Deep discharge

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Under deep discharge of a rechargeable battery refers to the state after current draw until the almost complete exhaustion of the capacity . or below a certain voltage. Since deep discharges can be harmful to the accumulator, they should be avoided if possible and the accumulator should be protected from them.

Basics

In the case of deep discharge, a cell of an accumulator is discharged with any current intensity to such an extent that the voltage drops below the end-of- discharge voltage . Depending on the battery type, the total discharge can cause different damage. If the cells are connected in series, the polarity of the cells with the lowest capacity can even be reversed. Depending on the battery type, a single deep discharge can destroy a battery. If the connected consumer does not switch itself off automatically when the voltage supply is too low, special care is required. Accumulators can also be deeply discharged when not in use, solely due to self- discharge.

Occur

The deep discharge of an accumulator begins when the voltage falls below the final discharge voltage. This is a fixed voltage up to which the accumulator can be discharged. The level of the final discharge voltage per cell depends on the respective battery type.

Accumulator type Typical end-of- discharge voltage for single cells
Lithium polymer battery 3.30 volts
Lithium-ion battery 2.50 volts
LiFePO battery 2.00 volts
Lead accumulator
  12 V battery		 1.75 volts
10.5 volts
Nickel-zinc accumulator 1.20 volts
Rechargeable alkaline manganese cells (RAM cells) 1.00 to 1.10 volts
Nickel-metal hydride battery 1.00 volts
Nickel-cadmium battery 0.85 to 1.00 volts

Reasons for a deep discharge

There are various reasons for deep discharge of a battery (examples):

  • Accumulator outdated (end of service life)
  • Accumulator is not charging properly
  • Charger not suitable for the accumulator
  • Charger defective
  • Passive power consumption by the device
  • The device is not switched off when the voltage falls below the final discharge voltage and does not switch itself off

There are other reasons for use in motor vehicles (examples):

  • Short circuit in the electrical system
  • Wrong accumulator installed
  • Vehicle lights left on
  • Operation of larger consumers while stationary (auxiliary heating, radio, cool box, etc.)
  • Defective alternator
  • Alternator regulator defective
  • Overload of the on-board network from additionally installed consumers
  • ECU error
  • Frequent short trips

Effects

Lead battery

With lead-acid batteries , the effects depend on the type of battery. Starter batteries are not suitable for constant deep discharge, as the active mass of the plus plates is stressed too much in the event of deep discharge. In lead-acid batteries, deep discharge can lead to sulfation of the active substance and thus to a loss of capacity. Furthermore, the lower acid density and the higher temperature lead to corrosion of the electrodes. If deeply discharged lead batteries remain in this state for a period of several days, recrystallization leads to the formation of coarsely crystalline lead sulfate. For this reason, deeply discharged lead-acid batteries should never be stored uncharged for a long time, as this will lead to irreversible damage to the battery. Multiple deep discharges lead to an irreversible hardening of the active material in the electrodes and to a greatly increased sludge removal of the active material from the positive electrode. This ultimately leads to short circuits between the plates due to so-called through- growths .

Lead-gel battery

Gel batteries are more secure against deep discharge, they withstand deep discharges much better than normal lead-acid batteries. Deep discharge is therefore possible to a limited extent with gel accumulators. With traction batteries , extra thick plus plates (so-called armor plates) are installed and special separators are used. This type of accumulator is less sensitive to deep discharge.

Lithium battery

With lithium-ion accumulators , a deep discharge to below 2.4 V leads to irreversible damage and a loss of capacity. If the voltage of a cell falls below 1.5 V, it should no longer be used. It is very likely that copper bridges have formed, which then lead to a short circuit. In this state, the cell becomes unstable and heats up very strongly, creating a risk of fire. Deeply discharged lithium-ion batteries should no longer be used for safety reasons.

NC battery

Nickel-cadmium batteries are robust against deep discharge. They can also be stored in a discharged state for several years without being damaged. Nickel-metal-hydride batteries are relatively robust against brief deep discharges.

Identifying features of a deeply discharged accumulator

First of all, the total discharge can be recognized by measuring the voltage of the accumulator. If the battery voltage is below the final discharge voltage, the accumulator has been deeply discharged.

In lead-acid batteries, the total discharge can also be determined by measuring the acid density . If the acid density is well below 1.1 kg / l, the battery has been deeply discharged. A deeply discharged lead accumulator with strong sulphation and completely used sulfuric acid can be recognized when charging: the initially high charging current of the accumulator drops very quickly to very low values.

The deep discharge of lithium-ion batteries can lead to cell short circuits during subsequent charging and is therefore potentially fire hazard. If cells are already short-circuited, the end-of-charge voltage is reduced and there is a risk of overcharging, which is also dangerous. NiCd batteries can suffer from deep discharge due to polarity reversal of the weakest cells and can even have cell short circuits. This can be seen from the atypical open circuit voltage after brief charging.

Preventive measures

Accumulators have to be protected against deep discharge, depending on the type. For this there is a deep discharge protection specially tailored to the respective battery type. This automatically switches off the consumers as soon as the battery voltage falls below a set limit value. Accumulators should not be discharged more than 80% depending on the type, therefore charging the accumulators in good time prevents deep discharge.

