Dual Circuit 2x Aux Batteries

It is simply a Voltage Sensitive Relay system having two relays each of 40 Amp capacity. The relays are both driven simultaneously by the output of the voltage sensing circuitry. The relay contacts connect each auxiliary battery to the source (main battery/alternator) when the source voltage is higher than the set point. At this time all three batteries are in parallel. When the voltage falls below the lower set point the relays de-energise and all batteries are isolated from each other.

An option is to use only two batteries (Cranking and auxiliary) and parallel the output circuits thus offering 80 Amps capacity.

Thanks Allan,

I thought/assumed that was probably the case.

A point worth noting - Although the relays may handle 40 amps (or 80A if wired in parallel), the actual charging current is determined by the state of the battery/s and especially the size of the wiring. It will be much lower than this.

Cheers

John

Yes John, good point to make. The actual charging current would certainly want to be determined by the battery state.
Imagine the effect upon a deep cycle battery by being regularly charged at near 80 Amps!

Hi John, decent size cable will make it easy for any alternator to fully charge house batteries back in a caravan or camper trailer and the instant you add a second battery, like bbuzz is planning on doing, your DC to DC inverter will mean the more power he uses, the longer he has to drive and he will need nearly twice the driving time to fully charge his two batteries using a DC - DC step up than if he just charged straight off his alternator.

Hi Drivesafe -

'decent size cable will make it easy for any alternator to fully charge house batteries'

NO!! Even with the house battery sitting beside the alternator and coupled by very heavy cable, the voltage output from the alternator is not sufficient to FULLY charge the house battery. The deep cycle house battery requires a higher charging voltage than the cranking battery does and the alternator is set up to suit the cranking battery.

When the house battery is quite some distance from the alternator, there will be significant losses in the cabling. Big cables will reduce this loss, but even a small loss will significantly reduce the rate of charge and ensure that the house battery is never fully charged. A second factor also enters in here - the alternator output is reduced as its temperature rises in order to suit the temperature characteristics of the cranking battery. If the house battery in the trailer isn't subject to the temperature increases seen by the alternator, it will lose out again.

A dc-dc charger takes the available voltage and increases it to the level required to properly charge the battery. There is a limit to how useful this can be - the charger draws extra current which it translates into a higher voltage, but the extra current drawn will drop the available voltage further, calling for extra current to correct, causing a further voltage drop...... Heavy cabling is required to keep this effect in check.

"he will need nearly twice the driving time to fully charge his two batteries using a DC - DC step up than if he just charged straight off his alternator." No, that's really not right Drivesafe.

Cheers

John

Sorry John but that’s not correct, with the exception of Calcium/Calcium batteries, ALL other batteries can be “properly” and fully charged with as little as 13.2v

I’ve carried out extensive R&D involving charging batteries with as little as 13.2v and the only requirement is time, and not weeks but just an hour or so more.

Furthermore, even your DC-DC inverters DO NOT charge at higher voltages until the battery is nearly fully charged and even with a single auxiliary or house battery set up, if the battery is low, at the beginning of the charge cycle, an alternator will easily supply both a higher voltage and more current than you DC-DC inverter will.

These things are great for topping batteries off but if the set up is getting a heavy use, particularly when two or more batteries are needing charging, there is NO WAY a DC-DC inverter could hope to compete with what an alternator can do.

The only exception is something like a Sterling inverter and as for Calcium/Calcium batteries, even these can be charged up to 95-97% using nothing more than an alternator.

Hi Mick O, with the way the SC80 works, it means that the SC80 set up will replace more used power in less time so you will actually have fully charged batteries in a shorter driving time and that if you have only one auxiliary battery.

In bbuzz’s set up, with two auxiliary batteries, an SC80 could fully charge his batteries in about half the time a DC to DC inverter could, and for heaps less set up expense.

And I might add, this includes when fitted to vehicles with low operating voltages.

But a 140A relay is only $15.
And can be powered by most alternators.....

LOL! I like putting costs into perspective. Plus redundancy issies....
Once you find the charge lamp circuit, it is a simple bit of wiring where almost any relay will do. (Although for multi-100A capacities, maybe a small relay to power the mega-Amp relays...)
Then you just carry a spare relay (normal $5 for 30A, $10 for 60A, $15 for 140A etc) else get one from wherever.
Or you go solid state for sensitive alternators. A MOSFET to switch 60A or more relays....
And none of the above have any timing issues associated with (expensive) voltage-sensing solutions!

Hi ChipPunk, there's quite a lot more to it than just current rated relays.

