Charging method
As a charging method , the various strategies of control of current and voltage during charging of accumulators , respectively. The aim of charging processes is to fully charge the battery within its operating limits. In addition, there are various options for maintaining the battery and maintaining the state of charge, depending on the battery type and charging technology. The charging process and its implementation have a significant influence on the performance and service life of a battery.
The assemblies used to control the charge, often integrated circuits , are called charge controllers . They exist as independent devices (so-called chargers ) or are permanently installed in the battery-operated device. In motor vehicles they are located on the alternator . In complex systems, charge controllers are part of the battery management system .
When specifying the charging current, the C-factor is used . This indicates the current in relation to the nominal capacity.
Constant current charging method
The accumulators are charged with a constant current I K over the entire charging time . To avoid overcharging, a suitable shutdown procedure must be used when fully charged.
In the simplest case, a fixed time is switched off or after trickle charging switched. The charging time t to be observed for fully discharged batteries results from the quotient of the capacity of the battery Q and the charging current I K , multiplied by a factor c to take into account the charging efficiency .
For example, nickel-cadmium batteries are usually chosen. This simple way of ending charging is only permissible for small charging currents of a maximum of C / 10. For higher charging currents, there is a risk of permanent damage to the accumulator in the event of overcharging due to the resulting heating and the increase in pressure.
This method of slow 14-hour charging with C / 10, which is used in simple chargers, was for a long time the only easy-to-implement charging method, but is no longer state of the art. This applies in particular to NiMH batteries, which are very sensitive to overcharging. If they are heated to more than 40 ° C, their charging capacity is irreversibly reduced.
Pulse charging method
This method is a special case of constant current charging, since it is charged with pulses of constant current. The advantages are:
- The charging voltage can be measured in the currentless pauses between the pulses, so there is no falsification of the measurement result due to contact and line resistances.
- By varying the pulse / pause ratio as part of a pulse width modulation (PWM), different phases of charging can be implemented in a simple manner without having to change the constant charging current during the pulses. This can be used for testing of the connected battery at the start of charging and after charging to the realization of a maintenance charge are used. Trickle charging is carried out in the form of short current pulses with long pauses and is advantageous over long-term charging with low current, since the risk of dendrite growth is reduced, which can lead to an internal short circuit in the battery.
Reverse current charging or reflex charging
Pulse charging process in which short discharge current pulses are inserted between the individual current pulses.
→ Main article: reverse current charging
Constant voltage charging method
In this method, the charging voltage U L is kept constant over the period t L. As the charging progresses, the charging current decreases due to the decreasing voltage difference between the accumulator and the charger. Ideally, the charging current would drop to zero; in practice, a residual current, dependent on the battery capacity, flows to compensate for self-discharge .
Commitment:
- Lead accumulator
- Lithium-ion battery
- RAM cells (Rechargeable Alkali Manganese)
IU charging method (CCCV)
The IU charging method, also known as CCCV for constant current constant voltage , combines the constant current charging method with the constant voltage charging method. In the first phase of charging, a constant current limited by the charger is used. Compared to the pure constant voltage charging method, this results in a limitation of the otherwise high initial charging current. When the selected end-of-charge voltage is reached on the battery, the system switches from current to voltage regulation and continues to charge with constant voltage in the second charging phase, with the charging current automatically decreasing as the battery level increases. As a criterion for completing the charge, lead and Li-ion batteries can be used if the selected minimum charge current is not reached.
Commitment:
IUoU charging method
Chargers with IUoU characteristics work like the IU (CCCV) devices described above, but after charging up to the charging voltage, the system switches to trickle charging. When trickle charging (often pulsed and temperature-monitored), the self-discharge of the accumulator is counteracted.
Chargers with this characteristic are suitable for permanently charging lead-acid batteries.
IUIa charging method
Chargers with an IUIa characteristic work like the IU (CCCV) devices described above, but if the charge current falls below a certain level, a lower constant current (Ia part) is charged again until it is fully charged. This method is z. B. used in traction batteries (lead). It achieves charging times of less than ten hours, which are necessary in shift work. Due to the increased gassing, the process is only suitable for batteries with a water refill system. Due to the increased gases, the active mass of the lead plates can be removed and the service life or capacity decrease.
Lithium-ion accumulator
In addition to the combination of constant current charging and constant voltage charging, a lithium-ion battery also requires the consideration of a deep discharge state at the beginning of charging, in which it cannot handle the full charging current. Since a deep discharge also affects the service life, this state of charge should be avoided. Adapted charge controllers therefore measure the no-load voltage at the beginning or increase the charging current up to the intended maximum value.