Special electronic load shedding relays are installed in accumulators that are installed in vehicles and in battery systems. These relays measure the vehicle electrical system voltage by means of built-in electronics and switch off the consumers if necessary.

With so-called battery packs z. In the event of a deep discharge, an internal fuse disconnects the battery, so it cannot be discharged any further.

Regeneration of the accumulators

Lead accumulator: Deeply discharged lead accumulators should be charged with a small charging current over a longer period of time. Following full charging, the accumulator must be charged further with an equalization charge in order to prevent insufficient charging. With this type of charge, the sulphation of the active material is eliminated again in "light cases". It can happen that the charging voltage increases to over three volts and still no charging current flows. If the damage to the cells is not too great, the insulating lead sulphate will break down during the charging process. A charging current then flows and the charging voltage drops back to normal values ​​of 2.7 volts per cell. The battery becomes unusable if there is strong sulphation with completely used sulfuric acid .

Li-ion battery: Deeply discharged lithium-ion batteries pose a risk during subsequent charging (short circuit, fire). As a rule, they cannot be regenerated. With electronic chargers for lithium-ion batteries (battery packs) it can happen that the chargers do not recharge the battery because there is no voltage at the external contacts when the internal protective electronics disconnect the discharged battery from the contacts, until a there is sufficient charge voltage. However, the chargers start with a very low charging current when the battery is discharged, which is sometimes not enough to reset the electronics.

See also

literature

Reference books

  • Norbert Adolph: Car electronics. Basics and building proposals . Publishing company Schulfernsehen, Cologne, ISBN 3-8025-1128-X .
  • Jürgen Kasedorf, Richard Koch: Service primer for vehicle electrics . Vogel Buchverlag, ISBN 3-8023-1881-1 .

Technical brochures

  • Battery guide. Information on the subject of batteries. Robert Bosch GmbH, Stuttgart.

Web links

Individual evidence

  1. ↑ a b Helmut Weik: Expert Practice Lexicon Solar Energy and Solar Technologies. 2nd completely revised and updated edition. Expert Verlag, Renningen 2006, ISBN 978-3-8169-2538-5 , p. 330.
  2. ↑ a b c d Volker Quaschning: Regenerative energy systems, technology calculation simulation. 8th updated and expanded edition. Hanser Verlag, Munich 2013, ISBN 978-3-446-43526-1 , pp. 222–228.
  3. ^ A b Günter Springer: Electrical engineering. 18th edition, Verlag Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9 , pp. 487-488.
  4. ↑ a b c Bosch: Technical briefing on batteries. Robert Bosch GmbH, Stuttgart, VDT-UBE 410/1.
  5. ↑ a b c d GNB Industrial Power (Ed.): Manual for stationary lead batteries. Part 1: Fundamentals, construction, operating modes and applications, Edition 6, 2012, pp. 65–67.
  6. ↑ Joachim Specovius: Basic course in power electronics, components, circuits and systems. 6th edition. Springer Vieweg Verlag, Wiesbaden 2013, ISBN 978-3-8348-2447-9 , pp. 361-367.
  7. ↑ a b Robert Bosch GmbH. (Ed.): Battery Tips. Bosch Vocational School Info, Stuttgart 2005, pp. 1–2.
  8. ↑ a b Heinz-Albert Kiehne: Device batteries. 3rd completely revised edition. Expert Verlag, Renningen 2001, ISBN 3-8169-1470-5 , pp. 55-57, 128.
  9. ↑ a b Thomas Kaufmann: Industrial trucks. Optimal planning and efficient use. Beuth Verlag GmbH, Berlin-Vienna-Zurich 2013, ISBN 978-3-410-22065-7 , pp. 74-75.
  10. ↑ a b c d e ZVEI: Requirements for battery discharge indicators for lead drive batteries to achieve a high level of economic efficiency. Leaflet of the Batteries Association, Frankfurt am Main 2008.
  11. ↑ a b Dietrich Naunin: Hybrid, battery and fuel cell electric vehicles. 4th edition. Expert Verlag, Renningen 2004, ISBN 978-3-8169-2625-2 .
  12. ↑ Solarlink GmbH (Ed.): Manual for sealed gel lead batteries. Part 1: Basics, construction, features, Bad Bederkesa 2003, p. 18.
  13. ↑ a b c Michael Trzesniowski: racing car technology, fundamentals, construction, components, systems. 3. Edition. Springer Vieweg Verlag, Wiesbaden 2012, ISBN 978-3-8348-1779-2 , pp. 808-816.
  14. ↑ Federal Guild Association for the German Two-Wheel Mechanic Craft (Ed.): Handling lithium batteries in two-wheelers. Bonn 2011.
  15. ↑ VARTA: Gas-tight Ni-Cd accumulators. VARTA Batterie AG (Ed.), Hanover.
  16. ↑ https://www.elektroniknet.de/elektronik/power/gefaehrdungspotenzial-von-li-ionen-zellen-92479.html Elektroniknet / BMZ Batteries-Montage-Zentrum GmbH: Text on dangers with Li-Ion batteries, accessed on Feb. 8, 2019