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May. 20, 2024

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Choosing the right relay

To get the most out of the BMS and protect your electrical installation you need some heavy duty relays. They open or close the power circuits, can handle high current and are remotely commanded by the BMS. In this article I look at the two main categories of heavy duty relays:

contactor or electro-mechanical relay (EMR)
solid state relay (SSR)

Typical use for heavy duty relays are:

main isolation of a battery bank
disconnect one or more charge sources
disconnect one or more loads
parallel two battery banks

Disclaimer: all the products I mention in this article are just examples. I do not endorse nor recommend any specific product. I do not have any interest in the brand names I mention. The choice of a heavy duty relay is yours based on their usage and the characteristics of your electrical installation. If in doubt, I strongly suggest that you get advice from a professional.

Contactor (EMR)

An electro-mechanical device where the contacts are closed or opened by a mechanical system

the power circuit is closed by mechanical contacts
when the control circuit is not powered, the spring keeps the contacts open
powering the electromagnet (control circuit) closes the contacts

Important characteristics of contactors:

voltage rating:
the voltage of the power circuit controlled by the SSR
continuous rated current:
the maximum current that can safely flow through the power circuit – should be 20% above the maximum current you anticipate
max braking current:
the maximum current the relay can interrupt without arcing (if arcing happens, the current is not interrupted) – should be greater than the rating of the fuse on the power circuit
contact resistance:
the resistance introduced in the power circuit when the contacts are closed – the smaller, the better as the heat generated in the relay is proportional to that resistance (for example a resistance of 0.5 mOhm will produce 5 watts of heat with a 100 A current, and 20 watts with 200 A)
coil power:
the power consumed by the control circuit after the relay is closed (more power is consumed during a short time to initially close the contacts) – it depends on the voltage and the conception of the control circuit (some have two coils or electronics to limit power once the contacts are closed)

Examples:

TE Connectivity
High current relay 150
price: ~ 70 USD
continuous rated current
max braking current
contact resistance
coil power
180 A @ 23°C
300 A
0.7 mOhm
4 W (0.33A@ 12v) TE Connectivity
IHVA200
price: ~ 100 USD

continuous rated current
max braking current
contact resistance
coil power

200 A
2000 A
0.3 mOhm
5W (0.42A @ 12V) AliExpress
SEV200ADXL
price: ~ 35 USD continuous rated current
max braking current
contact resistance
coil power 200 A
???
> 0.8 mOhm
? 2 – 7 W Blue Sea
ML-RBS 7713
price: ~220 USD continuous rated current
max braking current
contact resistance
coil power 500 A
1450 A
???
0.16 W (0.013A @ 12V) Gigavac
GX14
price: ~ 115 USD continuous rated current
max braking current
contact resistance
coil power 300 A
600 A
0.15 to 0.3 mOhm
2.8 W (0.23A @ 12V) Dongya
DH250L
price: ~60 USD? continuous rated current
max braking current
contact resistance
coil power 250 A
2000 A
0.3 mΩ
2.2 W (0.18A @ 12V)

What you need to check when selecting a contactor (EMR):

diameter of power studs and distance / isolation between them (critical for high currents / big cables)
some can only be mounted in one direction
check the voltage of the control circuit to match your installation
manual override (OFF / BMS / ON) – can also be done with a 3-way switch and some wiring on the control circuit
presence of a circuit to suppress high voltage spikes (if not you will have to add a spike suppression diode in the control circuit)
most contactors are mono-stable (they return to the open state when the control circuit is no longer powered), but some that are bi-stable (they stay in the last state even after the control circuit is no longer powered) – they have two control circuits (one to open and one to close) that are activated by an impulsion – the advantage is that they do not consume any power once switched in the desired state, but additional logic is required to control them (may not be the best choice for safety applications where you want the failure mode to be in the open state)

Benefits:

low contact resistance (low heat dissipation)

Other installation considerations:

if this a main contactor that is activated most of (all) the time you need to consider the power consumption of the control circuit (0.42A corresponds to 10Ah per day) and select a contactor with low coil power
the control circuit is inductive (coil) and an unacceptably high voltage spike can be produced in the control circuit when the device is de-energized – this may damage the controlling electronics and you should insert a protective device (diode) in the control circuit (some contactors are already equipped with such protective device and do not need external protection)
an arc is produced when the contactor interrupts the current – they cannot be used in environment with flammable gas or where equipment is sensitive to electromagnetic interference (EMI)
being mechanical devices you need to mount them only in a position approved by the manufacturer and they may be sensitive to shocks and vibrations leading to potential unreliable or erratic operation

