Battery isolating relay

A battery cut-off relay is a special relay that is used in motor vehicles when a second battery is installed in addition to the starter battery. The battery isolating relay disconnects individual lead accumulators (hereinafter referred to as batteries ) if necessary and reconnects them for charging.

Basics

Consumers are often operated with high power in the electrical systems of today's motor vehicles. If these consumers are operated for a long time with the engine stopped, the starter battery quickly reaches its capacity limit. So that the motor of the motor vehicle can be started again even after these consumers have been in operation for a long time, it makes sense to install a second battery in the motor vehicle. However, this second battery should not be connected directly in parallel to the starter battery because:

• Double the battery capacity (actually electrical charge ) can also be used up
• The batteries can discharge each other during longer periods of inactivity
• the starter battery will be affected if it is defective

Rather, it makes sense to set up a second on-board network and thus the secondary consumers such as:

• Powerful HiFi amplifier
• Cool box or refrigerator
• Living area electrics in mobile homes

to be electrically separated from the actual primary electrical system. The negative poles of the batteries are connected directly to ground and the positive poles are switched.

Alternatively, some devices have voltage monitoring . These switch off the device as soon as the supply voltage falls below 12.7 volts, for example.

Operating states in the vehicle electrical system

With regard to the load on the on-board network including the power output of the alternator , four operating states can be distinguished:

• Base load
• Partial load
• Full load
• Overload

For vehicles with 24-volt electrical systems, e.g. B. large coaches, the voltage values ​​are twice as high in the respective operating states.

With the basic load , the only consumer is the ignition system. The starter battery is well charged by the alternator. The vehicle electrical system voltage rises to over 13.6 volts. At 14 volts, the alternator regulator switches off. Since there is now enough charging energy available, the second battery can be charged without any problems.

In the case of partial load , more power is required from the alternator by connecting several large consumers (lights, rear window heating, fan). The vehicle electrical system voltage drops to 13.4 to 13.6 volts. The starter battery is being charged. The second battery should only be charged if the starter battery has received sufficient charging energy while driving. This is usually the case when the on-board electrical system voltage rises above 13.4 volts.

At full load , so many consumers are now switched on that the alternator has reached its performance limit. The on-board network voltage drops below 13.4 volts. The starter battery is no longer charged. If the second battery is now switched on, equalizing currents from the starter battery can occur, depending on the state of charge of the second battery.

In the event of overload , so many consumers are switched on that the alternator can no longer supply the required electricity on its own and electricity has to be supplied from the starter battery. The on-board network voltage now drops to 12 volts. The starter battery can be discharged in the event of prolonged overload. The second battery should not be switched on under any circumstances so that the on-board network is not additionally loaded.

Methods of battery disconnection

So that two on- board networks can be operated separately in a vehicle , the batteries must be disconnected if necessary. The following methods are suitable for this:

• Separation by means of a battery switch
• Separation by means of isolating diodes
• Separation by means of a vehicle relay
• Separation by means of a battery isolating relay

The separation by means of a battery switch is a very trivial method in terms of circuit complexity. However, the monitoring of the operating conditions and the associated switching on and off of the switch is a considerable additional burden for the driver.

When separating by means of isolating diodes, the two batteries are charged via special Schottky diodes . However, special control electronics are required to compensate for the threshold voltage (0.6 volts). In addition, in some cases a different alternator regulator must be installed. This method is very expensive because of the electronics.

When disconnecting by means of a vehicle relay, a commercially available vehicle relay with the coil is connected to the ignition starter switch and the two batteries are connected to one another via the normally open contact. The relay should be installed near the batteries. Must also before the NO contact from both sides a 40 Amp - Backup run because the contacts of automotive relays can switch up to only 40 amps. Relays with lower switching currents (e.g. 25 amps) are unsuitable for this purpose. So that the relays are not switched on during the starting process, these relays must be locked against the starter by means of a locking circuit. In addition, it is recommended to install a switch to switch the relay off again when the on-board network is fully loaded.

In other circuits for disconnection by means of battery disconnection relays, special relays that were specially developed for battery disconnection are installed. The relays have switching contacts that can switch a higher current. In addition, they are connected differently and usually have a so-called protective circuit to protect the sensitive on-board electronics.

