Interaction between 2 relays - Automotive battery isolator circuit

Thread Starter

tal mann

Joined Apr 23, 2015
11
I'm trying to understand relays in this circuit. It's a kill switch operated battery isolator. R1 is a TE high current relay and R2 is a 30 amp automotive relay.

The battery is connected to terminals 5 (not labelled) and 2 of R1 and 86 of R2. In the resting state, the coil circuit of the R1 relay and the R2 relay are both not energised and there is no ground path. No power out to the car/starter.

I'm confused about continuity. Terminal 1 of R1 and 30 of R2 appears to be connected to terminal 2 on R1 through the coil circuit of R2. Likewise, terminal 85 and 30 of R2 and terminal 1 of R1 appears to be connected through the coil circuit of R1. I assume R1 and R2 are not activated because there is no current flow to ground. I assume it is wired this way as it is ground dependent, ie through S1 and/or S2/S3.

Now, pushing S1, the 'Green On' momentary switch, energises the coil of the R2 'Hold on' relay. This creates a ground path through pin 87 of the R2 relay to energise the coil circuit of R1 through pin 1. Power is sent to the car/engine. R2 is held on by ground path 85 to 30 through the relay to 87 to ground. Is this correct?

If either of the 'Red Off' switches are pushed, the ground path through pin 87 of the 'Hold on' relay is cut. Power is cut to the coil circuit of R1, cutting power to the car, starter motor and the 'Negative Cut trigger'.

Now I assume the 'Negative Cut trigger' is wired to the terminal 85 of a conventionally wired relay, eg a fuel pump relay or a coil relay to kill the engine.

 
Last edited:

Ya’akov

Joined Jan 27, 2019
5,997
That is a really bad schematic, It is very hard to read and the layout is confusing.

From what I can gather, it is just a latching relay arrangement. I am not sure why it is wired as it is. I really struggle to follow the circuit in that layout and with the low resolution.

Why do you need to use this circuit? What are you trying to do with it?
 

crutschow

Joined Mar 14, 2008
30,094
Yes, it is a latching arrangement where pushing S1 grounds the two relays' coils, turning them both on.
Relay R2's contacts then close and keeps the line grounded through the S2 and S3 NC switches.

Then if either S2 or S3 are opened, relay R2's coil current stops, opening its contacts, when also de-energizes relay R1.

The diode pulls low when the relays are energized and goes high through the relay coils when de-energized.
 

LowQCab

Joined Nov 6, 2012
2,167
It's always a bad idea to add an extra Switch inline with a Car Starter.
It probably won't last for very long.

You stated that this is a "Battery-Isolator",
where's the second Battery that You want to "Isolate" ?

Cutting the Power to your Car's Fuel-Injection-Computer can cause "re-learning" aggravations
as the Computer will have to "re-learn" Idle-Settings, and Fuel-Trim values each time Power is cut.
.
.
.
 

Thread Starter

tal mann

Joined Apr 23, 2015
11
That is a really bad schematic, It is very hard to read and the layout is confusing.

From what I can gather, it is just a latching relay arrangement. I am not sure why it is wired as it is. I really struggle to follow the circuit in that layout and with the low resolution.

Why do you need to use this circuit? What are you trying to do with it?
Apologies for the poor resolution. A direct link to an image with better resolution is: Link

Local competition regs for motor racing requires "a battery isolation (master) switch which isolates the battery and stops the engine" with two kill points, reachable by the driver and externally at the front A pillar.

There are a number of ways of latching and I was trying to understand how this circuit would work.
I'll possibly use this method: Link
 
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Thread Starter

tal mann

Joined Apr 23, 2015
11
It's always a bad idea to add an extra Switch inline with a Car Starter.
It probably won't last for very long.

You stated that this is a "Battery-Isolator",
where's the second Battery that You want to "Isolate" ?

Cutting the Power to your Car's Fuel-Injection-Computer can cause "re-learning" aggravations
as the Computer will have to "re-learn" Idle-Settings, and Fuel-Trim values each time Power is cut.
.
As above, it's for motor racing that requires "a battery isolation (master) switch which isolates the battery and stops the engine. This is not for starting or stopping the engine, just for emergency use. The high current relay handles the power to the starter and the rest of the car and FWIW an aftermarket ECU is fitted.
 

Thread Starter

tal mann

Joined Apr 23, 2015
11
Yes, it is a latching arrangement where pushing S1 grounds the two relays' coils, turning them both on.
Relay R2's contacts then close and keeps the line grounded through the S2 and S3 NC switches.

Then if either S2 or S3 are opened, relay R2's coil current stops, opening its contacts, when also de-energizes relay R1.

The diode pulls low when the relays are energized and goes high through the relay coils when de-energized.
Thank you
 

MisterBill2

Joined Jan 23, 2018
12,396
There are switches made for the exact purpose of switching off all battery power, They were mandatory in demolition derby cars 50 years ago and may now be mandatory in all racing cars. The better ones used a Boden cable (Push-pull cable) so that they could be close to the battery and avoid extra wire length. The big problem with all llatching relay circuits is the constant drain from the relay coils.
Unfortunately the term "Battery isolation" is mostly used in reference to multiple battery installations, hence the confusion. This is actually a battery cutoff circuit.
 

MisterBill2

Joined Jan 23, 2018
12,396
Now, to make the drawing complete and totally useful to anybody and everybody, it needs the terminal numbers from those relay symbols, (30, 85, 86, 87, #1, #2 ) I am aware that the numbers seem to be common on some brands of the cube relays.
And in the race cars that I have seen close enough to see the details, aside from the instruments all the wiring was about the engine and fuel pump. The only high current draw was the starter motor. The circle-track racers all had the ignition switch on the roll cage, some of them had a starter button there as well. I guess the electric fuel pump came on with the ignition system power. The dragsters were all push-start because of the high compression and no big battery or need to restart the engine.
 

crutschow

Joined Mar 14, 2008
30,094
Here's the redrawn conventional schematic that makes it easy to understand.
Yes.
And I understand it won't work as drawn.
Once R2 closes, there is no way to interrupt the current to its coil.
S2 and S3 need to be in series with R2's contact, not in series with S1.
 
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