200 series starting battery (split)

CSeaJay...that was the first thing I looked for...wtf did i did leave on???

nothing

you must understand that the trip to the beach is sand and I was only going slow so I must wonder if the alternator has enough grunt at slow speed to top it up.??

1500rpm..it hardly does that at 100...and it's auto trans, so rev's is dictated to me and not me to it....

But I get what your saying.

The Aux is for fridge only and is isolated when ignition off....and I understoood or so I thought, the dc dc charger is not supposed to charge aux until it sense's the main is near full.

It was explained to me on another forum that most of these units can't actually tell when your main is charged unless it has been fitted between the alternator, battery I think they said from memory. Most isolators and bc/dc or dc/dc chargers are connected to the starter battery so when the car is started the alternator voltage increases and that is picked up by the isolator or charger so it is tricked into thinking the main is charged when it actually isn't. You can test that and you will find it's true.
I originally had a Baintech dual batt isolator which claimed it switched over when the main was charged.I contacted them about their claim and I have and email which they admitted their claim was not true that is was just a simple way to explain it to the customers that it cannot detect when a battery is charged.
I have ditched it and run a Redarc bc/dc charger which is also suppose to isolate the main no problems have occurred yet.
With the rpm and charging I done a quick search before I replied originally it said approx 2500rpm for max alternator output so locking it into a lower gear and bringing up the revs will help for shorter trips within reason.

Batt's
a dc-dc charger shouldnt be be giving a signal back to alternator as voltage flow etc is only in one direction thru dc-dc charger...cant be in reverse, meaning that aux battery cant send voltage back thru charger to starting battery...can it?

I expect that's correct from what I read.

same car.... had the redarc changed to DC DC charger

both were and are soI have been told, are wired to only charge the aux when alternator is running.

The first time it happend I was on the beach for several hours fishing.
went to start the car and it was dead.
Auxillary was still running the fridge.

basically same thing when we were back in camp.

HTH
seen that in another thread
might give him a ring

Nick
Splitting the batteries and REPLACING them is the best way to get Aux for running a fridge...IMO!!
the starting battery (passenger side) has been upgraded as has the drivers side to a deep cycle.

I dont want to get into a gun fight with you but its dual!...not duel

Whats a letter amongst friends hahaha
Anyway good luck with it you might find some help in those links from other post

have not checked alternator output but it does not give any rpoblems around town with fridge running. But I am going to get it checked as a matter of course
Service last week by dealer didnt reveal anything and they were asked to check it

I wonder..??
definitely food for thought

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That hypothesis does not hold up Bantam.

Keep in mind that the 100% charge state of a battery is considered to be 12.6 volts.

Typical specs for a solenoid isolator are Turn On = 13.2V and Turn Off = 12.7V
Accordingly, the solenoid will disconnect the second battery if the terminal voltage falls below 12.7V

And whilst ever the alternator is maintaining the terminal voltage above 12.7V no current will flow out of the first battery as its internal voltage will be somewhere below the Full-Charge voltage of 12.6V. Current cannot flow from it into a bus at 12.7 volts or higher and if the bus is below 12.7V then the second battery is disconnected. This is the very principle of voltage-sensing isolators.

I would not try to diagnose Member No1's problem without making voltage and current observations.

Although I am not suggesting that the following is the problem that Member No1 is experiencing, something does occur to me that may well be overlooked in modern vehicles with engine and alternator control technology.

When charging, the second (auxiliary) battery is considered by the system as an "accessory" load and accordingly the negative terminal of the second battery must not be connected directly to the negative terminal of the cranking battery but must be connected to the stud on the body where the cranking battery negative is also connected. This is to ensure that the charging load of the second battery is sensed by the current sensor.

The following is from the Redarc notes:
"It is important to note that current sensing in the vehicle’s electrical system means that all additional electrical accessories must be grounded to the vehicle chassis or body, not to the main battery negative terminal."