Structure and function of battery isolating relays

Block diagram of the electrical system of a motorhome with battery isolating relay (black box illustration)

So-called power relays are used as battery isolating relays. These are relays with contacts that, depending on the design, can safely switch currents between 60 amps and 250 amps. Most battery isolating relays can handle up to 10 ⁶ switching cycles. It is advantageous if the battery isolating relay is installed close to the batteries. The switching contact of the battery isolating relay is usually not fused at the factory, but the main lines must be laid so that they are protected against damage. The switching contacts of high-quality relays are made of special tungsten alloys and coated with a hard silver- nickel alloy. Due to the special alloy materials , the contacts are self-cleaning. The relay contacts of relays with high switching capacity are installed in a hermetically sealed tube filled with noble gas. This design protects the relay contacts from environmental influences. In order to minimize contact wear, some relays are equipped with so-called leading and trailing contacts .

In the case of battery isolating relays, where this material expenditure is not used, the switching contacts can stick due to the high current flow . As a result, the switching contacts of these relays can no longer open and the batteries are constantly connected in parallel.

The installation of battery isolating relays does not solve the problems of excessive power consumption in the vehicle electrical system. Alternator output and battery capacity must be matched to one another. If the battery capacity is too large, a too weakly dimensioned alternator is overwhelmed and the batteries may not be fully charged. This leads to problems with a cold start, especially in winter. The only remedy here is a more powerful alternator that is tailored to the battery capacity . However, this in turn can lead to slightly higher fuel consumption .

Adaptation of the contact load and alternator size to the battery capacity (charge)

Sum of the electrical charge of the built-in batteries in Ah	Maximum alternator current in A.	Rated current of the relay contact in A
33	40	60
55	55 ... 70	70
66	80	100
100 ... 150	90	100
180	110	150
300	120	180

Swell:

There are two types of battery disconnect relays:

1. simple battery isolating relays
2. electronic battery cut-off relays

Simple battery isolating relays

Until around 1996, the simplest circuit variant of some caravan models was the battery cut-off relay which was connected directly to the ignition switch and electrically locked against the starter. As a result, the batteries were always switched in parallel when the ignition was switched on.

Simple battery isolating relay with locking against the starter

As an extension, an additional lock against the starter can be implemented. To do this, the relay coil is connected to terminal 50 of the starter instead of directly to ground. The relay only picks up when the ignition / starter switch is in the "ignition" position again. These measures prevent part of the starter current from flowing through the isolating relay. This circuit also improves the self-excitation of the alternator, because an additional positive potential is passed on from the starter relay to excite the alternator via the coil of the battery isolating relay during the starting process. In order to be able to switch off the relay manually, an additional switch with switch-on control lights should be installed. Self-starting assistance is not provided for this type of isolating relay.

In another circuit variant, simple battery isolating relays are connected between the alternator regulator connection D + and ground. This ensures that the relay only switches through when the alternator rotates and the average alternator voltage is above the battery voltage. After the engine has been switched off, the voltage emitted by the generator drops immediately, so that the relay switches off immediately and the switching contact opens. Both batteries are then separated from each other again.

If simple battery isolating relays are connected directly to the "D +" connection, a device for damping possible voltage peaks through self-induction when the isolating relay solenoid is switched off is recommended. This protects the generator's excitation diodes from overvoltage.

Electronic battery cut-off relays

Electronic battery cut-off relay with self-starting aid via a timer

In the case of electronic battery isolating relays, the connection is much simpler. The relay coil is connected between the ignition switch (contact ignition) and vehicle ground. The relay contact is switched between both battery positive poles. The relay is equipped with electronics that monitor the level of the vehicle electrical system voltage and switch the relay accordingly. The relays switch the relay on when a certain voltage, the so-called switch-on threshold, is reached, and the relay off when the so-called switch-off threshold is reached. The values ​​are slightly different depending on the manufacturer.

In the case of 12- volt battery isolating relays, this is:

• Connection threshold between 13.4 and 13.6 volts
• Switch-off threshold between 12.7 and 12.6 volts.

In the case of 24 volt battery isolating relays, this is:

• Connection threshold between 27 and 27.2 volts
• Switch-off threshold between 25.4 and 25.2 volts

Protective circuit - simple battery isolating relay

By automatically connecting and disconnecting the battery blocks, the second battery is always charged when the alternator supplies enough energy. Electronic battery cut-off relays detect an overload via the electronics and switch off the relay in good time. Some relays are equipped with an additional function for so-called self-start help. The two batteries can be temporarily switched together in parallel using a timer. Electronic battery cut-off relays are to be preferred due to their switching method, especially when the "D +" connection of the alternator is difficult to reach.

Fully electronic battery isolating relays

While electronic battery isolating relays have an electronic control of a mechanical relay, fully electronic battery isolating relays manage without mechanical relays. They switch the current wear-free via power semiconductors and thus achieve a significantly higher number of switching cycles.

