My Dual Battery Install

[Symo Advanced Member Find all posts View Profile Close Symo]

I'm over on Fraser right now running a 60l freezer off the optima.
The first night I had an 80l engel as well. The battery was still running them in the morning at 9v started the car and 30 mins latter 12.9v at rest.

Awesome battery!

[AK7 Senior Member Find all posts View Profile Close AK7]

A surprise with your configuration is the 55AH Optima. Its only 20kg in weight - is that why you used that battery?

That's one of the reasons. I won't be fitting a steel/alloy bullbar or winch to my car because of the added weight. Size is another reason. The aircon pipe behind the driver side headlight is extremely fragile and limits the size of battery that can be used. Quality is another reason. Optima batteries are very well regarded.

Does your system charge the main battery first, and then switch to charging your battery?

Yes. You can get the SBI12 Manual from the Redarc website.

Could you use the main battery for power is it was a dual battery?

Yes, however it will disconnect the main battery once it drops to 12.7V (a safety feature). You can also use the "Override" switch to link the two batteries manually.

Can you utilise the spare (Optima) battery to either start, or assist starting?

Yes. Redarc would probably recommend using the SBI212 instead because it can handle the higher currents that may be drawn when starting the car. I suspect my setup would fail to work in this situation because the 100A circuit breakers might trip when they see the initial inrush current. I still have the option of putting my jumper leads between the two batteries.

I am considering battery issues at the moment ...

You're going the right way about it. The more you read about it, the better informed you'll be

I'm over on Fraser right now running a 60l freezer off the optima.
The first night I had an 80l engel as well. The battery was still running them in the morning at 9v started the car and 30 mins latter 12.9v at rest.

Awesome battery!

Sweet Still running at 9V is interesting. How's your car handling the soft sand? I'll be heading up there next weekend.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

The manufactures statement about charging the main first is tad misleading.

If the cranking battery was dead flat and you had a 30A alternator, go back about
20 years then yes the main would need to get some charge before the VSR would kick
in, so yes it would take a little while for the battery voltage to get to 13.2V or thereabouts ie a typical VSR kick in voltage. Note 13.2V is nowhere near fully charged.

With a modern vehicle the alternator will force the battery terminal voltage past the VSR cut in point within seconds of the vehicle starting even with a dead flat main, so in reality it does not charge the main first.

The only units that will are ones with a timed switchover such as the no longer available Rotronics unit. With that unit it waits till the cranking battery reaches the VSR kick in voltage it then connects the aux and starts a timer, it continues to charge both batteries until it determines the main has reached a reasonable state of charge and that the aux is "good" and charging, it then disconnects the main and directs all available charging current to the aux.

It also continues to monitor the main and reconnects it from time to time to top it up if required.

I don't know of any other VSR that have a timed charge capability , but there are a couple of programmable ones available that may offer it, but in practice with a simple VSR it really is not required, you still have to fully charge the main if you don't want premature failure, it can take up to 2 of more hours for the main to fully charge even from a quick start so in reality waiting for the main to charge is not practical, with a very small alternator the kick in voltage level will ensure the main gets some charge before the aux is connected, with newer vehicles with big alternators it really is not going to be an issue apart from lengthening charging times slightly assuming both battery are good.

That was another advantage of the Rotronics system, it monitored the health of the aux and would not connect it unless it was happy the aux was healthy thus ensuring the cranking battery wouldn't be comprised by a sick aux.

Alas no longer available.

[AK7 Senior Member Find all posts View Profile Close AK7]

I've just added some more pics of the accessory sockets I ran to the back of the car...

The manufactures statement about charging the main first is tad misleading.

I questioned that too. Correct me if I'm wrong, but the Redarc simply connects the batteries in parallel provided the factory battery is producing 13.2VDC or more, in which case both batteries will receive charge simultaneously (and given enough time should end up close to the alternator's output Voltage?)... I'd also expect a flat aux battery to pull down a main battery (ie. could pull it below 12.7VDC) - is it foreseeable that these isolators could then cut out before the aux can get charge or is the pull-down too slow for that?? Also, when you say 13.2VDC isn't anywhere near 100% charged, what would you say is? ~14VDC? I don't expect my car batteries to read outside the 12-13VDC range when cold... Pardon my ignorance!