The "hypothesis most certainly does hold up ....... the cut in and cut out voltages vary from manufacturer to manufacturer AND it must be considered that the behaviour of the battery differs considerably when there is a source of charge compered to when there is none AND the behavoir will be strongly influenced by the various very low measured resistances in the system.

When a battery that is at a low state of charge .... it will have a low resting terminal voltage ...... but as soon as a source of charge is applied its terminal voltage will rapidly increase, the terminal voltage will rise way ahead of the stste of charge ....... the internal resistance of the battery has much influence in this situation. ...... an older battery will have a much higher internal resistance, lower charge acceptance and its terminal voltage will rise very raidly ... but its state of charge may rise very slowly.

Both the VSRs and the DC to DC chargers are designed to detect that the battery A/ has a source of charge or B/ is in a resting state. ... no more.

My VSR, if both batteries are fully charged and there is no load on either system can take 10 to 15 minutes to drop out after the engine is stopped and the source of charge is removed ...... it takes the battereis this long to return to a resting state and drop below the low threshold.

neither VSRs nor DC to DC chargers can or do detect that the primary battery is at full or even approaching full charge ...... or for that matter any state of charge.

IF the device was detecting full charge or anything like, it would have a threshold well above 13.5 volts ( more like 13.8 to 14.5 volts, what is full charge is an endless argument) ... and in reality would have to monitor current flow.

They detect that the terminal voltage has risen ...... above somewhere around 13.xx ish Volts ( depending non the brand), even a fairly flat battery will rise to a 13.5 ish Volts within minutes of the engine starting ......... now remember hysteresis ....... once the engine has started and the main battery has reached threshold it WILL NOT disconnect till it drops below the bottom 12.5 ish Volt threshold.

In a modern car with a 100- 120 amp alternator even at low RPM it will be capable of kicking out 50+ amps ... enough to keep the main battery and the now connected aux battery above the bottom threshold .......

The primary battery will still have a higher terminal voltage than the AUX battery .... thus current MUST flow toward the AUX battery from the main battery ...... IN ADDITION ..... the majority of the current supplied by the alternator MUST flow to the battery with the lower state of charge.



The alternator holds both batteries above the lower threshold while the AUX battery takes the majority of the current from the alternator and current is shunted from the main battery till both equalise ....... the result in the short term is a nett discharge of the main battery, until the point of equalisation then both will charge.

So you now have around 160 to 200 AH of battery charging from an alternator that can provide 120 is amps MAX ....... so 2 hours at highway speed to even get both batteries close to a reasonable state of charge and considering current tapering 6 to 8 hours ( perhaps more) to fully charge.

Current shunting is something a lot of people simply do not want to acknoweledge happens ....... but it must.

cheers

Hi folks,

Now I'm confused!

Can somebody please enlighten this poor electrical troglodyte regarding how on earth an auxiliary battery, connected to the SAME stud on the body that the cranking battery is connected to, NOT be connected directly to the cranking battery negative? Wouldn't that be providing a direct connection? :-) :-)

I can't remember exactly what Mr Redarc says in his brochure or installation instructions for his SB12 isolator (and yes, I realise that's not what the OP is using), but my recollection is that the cranking and auxiliary batteries remain connected until the cranking battery voltage drops below that of the auxiliary battery. I do know that all of my batteries remain connected for some considerable time after I park and turn off my own vehicle if I have nothing drawing current on either system. While I will check this out in the next day or so, I'm certain that, if I'm mistaken, someone on here will be sure to let me know! :-)

Regards,

.
Hi Charlie,

1) In vehicles with engine and alternator control technology, current flow to and from the cranking battery is measured and becomes a parameter to control the alternator. This current measurement is made in the cable that connects between the cranking battery and the stud where this cable connects to the body/chassis. By connecting the aux battery negative to that stud point the current flow from the alternator to the aux battery is measured and taken into account. If the main negative cable from the aux battery were to be connected directly to the negative post of the cranking battery (and hence to the alternator) then it would bypass the current sensor and not subscribe its current to the charge control system.
Does that clarify it?