Protective circuit

Most battery isolating relays are equipped with a protective circuit to eliminate voltage peaks that could damage the on-board electronics. For this purpose, either a resistor or a fast-switching diode is connected in parallel to the relay coil by the manufacturer. For relays with a diode , the polarity of the relay is marked. Some manufacturers connect a diode in series to the relay coil to protect against polarity reversal. With some relay types, an RC element is connected in parallel to the relay contacts for radio interference suppression .

Problems with parallel switching

The parallel connection of starter batteries with the help of relays is not easy.

Especially with commercially available relays, when the on-board network is fully loaded, the batteries are not sufficiently charged. So that the starter battery is always supplied with power, less important consumers should be switched off or the second battery should be manually disconnected from the on-board network. In the event of overload, the second battery is fed back into the on-board network. This discharges the second battery and cannot be used sufficiently during standstill phases. Manual separation should also take place here.

If the second battery is heavily discharged, the battery isolating relay can switch in parallel to higher equalizing currents that quickly subside. After a short time, the voltage level between the two on-board networks is the same, and the secondary battery is charged by the charging current of the alternator. However, higher equalizing currents can push the contacts, especially of smaller relays, to their limits.

There are three ways around this problem:

• Fuses
• Resistance to limit the current
• powerful relay

Fuses

The subsequent installation of two fuses is a simpler variant, but it also has its pitfalls. On the one hand, motor vehicle fuses are only available to a limited extent in terms of current strength. On the other hand, the second battery is not charged if one of the two fuses blows. Fuses are also available in standardized gradations which do not always match the rated current of the relay contacts.

Electrical resistance to limit the current

When installing a series resistor in series with the relay contact, the heat load that occurs is not negligible. The power loss that occurs results from the interlocking formula for Ohm's law and the formula for electrical power :

${\displaystyle P_{v}=I^{2}\cdot R}$

With

P: Power loss of the series resistor in watts (size)

I: maximum charging current in amperes

R: selected size of the current limiting resistor.

The resistance value itself is typically small, approx. 50 mΩ, but the current is large at up to a few 100 amperes.

The charging current depends on the terminal voltage of the accumulators. This continues to decrease near the end of charge voltage and ultimately approaches zero. The voltage difference that occurs at the series resistor, which can be used for measurement purposes, is directly proportional to the charging current.

It is therefore most practical to dimension the series resistor with the aid of the maximum permissible current of the relay and, in the worst case, the difference between the voltage levels of the individual accumulators.

example

Commercially available automotive relay with 30 ampere contact current rating

A commercially available isolating relay is designed for a contact current load of 70 amps. The alternator has fully charged the on-board battery with 14.4 volts. The second battery, on the other hand, is completely discharged with 12 volts.

It is therefore important to limit the current to 70 amps at a voltage difference of 2.4 volts.

In other words: At 2.4 volts, a current of 70 amperes should flow through the resistor. The resistance value required for this purpose results directly from Ohm's law:

${\displaystyle R_{erf}={\frac {\Delta {U}}{I}}={\frac {2,4{\text{ V}}}{70{\text{ A}}}}=0,034{\text{ Ohm}}}$

This value of the total resistance includes all partial resistance values such as As contact resistance , line resistance , contact resistance and not least the internal resistances of the batteries. In particular, contact and transition resistances are difficult to determine because they are not constant over time.

Neglecting the partial resistances, which are difficult to determine, is beneficial to safety, since the sum of all partial resistances including the series resistor ultimately reduces the amount of current flow.

The current flow through the resistor causes a proportional voltage drop and thus a power loss. To avoid the risk of fire due to overheating, the heat must be dissipated. The electrical power to be dissipated is:

${\displaystyle P_{v}=U_{Vorwiderstand}\cdot I_{Vorwiderstand}=2,4{\text{ Volt}}\cdot 70{\text{ Ampere}}=168{\text{ Watt}}}$

This results in the required size of the resistor.

Neither the resistance value nor the performance class are commercially available. However, it may by suitable parallel and series connection of individual high-load resistors are implemented with, for example, 11 Watt power loss of the individual resistor.

For example, wire high-load resistors in a ceramic housing with a permissible power loss of 11 watts and a resistance value of 0.68 ohms are commercially available. By electrically connecting 20 individual resistors in parallel, the desired resistance value of 0.034 ohms is achieved and the possible total power loss of 220 watts exceeds the required value.

However, high-load resistors in metal housings up to a load capacity of 100 W are now commercially available. A recalculation of the arrangement is necessary, but the use of such resistors results in a much more compact arrangement.

