Battery Isolators - Charge or Drain isolation?

Hey Guys,
I've noticed many battery isolators claim to be able to isolate the main battery from the aux battery during charge.
In other words prioritizing the charge of main battery before it switched to the aux battery, as in the below link.

http://www.baintech.com.au/the-shop/...y-12v-24v-100a

It's obviously B/S to claim an isolator that has no direct connection to the alterantor or does not connect to the alternator before the batteries, are able to prioritize main battery charging. Electrically and electronically impossible task from the above design, 'cos there a one positive terminal for the main battery, another positive terminal for the aux battery and the third terminal for the Earth. This clearly indicated isolation for draining, not for charging. So, the unit prevents the main battery from getting drained while the aux battery is used for appliances(engine not running).

Few questions popped into my puzzled brain, which I'm hoping an electrickery expert here can provide me with an answer or two.

1)Do most of the VSR battery isolators make the bogus claim as above? (I have a Matson one that doesn't claim as the above brand one. But, I thought it was capable of prioritizing the main battery charge, until I looked at the isolator, which only have two positive terminals for each battery and an earth terminal, which clearly indicated it couldn't do such thing as I was expecting.)

2) Are there any genuinely charge isolating(prioritizing the main battery) battery isolators out there??!! Do they exist? Any links to that sort of isolation unit?

3)Do they(no 2 above) actually necessary? Are there any benefits of charging the main battery first and then allowing the charging process to the aux battery? One benefit I can imagine is that with a such kind of charge isolation, the need for high amp alternator may be reduced.

Thanx
Dom

The answer lies in the name - "VOLTAGE SENSING Relay". A lot can be determined by the battery voltage.
The unit monitors the battery voltage to determine the state of charge. It does not really 'switch' charging to the second battery; rather it connects it in parallel with the main battery once it determines that the main battery is sufficiently charged.
The main idea of this is to make sure you can (almost) always count on the starter battery having enough grunt to get you going.
Read the text in the link you provided - it does explain itself:
'The relay system manages the split charging of both primary/start battery prior to charging the auxiliary battery bank. The Baintech relay remains open (off) until the start battery reaches a preset voltage. Once the voltage is achieved on the start battery the relay will close (turn on) to allow charging to the auxiliary battery bank.'

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Originally Posted by Brissieboy

The answer lies in the name - "VOLTAGE SENSING Relay". A lot can be determined by the battery voltage.
The unit monitors the battery voltage to determine the state of charge. It does not really 'switch' charging to the second battery; rather it connects it in parallel with the main battery once it determines that the main battery is sufficiently charged.

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Yeah, I understand that there's no switching mechanism from one battery to the other(which would defeat the purpose anyway, 'cos I don't think it's a good idea to disconnect the main battery from the alternator. Besides the above isolator is wired in a way that the main battery is always connected to the alternator). I used the word "switch" in the third question with the meaning of "turning on the switch" to the aux battery. I probably shouldn't have worded it like that. Thanx for pointing that out.

How does it "determine" that?!
VSR relay reads the voltage that comes to the main battery during charging/engine running, and that voltage is the alternator output voltage.
It can't possibly read the main battery voltage until the engine's not running.

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The main idea of this is to make sure you can (almost) always count on the starter battery having enough grunt to get you going.

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Yeah, but not buy prioritizing the charge of the main battery, but rather by isolating the main battery from the aux battery, during the battery drain process.

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Read the text in the link you provided - it does explain itself:
'The relay system manages the split charging of both primary/start battery prior to charging the auxiliary battery bank. The Baintech relay remains open (off) until the start battery reaches a preset voltage. Once the voltage is achieved on the start battery the relay will close (turn on) to allow charging to the auxiliary battery bank.'

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Yeah, but the million dollar question is when the alternator is putting some 14.4 volt, how does the VSR manage to read the main battery voltage while the engine's running??!!
14.4 V coming to the main battery means, that's what the VSR relay reads(when the engine's running). So, it couldn't possibly "determine" the charge status of the main battery while the engine's running. Whatever the reading it would get from the main battery while the engine's running is a false reading of the main battery voltage.