2) Cranking and auxiliary batteries remain connected via the isolator until the voltage on the bus that connects the alternator/cranking battery/aux battery has fallen below the isolator "drop-out" voltage of (nominally) 12.7 volts. Due to the phenomena of so-called "surface charge" at the terminals of a recently charged battery, this may be of the order of 13 volts or more, so the isolator remains energised. This surface charge slowly decays and the isolator will drop-out when it falls below about 12.7 volts. There is little energy in this surface charge and it will more quickly decay with some load on the battery such as turning on the headlights. It is simple and normal behaviour of such a system and plays no functional part in the system's behaviour. There is also a few seconds delay built into the Redarc and other solenoid isolators to prevent undesired cycling of the isolator.

.
Ahh Mr Bantam, all the waffle in the world will not change the fundamental laws of electricity that disallow current to flow out of a nominal 12V battery to an alternator fed bus that is being maintained at a voltage of approximately 13.5V.
Certainly, when the alternator ceases to charge, then a battery connected to another of lower state-of-charge will transfer some energy at a rather low rate but the opening isolator would soon put a stop to that.

I am not going to refute your argument point-by-point and generate a peeing contest. But if you actually believe what you have written above then I am afraid that you are not understanding fundamental electrical principals.

Hi Allan,

Thank you very much for your detailed and lucid explanation(s).

NOW it all makes sense! :-) :-) Well, at least enough for me to understand that the "direct' connection to the cranker I was concerned about, actually isn't exactly direct, because of the sensor in the cable connecting the negative terminal of the cranking battery to the earth on the vehicle body.

Re (2) - For some time I'd been concerned that, because the isolator didn't seem to be isolating my auxiliary from my cranking batteries for quite a while after vehicle shut-down, my Engel might still have been draining current from my cranking batteries over that period. While it seems this may occur for a short period while the entire bank is connected as a unit, it appears as though my fears are entirely unfounded, unless the isolator goes out to lunch, as happened to me with an earlier isolator. Needless to say, I'm keeping my fingers crossed! :-)

Thanks once again for taking the time to explain the situation for me.

Kindest regards,

.
Anytime Charlie.

Incidentally, sometimes you will not see any physical sensor in or on that cable. In a few cases they make use of the very small resistance of the cable itself to act as a current shunt for measurement. In which case if there is a need to replace the cable it must be of the same specification to measure accurately.

Alan .... there is nothing to do with any "alternator field buss".

Most dual battery systems there is a direct connection from the main battery positive thru what ever dual battery controller to the positive of the second battery.

If the dual battery controller remains enegrgised and the circult remains closed ....... CURRENT MUST SHUNT between the battery of higher charge( higher voltage) and the battery of lower charge( lower voltage) .......

If the second battery is at a lower state of charge the vast majority of the alternator's charge current MUST enter the battery in the lower state of charge

.
Bantam, do you have some problem with perception of the written word or do you intentionally distort what others have written? Nowhere have I written "alternator field buss". I did write "an alternator fed bus". i.e a bus fed by or from an alternator.

If you actually understood electrical engineering you would appreciate that a "bus" describes an electrical conductor (cable or busbar) that has multiple connections to distribute current between its connections. In this case it is the common cable connecting the alternator, cranking battery and auxiliary battery (via an isolator). It was an appropriate term to use here as it describes a common conductor where the voltage is uniform.

As to your expression regarding current "shunting" between batteries, this is the very purpose of voltage-sensing isolators. Within a short time following charging the "surface charge" has fallen to (typically) 12.7 volts and the isolator opens to disconnect the batteries from each other.

Your final paragraph has nothing to do with this subject. It is another of your Red Herrings and in any case is not entirely correct.

Bantam, you do have something to offer this forum but could you restrain yourself from simply trying to occupy the top roost in the henhouse?