Voltage Sensiing Relay Operation
Submitted: Sunday, Dec 20, 2009 at 12:17
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gazand CJ
I'm about to install a dual battery system in my new truck.
I've installed a system before using a relay with a delayed timer to isolate the batteries and so that the alternator gave priority to charging the cranking battery for a couple of minutes before diverting power to the auxiliary battery in a parallel arrangement.
I am contemplating using a voltage sensing relay this time as an isolator but I have a question about whether or not it gives any priority to the cranking battery when charging.
The reason for my concern is as follows:
My understanding of the relay is that it will isolate the batteries whenever the voltage in the cranking battery is below a nominated level (12.7v for instance). If it is higher, the systems are connected. To sense the charge, the relay must measure the voltage on the crank battery side of the relay so that it can open the circuit if needed.
When I test my existing dual battery system in a 120 series prado the voltage measured at the crank battery side of the isolating relay immediately after the engine is started is about 14.37v, even though the voltage in the cranking battery immediately before starting was 12.7v.
If this is the case, how does the voltage sensing relay give any priority to the charging of the cranking battery? My testing indicates that the voltage sensing relay would connect the batteries as soon as the ignition was on, irrespective of what the charge would be in the cranking battery once the ignition was switched off again.
I would have thought that measuring the amps going into the cranking battery would be a more sensible determinant of its state of charge when the alternator was operating rather than measuring the voltage of the charging source.
Cheers
Gary
Reply By: Member - Mike DID - Sunday, Dec 20, 2009 at 12:35
Sunday, Dec 20, 2009 at 12:35
Trying to estimate the state-of-charge of a battery while it's being charged is a total waste of time.
A lot of what you read about what this particular Battery Isolator can do is mainly fantasy marketing hype.
If you haven't used your Starting Battery for winching, there is no need to worry about charging it fully before charging the Auxiliary.
If your Auxiliary is flat and the Isolator connects it to the Main Battery, there will NOT be any significant transfer of charge to the Auxiliary.
AnswerID:
395809
Reply By: gbc - Sunday, Dec 20, 2009 at 13:06
Sunday, Dec 20, 2009 at 13:06
What you wrote gazand CJ is what I understand to be correct for a VSR - not instantaneous, but not more than about ten secs either for the solenoid to shut after starting.
There's a few too many of us around been running them for a bit too long now to assume they aren't any good though. Only better system imho is to run two alternators.
AnswerID:
395817
Reply By: oldtrack123 - Sunday, Dec 20, 2009 at 15:41
Sunday, Dec 20, 2009 at 15:41
Hi Gary
The usual operational points are pull in @ 13.2v approx drop out @ 12.6vapprox .This protects your main battery from discharging when engine OFF.
If the crank bat is near fully charged it will quickly rise above 13.2v Once the engine starts.
This may not occur immediately if you have a diesel with an after glow relay for the glow plugs. It may also drop in & out if your aux is extremely low &/or you have other loads
AnswerID:
395834
Reply By: Member - John and Val - Sunday, Dec 20, 2009 at 16:27
Sunday, Dec 20, 2009 at 16:27
Garry,
Sounds good in principle to charge the cranking battery first, then parallel in the auxilliary battery. In practice, with the alternator cold it will deliver to the cranking battery its peak voltage - i.e. 14+volts which will immediately trip a VSR into action to connect the auxilliary battery. The current flowing to each battery will be determined largely by the battery state, with some effect from the resistance between each and the alternator.
I don't use a VSR. I have my relay set up so that the aux is connected in parallel with the cranking battery whenever the ignition is ON AND the oil pressure warning light is OFF. This combination can only occur with the engine running which I think is all we really want to achieve. This has worked
well for me for the past decade or so.
HTH
John
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AnswerID:
395838
Reply By: gazand CJ - Sunday, Dec 20, 2009 at 17:03
Sunday, Dec 20, 2009 at 17:03
Thanks everyone for the quick replies. Seems just another way to isolate the batteries. Enjoy the rest of your weekend. Gary
AnswerID:
395841
Reply By: pop2jocem - Sunday, Dec 20, 2009 at 19:06
Sunday, Dec 20, 2009 at 19:06
Gary, I think what you are seeing on startup would indicate that your cranking battery is in very good condition or that your Prado is not drawing much current to start, or both, meaning all is good. The function of a VSR is to preserve the charge in your cranking battery for starting and allow other accessory loads to be carried by the other battery. If your cranker has not had to supply a lot of power then its state of charge will not need a very long charging time and the relay may activate fairly quickly. Try leaving your headlights on for a couple of hours without the engine running and perform the same tests that you have done. You may find that the relay will keep the batteries separated longer to build up the cranking battery which is now in need of a longer charging time.
Cheers
Pop
AnswerID:
395853
Reply By: Sand Man (SA) - Monday, Dec 21, 2009 at 07:29
Monday, Dec 21, 2009 at 07:29
There are many types of Isolators around from very cheap to not so cheap and they each provide different operating techniques.
To my way of thinking, an Isolator should do exactly what the name says.
A "smart" Isolator such as the Piranha DBE180S keeps both batteries electronically isolated from each other and is the safest way to go IMHO.
This is all done electronically and there is no need to "remember" to flick a switch to isolate or connect batteries together.
Bill.
AnswerID:
395901
Reply By: Nik & Dee - Wednesday, Dec 23, 2009 at 15:09
Wednesday, Dec 23, 2009 at 15:09
Just a thought, did you disconnect your altenator from the battery before you checked the volts across the cranker? If not you are seeing the 14V from the alternator. There should be no reason for a battery to increase voltage I know of.
AnswerID:
396195
Follow Up By: Member - Mike DID - Wednesday, Dec 23, 2009 at 23:03
Wednesday, Dec 23, 2009 at 23:03
If you disconnect the Alternator from the Battery while the engine is running, there's a good chance you'll blow up the Alternator and any electronics connected to it.
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Follow Up By: Maîneÿ . . .- Thursday, Dec 24, 2009 at 07:45
Thursday, Dec 24, 2009 at 07:45
Nik & Dee,
gazand states: "When I test my existing dual battery system in a 120 series prado the voltage measured at the crank battery side of the isolating relay immediately AFTER the engine is started is about 14.37v, even though the voltage in the cranking battery immediately *BEFORE STARTING* was 12.7v"
As Mike has said, "If you disconnect the Alternator from the Battery while the engine is running, there's a good chance you'll blow up the Alternator and any electronics connected to it" but in this case gazand has clearly stated he has not done this, he has checked the Cranking battery rested voltage before starting the vehicle.
Maîneÿ . . .
FollowupID:
665001