For a maximum service life, it makes sense not only to keep the depth of discharge but also the end-of-charge voltage well below the maximum values specified by the manufacturer, i.e. close to the nominal voltage of the cell. The upper voltage limit of the cell should be avoided, since in this zone processes start in the cells that irreversibly damage them and cause a rapid decrease in capacity. In current applications ( cell balancing by the BMS ), the upper charging voltage is often set high during charging, as this allows the charging status of the individual cells to be better determined. Since the battery is only slightly recharged when balancing, charging can be aborted prematurely to protect the battery, thus avoiding these voltage ranges.
With a serial interconnection of multiple cells to increase the voltage, for example. In a traction battery , to a recharging by Balancer secured a uniform voltage level of the cells and of the inevitable drift cell are met. Avoid using higher charging currents than the balancer can compensate. As a result, the cell voltage continues to rise in spite of the balancing and there is either a shutdown at an upper voltage limit without complete balancing of all cells, or an overload of individual cells.
Shutdown criteria
Stress criterion: the −Δ U method
Modern charge controllers monitor the voltage curve on the battery during charging. With increasing charge, the differential resistance of the accumulator decreases , but the voltage dropping across it ( primary voltage ) increases. When the full charge is reached, the supplied energy can no longer be chemically bound, the original voltage does not rise any further and the accumulator heats up. However, with every degree the differential resistance continues to decrease, and the charging voltage now decreases again (hence: −Δ U , "minus delta U"). The drop in voltage is clearly pronounced with NiCd batteries . In the case of NiMH , however, the drop in voltage after a maximum is only observed with sufficiently high charging currents.
Possible criteria for the end of the loading are:
- The drop in the charging voltage after reaching the maximum (−Δ U - or "Minus-Delta-U" method). 10-20 mV per cell are usual here.
- Reaching the maximum charge voltage (peak voltage detection). The charge controller calculates the first derivative of the voltage curve for this. It is switched off when the maximum charge voltage is reached with . In practice, however, a slight voltage drop can also be detected in devices with peak voltage detection. An exact detection of a stagnant voltage is not possible due to slight natural voltage fluctuations, contact uncertainties and measurement inaccuracies.
- Beginning of the flattening of the charging voltage curve. The charge controller calculates the second derivative of the voltage curve for this. The switch-off takes place when the turning point is reached with .
The voltage curve shows the charging of a rechargeable battery (NiCd, 800 mAh) in a microcontroller- controlled charger. First the discharge of the NiCd battery, then the charge until the −Δ U switch-off and finally the switch to charge retention.
Temperature criterion
In this process, a maximum temperature or the temperature profile during charging are used as a switch-off criterion.
As a rule, accumulators should not exceed a temperature of 55–60 ° C. If this temperature is nevertheless exceeded, the cell can lose its tightness and leak due to the increase in pressure inside. Switching off the charge at a certain temperature is used as a safety criterion in some chargers. However, this is not recommended as the sole switch-off criterion , as it is rather imprecise due to the indirect measurement and the temperature developments of NiCd and NiMH batteries also differ. In addition, the ambient temperature also influences the switch-off time.
End-of-charge voltage
| Accumulator system | End-of-charge voltage | annotation |
|---|---|---|
| Lead accumulator | ≈ 2.42 V / cell | Sustained charge 2.23 V. |
| NiCd / NiMH battery | ≈ 1.45 V / cell | |
| Lithium cobalt dioxide accumulator | 4.2 V / cell | colloquially often blurred as a lithium-ion battery referred |
| Lithium polymer battery (LiPo) | 4.2 V / cell | |
| Lithium polymer high voltage battery (LiHV) | 4.35 V / cell | |
| Lithium iron phosphate battery ( LiFePO4 ) | 3.6 V / cell | maximum 3.8 volts |
| Nickel-zinc accumulator | ≈ 1.90 V / cell |
With lithium batteries in particular, it makes sense to switch off the charge below these upper limit voltages (approx. −0.3 V) in favor of the service life. In the last charging phase, the cell voltage usually rises faster, the additional energy absorbed is low.
See also
Web links
- Advice on batteries and rechargeable batteries - Federal Environment Agency , extensive information brochure, PDF 3.4 MB
literature
- Ludwig Retzbach: Batteries and chargers . Neckar, Villingen-Schwenningen 2008, ISBN 978-3-7883-4142-8 .
Individual evidence
- ↑ Werbat battery chargers. (PDF) In: http://www.werbat.de/ . December 7, 2010, accessed March 12, 2018 .
- ↑ Charging of electrochemical accumulators. (No longer available online.) Archived from the original on March 12, 2018 ; accessed on March 12, 2018 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Jens Groot, Chalmers University of Technology, Göteborg, 2012: State-of-Health Estimation of Li-ion Batteries: Cycle Life Test Methods , PDF, accessed July 1, 2013