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

The VSR will kick as soon as the alternator forces the vehicles electrical system above 13.2V

With a modern vehicle the aux will never pull the main down, the alternator will
have more than enough capacity to hold its float voltage ie 14.4V therefore
the cranking battery will be forced to 14.4V within seconds of the car starting
the aux will take longer due to cable lengths, bottom line is once car is running both
batteries terminals voltages will be forced above there resting voltages, and it is not
possible for the aux to drain the main.

It is not possibly to tell the SOC of a battery under charge by using voltage as a reference, the alternator or charger will force the surface voltage up therefore volts can't be used as a guide.

This is why the kickin voltage of a VSR really is no indication of the SOC of the battery, a 30A alternator will force the voltage past the kick in point within minutes if a 100Ah battery was "flat" to start with, a 100 amp alternator will force it past the kickin voltage within seconds.

The amps going into the battery will give you some idea, ie if greater than 20A is flowing with a large capacity battery then one could reasonably assume the SOC of the battery is below 80% given around a 14.4V charge voltage. If lower than 20 amps then most likely it is above 80%
soc.

Cheers
Leigh

[AK7 Senior Member Find all posts View Profile Close AK7]

Thanks Leigh I'm certainly not the only one who appreciates the time you spend helping others on these forums!

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

No problem

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

First off, what is with this “KICK IN” voltage? Perhaps if you have some elcheapo Asian isolator, you may have to kick the living crap out of it to get it working but the correct terminology is “CUT-IN” and “CUT-OUT” voltage levels, and has been the terminology for 40 odd years.

There are some other factors to take into account when it comes to the CUT-IN voltage lever being reached at the cranking battery ( and the isolators input ). This is the size and length of the cable running from the alternator to the cranking battery.

While alternators have been getting bigger for a good few years now, it’s not been that long since all the manufacturers have started using decent sized cabling.

Plus as Leigh pointed out alternators use to be much smaller, usually in the 55 to 70 amp region, for the last 20 years. 30 plus years ago, they were around the 30-35 amp size.

Also many newer vehicles now monitor the State of Charge ( SoC ) of the cranking battery just after the motor is started and to do this, they often leave the alternator freewheeling, so it doesn’t put out any charge.

This is done so the vehicle’s Battery Management System ( BMS ), which will have the known current load of the starter motor as a factor, can first see how low the cranking battery drops while cranking and then see how well the battery's SoC recovers when the load of the starter motor is removed.

This BMS activity can take up to 3 minutes in some makes, but no matter how long it takes before the alternator actually starts charging and raises the voltage, you may need to allow about a minute or so before the voltage at the isolator reaches it’s CUT-IN level, and the isolator turns on.

Also note, there are a few brands of isolators, that have a deliberate CUT-IN delay, so this has to be allowed for as well.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

"First off, what is with this “KICK IN” voltage? Perhaps if you have some elcheapo Asian isolator, you may have to kick the living crap out of it to get it working but the correct terminology is “CUT-IN” and “CUT-OUT” voltage levels, and has been the terminology for 40 odd years."

Come on Tim, you need to chill a bit, if only correct terms must be used most would be scared to post anything

Last time I looked at a Redarc document they were using "turn on" and "turn off"
voltage.

Cheers
Leigh

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi again Leigh and that’s a strange statement from someone working in this industry.

Yes people do use the wrong terminology and in this case, it is trivial.

The problem is that in some cases, it can be down and out right dangerous.

So keeping to the accepted terminology is a simple thing to do and can be a safe thing to do.

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

I like kick it in the guts in lieu of 'start'.

[AussieAndy Find all posts View Profile Close AussieAndy]

I like kick it in the guts in lieu of 'start'.

Can we get a new start button made with this on it???

[Guest Close Guest]

Thread being watched gentlemen.. Stay on the topic.

[aridyne Lurker Find all posts View Profile Close aridyne]

Thanks AK7 for a very helpful post.. First up the diode situation now sorted.. Will order one from Leigh's website.. Since I already have a Century Marine Pro 630 installed and a Redarc SBI12D I want to ensure I get the best out of it.. Next is the Baintech power panel at the rear above the 240V outlet and having the inverter come off the 2nd battery instead. :0

[AK7 Senior Member Find all posts View Profile Close AK7]

No worries mate Hope it all works out smoothly Are you putting in the over-ride switch? If so, don't waste your money on a carling rocker switch. They don't fit I got switches from here.