A resistor made of a sufficiently long copper line is also conceivable. The electrical conductance of copper is 58  Siemens or the reciprocal value 0.01742  Ohm per meter of cable length and mm² conductor cross-section. With a conductor cross-section of 6 mm², a 12-meter-long cable is required, which must be laid airy at a distance in order to avoid overheating due to heat build-up similar to a cable drum due to the inadequate dissipation of power loss.

Alternatively, you can of course also use commercially available halogen lamps and instead of the above. Use high load resistors. They are available in different levels of performance, e.g. B. 10 W, 20 W, 50 W, 100 W and many more. The advantage lies in the signaling of the current flow (the lamp lights up when current flows) and in the small spatial expansion compared to high-load resistors. The fact that lamps are PTC thermistors can be an advantage here, because when the charging current drops, the brightness and thus the filament temperature are reduced at the same time. This in turn has the consequence that the resistance of the tungsten filament drops and the voltage drop is reduced. The battery can be charged more fully. Here, too, it is important to make further calculations in order to dimension the circuit properly.

Because of the current limitation, the second battery does not initially receive the full charging voltage. In the above example, depending on the charging current, 34 millivolts per ampere of charging current drops across the current limiting resistor. The second battery needs a longer time to reach the end-of-charge voltage. However, if the second battery is continuously drawn by a connected consumer, the charging voltage can never reach the level of the first battery according to Ohm's law:

${\displaystyle \Delta U=R_{Vorwiderstand}\cdot I_{Verbraucher}}$

Powerful relay

This is the most uncomplicated variant (no voltage loss, in the event of a fuse failure). However, more powerful relays are also more expensive than less powerful ones. The worst case can happen when one of the batteries in the charge network is at the end of its service life. Then the deposited lead sludge leads to cell short circuits, which in turn lead to low resistances within the battery. This in turn means that when the batteries are connected, a higher equalizing current (sometimes >> 100 amperes) flows through the contacts of the battery isolating relay. In order to avoid a possible fire hazard due to short-circuits as a result of insulation faults between the batteries, a fuse with a maximum length of 30 cm for each battery must be provided in the connection line, adapted to the current conductivity of the relay or the conductor cross-section. This ensures that insurance cover is also guaranteed. This prevents the necessary regular maintenance (e.g. checking the acid density ) and care of the battery , even if the necessary regular maintenance is neglected . The timely replacement of old batteries is highly recommended.

Function check

On-board network monitoring with voltmeter and ammeter

In order to constantly monitor the height of the vehicle electrical system voltage, a so-called voltmeter (must voltmeter ) be installed. A so-called ammeter ( ammeter ) can serve as a charge control . These special on-board instruments indicate whether the current is being drawn from the battery or fed into the battery. With some high-quality electronic battery isolating relays, a control lamp can be connected, which is installed in the dashboard of the vehicle. A second voltmeter is also required to monitor the secondary electrical system.

Application examples

The battery isolating relay should always be installed if two separate vehicle electrical systems are available or are being created.

This is usually the case with:

• Mobile homes
• Caravan with its own battery for autonomous operation
• Military vehicles (e.g. radio vehicles)
• truck
• Coaches
• Cars with heavy additional consumers (e.g. powerful hi-fi amplifiers)
• smaller motor yachts without an on-board generator
• Ambulance vehicles

Advantages and disadvantages

Advantages and disadvantages of the respective battery separation systems

Separation methods	Battery switch	Isolation diode	Automotive relays	simple isolating relay	electronic isolating relay	fully electronic isolating relay
advantages	little circuit effort	Automatic load monitoring	low voltage losses	high contact load capacity	only switches when setpoints are reached	only switches when setpoints are reached
	inexpensive	no switch contacts	inexpensive	only switches when the alternator is running	Self-starting help possible	Self-starting help possible
		no battery equalization currents		hardly any voltage losses	little circuit effort	little circuit effort
					simple function control	simple function control
						no contact wear
disadvantage	Increased burden on the driver	more expensive than simple systems	only for small streams	no overload protection	more expensive than simple systems	more expensive than simple systems
	no automatic switching	Threshold voltage	Parallel connection when ZAS is activated	higher circuit complexity	increased contact wear	Equalizing currents of the batteries
	Loss of voltage due to cable length	Power dissipation of the diodes	higher circuit complexity	no simple function check	Equalizing currents of the batteries
	Actuation can be forgotten	installation of a new controller if necessary	Equalizing currents of the batteries	Equalizing currents of the batteries
	Equalizing currents of the batteries	Self-starting help not possible	Self-starting help not possible

safety instructions

Work on the on-board electrical system may only be carried out by a specialist. In the case of improper handling or incorrect installation, short circuits can occur which both injure the fitter and cause serious damage to the vehicle, e.g. B. cable fire.

