
Cell chemistry
The cell chemistry is decisive for the function. It defines the most important properties of the battery. Here we will go into the three different cells that we use.
Lead-acid battery (AGM)
Many people know the lead battery from conventional combustion engines, where it serves as a "starter battery".
However, lead batteries for electric cars are specially designed for the requirements there. So-called AGM batteries (Absorbent Glass Mat, electrolyte stored in a fiberglass flow) are used in our electric vehicles.
Benefits: Disadvantage:
- Inexpensive - Shorter service life
- Easy to use - High weight
- Robust against wrong treatment - Lower power output
- Little control electronics required
- Simple SoC determination
The decisive factor for lead batteries is the charging curve. The more fully charged the battery, the higher the voltage. This end-of-charge voltage depends on the ambient temperature, which is why the charging electronics must adjust the charging voltage based on the temperature. Furthermore, the deep discharge of the battery is harmful. You should therefore always make sure that the vehicle is never completely empty. If the battery is almost empty, the vehicle should be charged as it also needs electricity when it is stationary.
The state of charge (SoC) of lead batteries can
can be determined relatively easily because the voltage curve is linear.
Such a voltage curve is shown in the graphic on the right.
This voltage curve applies at a temperature of 20 ° C and in
unloaded state. Changes the temperature changes, then it changes
Voltage. Even if at current is drawn from the battery, decreases
the tension off and falsifies the measurement.
The SoC (state of charge) can only be used in the lead battery when it is idle
(without the electricity being drawn) and taking into account the
Temperature can be determined.
The service life of an AGM battery is between 500 and 1500 cycles (discharged up to 20%, fully charged up to 100%) or around 5 to 15 years. It is best to operate this battery between 50% and 100% for a long service life.
Lithium battery (NCM)
This battery technology is used very often in electric cars. Lithium cells or rechargeable batteries are so named because the lithium ions are responsible for the electricity. To store them, graphite is used on the minus side and a mixture of nickel, cobalt, manganese and oxygen (hence NCM) on the plus side.
Benefits: Disadvantage:
- Very high storage density - Flammable
- Very high performance - Elaborate monitoring electronics
- Light weight - Very expensive
- Simple SoC determination
- Long durability
The charging curve of NMC batteries is similar to that of AGM batteries, which means that the SoC is easy to determine
(~ 1% deviation). This also makes it easy to estimate the remaining range.
Lithium batteries cannot be charged at temperatures below 0 ° C and are then also limited in their power output, which is why battery heating is necessary for permanent operation in our climatic region. Depending on the driving style, these batteries can also get relatively warm, which is why battery cooling is also recommended for applications with high power consumption. Batteries also heat up when charging at high power. Temperature plays an important role in fast charging (CCS). Temperatures that are too low or too high reduce the charging capacity.
The battery management system (BMS for short) is very important for all lithium batteries, as it prevents overvoltage and undervoltage in the individual cells. These occur when a cell is charged too fully or is discharged too deeply. Differences in the charge of the individual cells must therefore be balanced out in order to prevent consequential damage, which makes the monitoring electronics (BMS) more complex than with AGM batteries.
Lithium battery (LFP)
The lithium iron phosphate battery (LFP or LiFePo for short) is not as widespread as the NCM battery, but has some advantages over the NCM batteries. The negative pole of the LFP batteries is also made of graphite, but the positive pole is made of iron phosphate.
Benefits: Disadvantage:
- Very high performance - High weight
- High storage density - Complicated SoC determination
- Very long shelf life - Elaborate monitoring electronics
- High resilience - Expensive
- Non-flammable
Compared to the NCM battery, the LFP battery has the advantage of durability (up to 6,000 cycles), high load capacity, especially when charging, and it is very safe and non-flammable. The decisive disadvantage is the weight, because in order to achieve the same range as an NCM battery, the LFP battery has to be ~ 30% larger and heavier.
In terms of temperature, the lithium batteries behave relatively similarly.
Due to the linear charge / discharge curve in LFP batteries, the SoC determination is complicated and can deviate by almost 10%. But they are also a little cheaper than the NCM batteries.
Due to the very high cycle stability (3,000-6,000), the LFP batteries are still very attractive, as a vehicle with a range of 300 km (like our electric motorhome ) thus has a battery life of up to
1,800,000 km.
Conclusion: Regardless of which battery technology or cell chemistry is used, each has its advantages for certain purposes.
