Relay on ground wire of Redarc SBI12

Submitted: Sunday, Oct 04, 2015 at 16:41
ThreadID: 130503 Views:2101 Replies:5 FollowUps:7
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I realised recently that my deep cycle battery is feeding the crank battery until the isolator disconnects. I was thinking of putting a relay on the ground wire of the SBI12 that is controlled by the ignition, therefore when the ignition is off it instantly disconnects the 2 batteries, when ignition is on the isolator is free to work as normal. I feel like the crank battery is stealing power that could run my fridge for longer. Does anyone have any thoughts on this or have tried it?
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Reply By: Frank P (NSW) - Sunday, Oct 04, 2015 at 17:05

Sunday, Oct 04, 2015 at 17:05
ER,
I presume you have a simple and conventional setup where once the isolator operates the batteries are connected and charged in parallel.

Under this arrangement, when the key is off, any load on the electrical system will draw down both batteries until the system voltage reaches the drop-out voltage of the isolator, at which point the isolator disconnects the batteries.

Your crank battery won't steal from your second battery unless it is faulty and/or there are loads normally attributed to the vehicle, such as lights left on, radio, etc.

Likewise, your fridge connected to your second battery will draw off both batteries until the voltage (of both, because they are connected) comes down enough for the isolator to drop out, whereupon the fridge will run off the second battery alone.

If you want to disable the smarts in the SBI12, a simpler way than adding an IGN-controlled relay to the ground wire is to just remove the Redarc module and hook up the solenoid coil to an IGN controlled source.

If you do this you should add a diode across the coil terminals with the band at the positive side of the coil. This acts as a spike/surge supressor, a function normally done by the Redarc module.

Cheers
FrankP

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Reply By: Member - John - Sunday, Oct 04, 2015 at 17:06

Sunday, Oct 04, 2015 at 17:06
G'day, I see nothing wrong with the current set up, it disconnects at a set voltage of the crank battery (12.7Volts), with a delay of 10 seconds, the crank battery is supplying power to the aux battery, not the other way around. Cheers.
John and Jan

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Follow Up By: Member - Bigred13 - Sunday, Oct 04, 2015 at 17:20

Sunday, Oct 04, 2015 at 17:20
Yes agree with John,the isolator cuts out at 12.7V which is in fact 100% fully charged,so it will not affect your start battery one iota.
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John
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Follow Up By: Frank P (NSW) - Sunday, Oct 04, 2015 at 17:33

Sunday, Oct 04, 2015 at 17:33
The OP is not thinking about what affects his start battery. He is thinking about what is affecting his second battery.

After a period of driving both batteries will be at an equal voltage because the Redarc connects them in parallel.

When he turns the key off, they stay connected because the Redarc sees the voltage from the second battery AS WELL AS the voltage from the crank. The Redarc will cut out only when BOTH batteries have dropped voltage.

He wants to discuss whether or not that is the best way to do it, or should he somehow better control the Redarc so it disconnects as soon as the key is off.
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Follow Up By: Member - John - Sunday, Oct 04, 2015 at 17:57

Sunday, Oct 04, 2015 at 17:57
Frank, technically correct, but the Redarc is actually reading the start battery voltage and disconnects at 12.7 volts, (fully charged battery) protecting the start battery from low voltage. If both batteries are over 12.7 volts, the fridge or what ever is drawing current from both, until the start battery drops to 12.7 volts and then disconnects. In my mind a good thing.
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Follow Up By: Frank P (NSW) - Sunday, Oct 04, 2015 at 17:59

Sunday, Oct 04, 2015 at 17:59
That's right, John. See my reply at the top of the list.

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Reply By: HKB Electronics - Sunday, Oct 04, 2015 at 17:58

Sunday, Oct 04, 2015 at 17:58
I wouldn't worry about it, most VSR type isolators work on the same principal, both batteries
will remain connected till the surface charge drops. Assuming no accessories are running this may take a few minutes but very little power will flow from the aux to the main as the only load on the main will be ECU back up power etc. Your more likely to loose a small amount from the main to the aux if a fridge is running for example but again very little charge as the isolator will drop out fairly quickly and your only talking milli amp capacity not amps. Same the other way around, as soon as you put any load on the main ie turn the car headlights on the surface charge will drop and the isolator will drop out within a few seconds, and you would loose very little aux charge.

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Follow Up By: Frank P (NSW) - Sunday, Oct 04, 2015 at 18:09

Sunday, Oct 04, 2015 at 18:09
Thanks HKB, you are correct. In my reply at the top of the list I hadn't considered the surface charge aspect. It is the loss of surface charge that will cause the isolator to disconnect.

In the great scheme of things loss of surface charge on the second battery is a miniscule loss of capacity.

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Reply By: Member - Phil G (SA) - Sunday, Oct 04, 2015 at 18:33

Sunday, Oct 04, 2015 at 18:33
I have done what you have suggested in the past. The Redarc isolators used to drop out at 12.5V and while connected, the solenoid used something like 110mA (from memory). The issue I found was not with camping, but when stored at home - both batteries would get drawn down to 12.5V even though they had no load on them. Made it impossible to keep your batteries fully charged.

Redarc addressed the problem about 5 years ago by lifting the cutoff voltage to 12.7V. With wet cell batteries, I am OK with this. But if I had a pair of Optimas (or AGMs) in there which sit at a higher voltage, then I would fit the relay.
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Reply By: Enormous Racing - Sunday, Oct 04, 2015 at 18:39

Sunday, Oct 04, 2015 at 18:39
I just find that the crank battery voltage slowly decreases to below 12.6v, the deep cycle holds a higher voltage of 12.85 for longer. My isolator disconnects at 12.6v. So the crank battery is slowly pulling power out of the deep cycle until they both reach 12.6, then it disconnects. When the car is not being used for camping & the fridge isn't switched on, would this power transfer be reducing the life of the deep cycle?
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Follow Up By: Frank P (NSW) - Sunday, Oct 04, 2015 at 21:57

Sunday, Oct 04, 2015 at 21:57
ER
I don't think it would reduce the life.

It might take a few amp-hours out of the second battery, but IMHO not enough to worry about.

In the relentless pursuit of perfection you could do as you suggested in your OP, or as I suggested in my reply, to prematurely disconnect the isolator but I'm not sure there will be a material benefit.

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Follow Up By: Member - Phil G (SA) - Monday, Oct 05, 2015 at 17:04

Monday, Oct 05, 2015 at 17:04
You say "I just find that the crank battery voltage slowly decreases to below 12.6v, the deep cycle holds a higher voltage of 12.85 for longer. " - It doesn't quite happen that way - the 2 batteries are connected while the Redarc isolator is active, so are at the same voltage until they disconnect. Can I suggest that they disconnect at 12.7 and ongoing parasitic losses reduce the cranking battery a little further (seems to happen more on the modern vehicles) while the aux voltage creeps back up.

With regard to cranking battery voltage during storage, I agree that 12.7 doesn't matter, 12.6 should be OK and 12.5 is getting lower than I would like, but as Frank says, I don't believe there would be significant damage done to your cranking battery.
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