Statutory provisions and other regulations

• BS ISO 7588-1: Road vehicles - electrical / electronic switchgear - relays and flasher units
• IEC 61810-1: 2003 Electromechanical elementary relays (electromechanical switching relays without defined time behavior) - Part 1 General and safety-related requirements

See also

• Battery switching system
• Jump start
• Deep discharge

Individual evidence

1. ^ ^{A b} Robert Bosch GmbH (Ed.): Autoelectrics Autoelectronics . 3rd updated edition. Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Wiesbaden 1998, ISBN 978-3-322-91537-5 , p. 106.
2. ↑ Technical Information Electronics - Relays. Hella KGaA Hueck & Co, Lippstadt.
3. ^ Norbert Adolph: Autoelectronics / Basics and Building Proposals. Verlagsgesellschaft Schulfernsehen, Cologne 1979, ISBN 3-8025-1128-X , pp. 46–51.
4. ↑ ^{a b} Jürgen Kasedorf, Richard Koch: Service Primer for automotive electrical. Vogel, Würzburg 2001 (14th revised edition), ISBN 3-8023-1881-1 .
5. ^ Günter Springer: Electrical engineering. 18th edition, Verlag - Europa - Lehrmittel, Wuppertal 1989, S 61 Circuit of voltage generators, ISBN 3-8085-3018-9 .
6. ↑ Two-battery on-board network function . Online (accessed February 7, 2012; PDF; 117 kB).
7. ↑ ^{a b} Jens Feddern: Theory and practice of on- board electrics . 5th revised edition, Delius Klasing Verlag, Bielefeld 2009, ISBN 978-3-7688-0913-9 , pp. 74-77.
8. ^ ^{A b} Robert Bosch GmbH (Ed.): Technical instruction on electrical starting systems. Stuttgart 1974, VDT-UBE 501/1.
9. ^ Andreas Weingand: Current and voltage in the mobile home. Calculation, generation, Norderstedt 2016, ISBN 978-383-707-689-9 , pp. 90-91.
10. ^ Robert Bosch GmbH (Ed.): Technical instruction on circuit symbols and circuit diagrams for vehicle electrics. 2nd edition, Stuttgart 1974, p. 48, VDT-UBE 001/10.
11. ↑ Switching and moving, battery isolating relay. Gruner AG, Wehingen .
12. ↑ Battery cut-off relay 12 V. Hagen Batterieservice Ing. CH. Corner.
13. ↑ Hella KGaA Hueck & Co (ed.): Relays and relay devices. Lippstadt, pp. 24-27. Online (accessed August 18, 2016).
14. ^ Robert Bosch GmbH (ed.): Technical briefing on batteries. Stuttgart 1974, VDT-UBE 410/1.
15. ↑ Philippi electrical systems GmbH: battery isolating diode . Online ( Memento of November 14, 2012 in the Internet Archive ) (accessed on August 18, 2016).
16. ↑ Mastervolt: Solid state battery isolating diodes and electronic battery isolating diodes . Online (PDF; 651 kB) (accessed on August 18, 2016).
17. ↑ Thomas Rücker: On the essence of isolating diodes In: Tom's electronics forge , MicroCharge .
18. ↑ Fully automatic battery cut-off relay . Online (accessed February 7, 2012; PDF; 321 kB).
19. ↑ ^{a b} BEOS electronics: Technology: battery isolating relay / load relay / load shedding relay . Prussian Oldendorf. Online (PDF; 262 kB).
20. ↑ Motometer - IVEKA Automotive Technologies Schauz GmbH (Ed.): Fully electronic battery isolating relay / load relay / load shedding relay. Lomersheim.
21. ^ Robert Bosch GmbH (Ed.): Autoelectrics Autoelectronics. Vieweg & Sohn, Wiesbaden 2007 (5th revised edition), ISBN 978-3-528-23872-8 .
22. ^ Book of tables Motor vehicle mechatronics . Volume 1. Bildungsverlag Eins, 2007 Troisdorf, ISBN 978-3-427-04950-0 , pp. 96 + 100.
23. ^ A. Senner: Electrical engineering. 4th edition, Verlag - Europa - Lehrmittel, 1965, pp. 275-279.
24. ^ Dieter K. Franke: VAG manual, Do it yourself, used car purchase-accessory installation-care. 1st edition, ADAC Verlag GmbH, Munich 1984, ISBN 3-87003-227-8 , pp. 139 + 140.
25. ^ Rudolf Hüppen, Dieter Korp: Auto electrics all types . Motorbuchverlag, Stuttgart 1968, ISBN 3-87943-059-4 .