If you have a 100 Ah equivalent cranking battery and a 100 Ah auxiliary battery then any one of your relays will give you around 70 Ah of usable accessories power and this goes for all other DBS isolators as well.

With the SC80 and the same battery set up, you have 120 Ah of usable power and the additional power cost no more than what the SC80 costs.

To add the additional 50 Ah of usable power to any of your relay set ups or any other DBS isolator, you not only have to spend more than what the SC80 is worth, just on an additional battery, you now have to make room for another battery plus what ever hardware you need to install the extra battery.

The SC80 actually works out heaps cheaper than your cheapest relay.

You are saying that a 30A relay is less than 120AH??
I think you need to reconsider.

I suggest just referring to Amps - IMO that is much simpler.

Or is the SC80 cheaper and more reliable than a $15 140A relay?

A vehicle's charging system is merely a constant voltage supply (by intent & design) to suit 6-cell lead acid batteries.

Hi again ChipPunk, you need to see how an SC80 works but none the less, your relay, which is switched on and off with the motor, will give you roughly 70 Ah of useable capacity from a 100 Ah battery, that is unless you intend to flatten the battery every time you use it, then yes you will get 100 Ah of use from the battery but two or three cycles like that and you can kiss your battery good bye.

I think you misunderstand the operation of both methods...

How will my system flatten a battery?
The alternator will provide whatever power the battery(s) will accept (assuming it has that capacity) - the relays will not limit that.

Isn't the SC80 a voltage controlled switch for secondary batteries - ie, a voltage switch isolation device that can switch up to 80 Amps (as opposed to 140A for $15; 200A for $25 etc)?

Or is the SC80 somehow boosting voltage as well?

Even though I am only using 60A relay and 50A circuit breakers, I have manually engaged mine when my main battery is a flattery for cranking. (I have yet to fit the intended parallel-cranking bypass contactors LOL!)

If others consider this an unwelcome diversion, please don't hesitate....
I am merely describing a simple system that has been in use for a while by those that realise its benefits over more expensive solutions (and the hype that goes with them).

Hi ChipPunk, your 60A relay is fine for a single auxiliary battery set up, even with two or more auxiliary batteries, the likelihood of the current ever exceeding 80 amps is extremely low and even then it would only be for a very short time period.

The use of any relay over 100 amps is nothing more than sales hype.

The potential problem with your set up is as I posted above, you cycle you battery to a flat state just a couple of times, because you forget to switch off your relay and your up for a new battery.

Electronic controlled isolators don’t forget and just the need to replace one battery because you forget, as I posted, and you cheap relay will work out costing you heaps more.

drivesafe - Like I said, you misunderstand the system/operation I described. And the relay capacity is irrelevant - that is determined by the application.

It IS fully automated.
The batteries are isolated if the vehicle is not charging.
There is no timing nor complication involved (unlike voltage sensors).

The system is easily extended to any number of batteries - just add another relay for each monoblock or bank (driven of the "master" relay - it's the same as extending the SC80 and any similar system).


I agree with the "sales hype" LOL! But the applications vary and people have used anything from $5 30A relays to USD$25 250A contactors and larger - the later typical when running big audio systems or winches off secondary batteries; dual alternators; etc.
But even the 250A contactors (even if they require a $5 30A driver relay) seem to be cheaper than voltage sensing and other commercial systems. And most seem to prefer carrying a spare relay....

For most battery isolator applications, "smart isolators" offer no advantage over the simple "charge light (circuit)" controlled relay - the former usually being subject to limited availability, greater circuit complication, higher cost, more hype and much misunderstanding.

Phew! I didn't understand what you were trying to convey. Other than sensing, the SC80 is equivalent to the Charge Lamp controlled - they both have whatever rated relays or FETs etc. (I think I said I currently (no pun) use a 60A relay with circuit breakers at each battery end.)

So, given that the system I mentioned cannot be "forgotten on", are there any issues?

[I haven't mentioned how easy it is to manually control that system, nor add a self-latching circuit with low-voltage cutout (using a commonly available $20 device and a few switches and optional diodes).... ]

Apologies to bbuzz - or is this ok?

Hi ChipPunk, your going round in circles.

Please go and read up on the SC80.

It does NOT work like other isolators and as I posted earlier, with your set up, and based on the batteries being used as an example, you would have a total accessible capacity of just 70 Ah, while an SC80 would give you a total of around 120 Ah.

And also as posted, to achieve the same capacity with any other isolator or any of your relay set ups, you would need to add an additional battery which, the cost of the battery by itself would make your cheap relay set up heaps dearer than an SC80 set.