Solid State Relay (SSR)

An electronic switching device that switches ON and OFF when an external voltage is applied across its control terminals.

the power circuit is closed by an electronic switch (usually a MOSFET for DC applications)
when the control circuit is not powered, the MOSFET does not conduct electricity (except for a very small leakage current)
powering the control circuit turns on a light emitting diode (led) – the generated light activates a drive circuit that closes the electronic switch (the MOSFET becomes conductive)

Important characteristics of a SSR:

voltage rating:
the voltage of the power circuit controlled by the SSR
continuous rated current:
the maximum current that can safely flow through the power circuit – should be at least 20% above the normal maximum current you anticipate
maximum surge current:
the maximum current that can flow through the power circuit for a very short time (should be well above the rating of the fuse protecting the power circuit)
on-state resistance

– Rdson

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(mOhm) / on-state voltage drop (V):
corresponds to the amount of energy the SSR will have to dissipate in the on state
max off-state leakage current:
even in the off-sate some current goes through the electronic switch (this could drain a battery over a long period)
control voltage:
the voltage of the control circuit with the minimum turn-on voltage and the must turn-off voltage

Examples:

Crydom
HDC60D160
price: ~ 150 USD
voltage rating
continuous rated current
max surge current (10ms)
on-state resistance (Rdson)
max voltage drop
max off-state leak current
control voltage
7 – 48 VDC
160 A
470 A
3.5 mOhm (90W @ 160A)
0.56 V
0.1 mA
4.5 – 32 VDC
CG Instrument
(AliExpress)
SSR-200DD
price: ~ 15 USD

voltage rating
continuous rated current
max surge current (10ms)
on-state resistance (Rdson)
max voltage drop
max off-state leak current
control voltage

4 – 220 VDC
200 A
???
??? (see heat sink!)
???
2 mA
3 – 32 VDC Kudom
KSJ30D100
price: ~ 55 USD voltage rating continuous
rated current
max surge current (10ms)
on-state resistance (Rdson)
max voltage drop max
off-state leak current
control voltage 30 VDC
100 A
260 A
6 mOhm (60W @ 100A)
0.6 V
0.1 mA
4 – 32 VDC

What you need to check when selecting a SSR:

power stud diameter and distance / isolation between them (most low cost SSR seem to have 4mm studs which is not suitable for power applications with large lugs)
power (heat) dissipation at operating current – this is wasted energy you cannot use

Benefits:

control circuit consumes very little power (less than 20 mA)
reliable – there are no moving parts (position insensitive, resistant to shock and vibration)
no spark / electric arc

Other installation considerations:

make sure you get a SSR designed for DC current (SSR for AC current have different switching principle and some may not open when used with DC current)
although they are very reliable, when a SSR fails, it is often in the closed state, so that may not be suitable as a safety device for some installations
SSR for DC current most often only work with current flowing in one direction – check the SSR characteristics if using it to isolate a battery that is charged and discharged
most SSR dissipate a significant amount of energy and need to be mounted on a heat-sink
– the Crydom HDC60D160 with an on-state resistance of 3.5 mOhm dissipates 90W at 160A
– the CG SSR-200D has no data about on-state resistance, or heat dissipation, but looking at the suggested heat-sink, it must be a lot!
if a SSR is mounted in the same closed compartment as the battery, you want to be sure that the generated heat will not impact the battery temperature too much (lithium batteries age faster at high temperature)

The four questions to ask yourself:

Does polarity matter?
- most SSR are designed to work only with current flowing in one direction (they can cut charge or load buses separately, but cannot isolate a battery)
- most EMR are insensitive to current direction

Is power dissipation (heat) a concern?
- EMR dissipate very little power even with high current
- some SSR will generate a lot of heat when the current increases and need to be mounted on a heat-sink – they may be more suited for lower current applications (or when high current is not frequent / does not last long)

Can you afford the power consumption of the control circuit?
- SSR’s control circuit consumes very little power
- some EMR’s control circuit consumes from 0.3 to 0.5 A continuous @ 12V and much more for a brief moment when they are switched ON:
  - you must validate that the control circuit can handle that current (if not, you need to add an intermediate low power relay to control them)
  - power consumption can be 7 to 12 Ah per day – if the system is unattended for a long time you need a charge source (if you keep the solar panel in float all the time, make sure the float voltage is low as a lithium battery should not be kept fully charged)
Where is the device going to be mounted?
- SSR can be mounted in any direction, but low cost models often have very small posts
- some EMR can only be mounted in one position (check specifications)

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