Smart dual battery isolators not so smart?

CJ, you pretty much have the important facts right in terms of how most of these systems work.

But there is no need to worry. Despite fears held and repeated by some people about charging dissimilar batteries in parallel, there is little to worry about. Just do it.

I have been using AGMs as aux batteries for over four years. I currently have five AGMs used as aux (3 in van and 2 in vehicle - one of which is in engine bay). I have never had a problem and one of the AGMs in the vehicle is now over 4 years old.

There are many 'old wives tales' about 12 Volt batteries. In my view, you have been told one of them.

Norm C

3)isolate the starter battery when ignition is off, power from au only


There's been a fair bit of discussion recently about this misconception also.
Some (most) voltage sensing isolators keep the batteries connected for some time after engine stopped, ignition off until the main battery voltage drops.

1) generally the "smart" systems keep the batteries isolated until the main battery reaches a pre-determined voltage level ie. charged
2) at this point, the second battery is electrically connected. Since the main battery is theoretically charged, it should not accept any further charging current to maintain the required voltage, therefore all remaining current should go to the second battery at this time.
3) This is the most important aspect - the whole point of having a second battery!

As someone else said, many of the so-called "smart" systems leave the batteries connected after the engine has been shut off, until the batteries drop back to the pre-determined voltage level. Some even click in and out sevel times a second if the aux battery is being used. This is because the load on the aux battery causes the voltage to drop. If it's still connected to the main battery, the main battery voltage drops too causing the system to isolate. This then relieves the main battery so the voltage increases again and the "smart" solenoid then reconnects the batteries and this process repeats... very bad for both batteries.

When I wired in my system, I did it in a far simpler way - they were isolated when the engine wasn't running (even if ignition was on) and they were connected as soon as the engine started. If you wanted to get fancy, you could add a timer so the batteries connected a certain time (ie. a couple of minutes) after starting the engine giving the main battery a chance to be topped-up after starting the engine before connecting the aux battery. And still isolate as soon as the engine is turned off.

Hi CJ

Well spotted and this thread will clear up your worries. We are working on this issue and fingers crossed will have our new isolator available by the end of the year.

We have decided to redesign the battery isolator from scratch and have a worked out a way using 3 integrated circuits to switch the alternator and not the batteries but still supply enough current to the main battery to run the car electrics. Load demand will shift the alternator back to the main battery should the load be greater than circuit 3 is supplying back to the main battery and once the load is reduced it flips over the other way again. All done with a 3 minute dwell to prevents any surges and spikes.

Regards

Derek from ABR

CJ,

Your assumption is not correct for some smart isolators, according to advice I received from Piranha.

The main starting battery is always protected and cannot be drained by a load connected to the auxiliary battery.

Once the controller has determined that the primary battery has reached a certain voltage, it switches in the auxiliary circuit to enable charging of the second battery.
The two batteries however remain electrically isolated from each other and a load applied to the auxiliary battery will not affect the primary starting battery in any way.

Other makes of Controllers may well perform differently, but are they Isolators, or merely Controllers.


Bill.

CJ
My Redarc is connected to the post on the alternator where the main battery cable is connected to so it is not after the main battery (in circuit terms) so it can actually switch the Aux on and off from the main circuit based on what it reads in the main circuit.
Cheers
Trevor

Watch out for a spectacular new RedArc product especially for caravans and campers due 1/7/09.

Cheers,
Peter
OKA196 Motorhome

CJ,
ROTRONICS make a battery Isolator that WILL charge the Auxiliary (Deep Cycle) battery Directly by the Alternator, with-out having the Cranking battery in the same charging circuit, so you are charging the Deep Cycle battery system independently.

The Cranking battery is charged and is then Isolated from the alternator.

Maîneÿ . . .

Guys,

Thank you all for your replies. It has cleared a few things up for me, I hope it has been of use to others

CJ

If you want to isolate your ?100series TD twin batteries, then you have two options:

#1 Simply fit a battery kill switch between the two batteries. Keep them joined most of the time. If you are camped more than one night, you might want to lift the bonnet and flick the switch to isolate them. The modern fridges don't use much power these days. This option costs $20 and works more reliably than any of the flash (???Smart) isolators. But none of the 4wd shops will sell this because they won't make money. Best to have two identical batteries (eg Exide Extremes) for this setup.

#2 Buy a seriously Smart isolator - eg Rotronics MH10 that will cost twice as much as a Redarc not-so-smart isolator. It is hooked into the IGN circuit, so the batteries are isolated when you switch the motor off. Costs about $300 for the isolator. Can use dissimilar batteries with this setup.

I personally favour #1 for a 2 battery system with my style of camping/4wding. I drive the vehicle every day and twin batteries crank nicely and last a long time. If you stay put for a couple of days, or really want an AGM, you might prefer option #2.

CJ,

One problem I had was, when the main battery was totally dead, there wasn't even enough charge for the smart battery system to switch over to run the aux in parrellel. I had to install a manual override switch as the smart battery system was connected to my standard 12v electrics. This works well though and a good backup nonetheless.

In my case, I have a number of auxiliary power points throughtout the vehicle that run directly of my deep cycle auxiliary battery and those outlets run irrespective of the key position. This works really weell as I have a light permanently plugged in whenever I am on the road.

Other than that, no problems with parrellel charging after many years. It does take quite a drive to get them both charged so I always leave home after leaving them on the 240V charger overnight.

Good luck.

Peter

Now I am totally confused-pretty electrically challenged for a start
Perhaps someone can help
Had a rotronics duel system fitted -seemed to work as intended
have a national lunar volt meter connected which shows volt condition of both batteries with a low vlotage alarm
last year alternator had a problem was reconed-batteriies didn't seem to retain charge at the level noted before-no leakage detected by auto elec
Suggesteed redarc was fitted.now batteries seem to charge simultaneously as i suppose is intenede -ie 2 separate solenoid clicks audible but my confusion is that in the morning both batteries seem to be equalisedat the same state .Ie crank and aux seem to draw downto same voltage
WHat's wrong?
At least rotronic system would reflect usage ie frig on aux
this way it's just both batts down to danger level overnight.
???
Please make replies in as easy to understand language as possible'
Thanks,
Mary

Hi mainey,
The overnght voltage of both batts is barely 12 volts now frig will cut out if left on for more than 12 hrs.Previously would get 2 days from aux without starting engine.Since the alt problem seemed to be losing charge but no leakage detected by 2 different testers.Am running delco calcium batts and even replaced these.Auto elec suggested Redarc but i feel that all it does is equalise voltage between bat and not really isolate crank at all,Always seem to be able to start but it's a bit slow sometimes.
This still occurs ater a drive of over 300k's .Do you think it's an alt problem?
Cheers,
mary