And one more point, in the set up bbuzz wants to do, the SC80 would be FAR easier to install and wire up than your relay set up would be, just 3 wires to connect and the SC80 is up and running.

If you can tell me where I can read up on the SC80 I will do so.

I asked if it was a dc dc converter - you did not answer.
I have asked how passing 80A exceeds the capability of passing 140A or 30A etc - you have no explained.
I have found nothing to suggest it is anything other than an isolator with dual outputs - but I have been concentrating on your incorrect statements.

But please - stick to Amps as does the SC80 literature I have seen. AH has nothing to do with how much something can pass - it has to do with battery and other powersource capacities.

I repeat again, for a mere $25, I can pass 250Amps. How many AH is that? Is it less than your 120AH?


As to ease of fitment - I'm not arguing that.
Some say it is far to difficult finding the D+ or L wire from the alternator - they prefer to spend the money.
Others consider it easy else worthwhile and prefer using common everyday relays that are cheaper and reliable than other isolator that have no additional benefits.
If users cannot find that wire or do the install, it can be done by others & professionals. Most will then be able to replace or add relays etc.
The choice is up to the user and their ability.
Maybe bbuzz is capable of tapping into his D+ or L circuit and considers that trivial compared to the extra cabling, fusing, housing etc. And maybe he has a relay that will do the job else doesn't mind spending $5-$10.
Let the OP decide, else request further information.

But it is a simple 3 wire circuit (+GND) - ie, the standard In & Out (heavy current) & earth/ground/chassis connections.
Then "merely" the connection to the alternator or voltage regulator's D+ or L "Charge Lamp" circuit - this being the only extra required compared to voltage sensing systems (normally a short length of thin wire from the nearby alt/reg else dash to the relay with an optional fuse). (Or no fuse if a MOSFET is used either instead of replace the relay or for modern alternators that cannot drive (say) 250mA @ 12V.)


I request that you not continue in circles - let readers decide what they want, or start another thread to invite continuance of this discussion.
Or supply a link to the relevant information....
Else describe WHY the SC80 is not like other isolators.
EG: How does the SC80 charge batteries and pass higher AH capacity than a near-zero resistance relay contact that connects the alternator to the batteries? Are you saying that is uses conversion?
(It isn't another one that claims "prioritised charging" and "monitoring of batteries". LOL - talk about hype - though the SC80 may be different, but not at that price!)

AFAIAC, I have merely suggested an isolator that meets all requirements and is as simple and reliable as it gets.
It overcomes parallel battery issues and provides the quickest charging for connected batteries. (And even a reverse polarity won't destroy its control LOL!)
It is easy to expand for additional batteries, manually defeat or isolate selected banks, manually activate and include a low voltage cutout. And all that with common off-the-shelf items (generally).


And whilst battery condition monitoring (BCM) and alarms can be provided as well as dc-dc converters to boost charge and even help restore batteries, I keep that separate from the isolator function. All of that can be added - it is compatible - but it is NOT part of the basic issue of installing multiple batteries in a vehicle/system and be able to charge them avoiding idle-parallel issues.


If anyone has questions or requires clarification, please ask.
But I find it pointless repeating the same over & over. (Did I include a diagram or provide a link? Ooops?)

Hi gmd, and your are correct in that the name “smart Charger’ is not a true meaning for VSRs but it is a well established generic name used to differentiate between a dumb relay type set ups and a VSR type set ups.

Hi Allan, bbuzz specifically asked a question about the SC80 and some of the answers were incorrect about the way an SC80 works, so, as the designer and manufacturer of these isolators, I simply posted up factual info relating to the SC80.

With apologies - I thought I hit the Post-Followup button....
This R9 should have been the FollowUp to drivesafe's FU1 to R8.


And gmd (R7), let's see what technical information is returned.

Alas "smart" is not defined.
The simple relay system I described has been called a smart isolator because it uses the smarts of the (alternator's) voltage regulator.
But it is the same as a typical voltage sensing system - it just uses the alternator's charging indicator instead of an on-board comparator, and timing delays, and logic.
I consider neither "smart", but that is IMO and by my definitions.


And I cannot stress enough how the "ideal" solution is very dependent on the application etc.

Most will have a dash voltmeter or alarm system to warn of charging issues.
Hence people know how their charging system is coping.
And most find that their alternator can output more power than what their vehicle loads and recharging batteries consume.
Ha! It's almost as good as an Artificial Intelligence system!

Mainey, Exaaaactly mate.!!!