The battery voltage will not be 14.4V under charge until it is well charged.
Yes, the alternator voltage is set to 14.4V (or somewhere near there) but it will not achieve that until the charging current is reduced as the battery approaches full charge. The alternator has a finite internal resistance which accounts for the lost voltage between 14.4V and the actual battery voltage. The regulator only sets the MAX voltage the alternator can put out.
Run a battery down a bit and measure the voltage as the state of charge is restored.
The VSR will 'prioritise' the charge by not connecting the second battery until the main reaches somewhere around 13.5V (depends on the unit) as well as saving the main from discharging by isolating it when no charge is available so ensuring only the second battery is in use.

Sorry, but have to disagree with an sort of useful prioritising by a VSR...
When the engine is running the VSR can only 'see' the Alternator output voltage.

The Alternator reg is now in a voltage regulated current limited state IE it will continue to increase current to try and maintain voltage.
Most vehicles have an Alternator that will easily supply sufficient voltage at idle to tip the VSR when no other major loads are present (Headlamps etc)

Unless the Cranker is discharged so extensively that the charge acceptance rate is high enough to drag the Regulated Voltage Output below the cut out voltage of the VSR then the VSR will happily parallel the batteries.

I have significant parasitic drain on my Cranker and the SOC will often get down to 70 ish percent and the Alternator Voltage and therefore the Redarc will pull in within a minute or two of starting.

Dom... short answers are

1. Usually Yes, most manuf will dance around the plain english so it reads that their gear will do it.

2. Yes, they are usually expensive and the only way it can be reliably down is by current splitting

3. In a vehicle not really because there is suprisingly low capacity drain to start a healthy vehicle in real terms.
Soooo.... because you have a VSR the Cranker is usually near full SOC anyway so when you fire up the Alternator it means either
a) good Cranker low Aux - even tho the majority of the current goes to the Aux there is still enough overhead for all that the Cranker will accept.
b) If the Aux is up near full SOC then the Cranker gets all it can accept anyway if it is low or or not because the Aux charge acceptance rate is very low

The only scenario where it is of any appreciable benefit to a vehicle is when both batteries are well down
You will know if you have a true prioritising device because it will have current monitoring and programmable charging maps
Enerdrive is one mob who I have heard good things about... they may be worth a try.

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Originally Posted by the evil twin

Sorry, but have to disagree with an sort of useful prioritising by a VSR...
When the engine is running the VSR can only 'see' the Alternator output voltage.

The Alternator reg is now in a voltage regulated current limited state IE it will continue to increase current to try and maintain voltage.
Most vehicles have an Alternator that will easily supply sufficient voltage at idle to tip the VSR when no other major loads are present (Headlamps etc)

Unless the Cranker is discharged so extensively that the charge acceptance rate is high enough to drag the Regulated Voltage Output below the cut out voltage of the VSR then the VSR will happily parallel the batteries.

I have significant parasitic drain on my Cranker and the SOC will often get down to 70 ish percent and the Alternator Voltage and therefore the Redarc will pull in within a minute or two of starting.

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Hey Evil Twin,
You wanna fix that parasitic drain asap. I reckon a cranker getting down to 70% can reduce it's life span a fair bit.
Thanx for the clear explanation above.

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Originally Posted by dom14

Hey Evil Twin,
You wanna fix that parasitic drain asap. I reckon a cranker getting down to 70% can reduce it's life span a fair bit.
Thanx for the clear explanation above.

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Agree it will affect the life but there are reasons the way I want it and have my radios and some other toys wired up.
Currently I run two crankers in my truck.
The primary Battery is to all intent factory wired and every thing runs off it.
The second Battery is a 'reserve' battery rather than a 'house' IE no load is normally connected to it (other than plugs and leads for odd jobs) so it is fully charged at all times.
The VSR removes the paralleling when the vehicle is parked up.
I usually run a fully programmeable Intervolt VSR but I am playing with another project so I have one of my Redarcs doing the duty ATM.
I switch the VSR in manually for winching or, if the Cranker is down too far, for starting

My batteries get a 'smart' Mains charge every couple of months and capacity and load tested every 6 months.
The cranker is an AC Delco about 3 years old and still going strong

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Originally Posted by the evil twin

My batteries get a 'smart' Mains charge every couple of months and capacity and load tested every 6 months.
The cranker is an AC Delco about 3 years old and still going strong

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Mine too. I do that once a month. I'm a firm believer in smart charging batteries.
I also put the two batteries through a pulse charger as well.
I have a secondnand el cheapo cranker that's still putting out the same CCA for over two years.
I believe all that 'cos of my smart charging them.