Batteries connection in parallel

Submitted: Sunday, Mar 31, 2019 at 14:41
ThreadID: 138088 Views:1839 Replies:7 FollowUps:34
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I have 3 batteries connected in my ute.
The first battery (starter battery) is connected to a auxiliary battery.
The auxiliary battery is connected to a 3rd battery.
The connection between the starter and auxiliary battery is also via Redarc isolation switch.
The 3rd battery is connected to a 30 amp solar charger.
I assume that the isolation switch will keep all 3 batteries charged whilst I am driving.
When I am stopped for a week or more I am relying on solar power to keep all 3 batteries charged.
The connection cables between the batteries are heavy duty battery cables.
Question to the electrical experts, is there a problem with this setup ?
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Reply By: Keith B2 - Sunday, Mar 31, 2019 at 14:57

Sunday, Mar 31, 2019 at 14:57
I think that when the auxiliary batteries come up to charge, the voltage on them from the solar should cause the Redarc switch to close, allowing the solar to charge all three.
That's the way it works on my rig, which has a 200 watt panel on the roof and a small auxiliary under the bonnet connected with an ARB switch.
Maybe check with redarc.
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Reply By: Member-George (WA) - Sunday, Mar 31, 2019 at 15:06

Sunday, Mar 31, 2019 at 15:06
Thanks Keith, I have 320 watt solar panel connected.
I also run a 1800w projecta inverter and an Engle fridge from the 3rd battery.
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Reply By: Member - Bigred13 - Sunday, Mar 31, 2019 at 16:36

Sunday, Mar 31, 2019 at 16:36
George. as I read it your first and second batteries are the auxillary batteries ,and with the isolator between the the start and aux batteries ,the start battery is only charged from alternator ,once motor is stopped the isolator will disconnect the start battery from the auxillary ones ,so you do not flatten battery ,so if it is isolated no current will flow back to starter battery .
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Follow Up By: Member-George (WA) - Sunday, Mar 31, 2019 at 18:20

Sunday, Mar 31, 2019 at 18:20
Hi, all 3 batteries are connected in parallel
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Reply By: RMD - Sunday, Mar 31, 2019 at 17:03

Sunday, Mar 31, 2019 at 17:03
I would imagine if the 3rd battery is solar charged and it is linked to the 1st aux, then to have solar charge ALL you would require A 2 way VSR between the main and the aux system so charge can happen both directions as required. The main will/should then be isolated when voltage falls to the OFF threshold and prevent main from discharging further. All will be then linked above the ON threshold voltage for charge to each.
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Follow Up By: bellony - Sunday, Mar 31, 2019 at 18:13

Sunday, Mar 31, 2019 at 18:13
RMD is correct.
If your Redarc is a SBI12 it is only one way, if it is a SBI12D it is dual sense and will charge the starter.
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Reply By: Allan B (Sunshine Coast) - Sunday, Mar 31, 2019 at 23:01

Sunday, Mar 31, 2019 at 23:01
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You tell us George....... are you having a "problem with this setup"?

If not, then there is no problem with this setup.

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Follow Up By: Member-George (WA) - Sunday, Mar 31, 2019 at 23:08

Sunday, Mar 31, 2019 at 23:08
Hi Allan, I guess that is the short and logical answer.
However, I have only done the connections today and not made any test runs.
Thought I would ask the question before I drive off.
According my limited knowledge it should work. Cheers
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Reply By: Member-George (WA) - Sunday, Mar 31, 2019 at 23:23

Sunday, Mar 31, 2019 at 23:23
Maybe I should me more precise..
The car alternator will charge the auxiliary battery once the starter battery has reach the "fully charged" status and theredarc switch will then also charge the auxiliary battery from the car alternator whilst the motor is running.
Because all 3 batteries are connected in parallel they should all charge to the same voltage level together.
When I stop and hook up the solar panels they, the 3 batteries, should again charge to the same voltage together and voltage level be maintained by the solar controller.
My only thought was, should I have a ON/OFF switch on the positive cable between the starter and auxiliary battery and switch off whilst the motor is running and back on when connected to solar,
I hope all this makes sense, George
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Follow Up By: Allan B (Sunshine Coast) - Sunday, Mar 31, 2019 at 23:43

Sunday, Mar 31, 2019 at 23:43
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Well George, your system could be described as.... an alternator connected to a starter battery which then connects to a paralleled pair of aux batteries via a Redarc isolator. Also, a solar panel connects to the aux batteries via a regulator.

From there, it depends on exactly which model of Redarc isolator you have used.

If it is the basic SBI-12 then it allows all three batteries to be charged from your alternator and the solar will charge ONLY the TWO aux batteries.

If it is the Dual Sensing SBI-12D then the three batteries will charge from the alternator and ALL THREE batteries will receive charge from the solar.

In either case, the Redarc will protect your auxiliary loads from discharging your starter battery. You do NOT need an on-off switch...... the Redarc isolator performs the necessary operations automatically.

Which Redarc isolator do you have?


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Follow Up By: Member-George (WA) - Monday, Apr 01, 2019 at 13:53

Monday, Apr 01, 2019 at 13:53
Hi Allan, the Redarc battery isolator is the BCDC1220. Cheers, George
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Follow Up By: Allan B (Sunshine Coast) - Monday, Apr 01, 2019 at 14:34

Monday, Apr 01, 2019 at 14:34
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Ah then George, that is more than an isolator. It is a dc to dc charger with a maximum output of 20A. Quite a good choice for charging your auxillary batteries. It also acts as an isolator to protect your starter battery from being discharged by the aux loads.
The Redarc BCDC1220 will however not allow the solar to charge your starter battery, only the pair of aux batteries.
In any case, there should be no need to charge the starter battery from the solar whilst camped unless you use a lot of car auxilliary power whilst stationary, such as radio, interior lights etc. Be careful.
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Follow Up By: Member-George (WA) - Monday, Apr 01, 2019 at 15:09

Monday, Apr 01, 2019 at 15:09
Hi Allan, when stationary for a week or more, the starter battery is slowly dropping voltage.
I thought that if it is connected in parallel with the other 2 batteries, the solar controller will keep it charged in line with the other 2 batteries ? Cheers
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Follow Up By: Allan B (Sunshine Coast) - Monday, Apr 01, 2019 at 17:23

Monday, Apr 01, 2019 at 17:23
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George,

Let's take this one step at a time.

1) If your starter battery is progressively dropping voltage then there are two possibilities........
a) The battery is self-discharging due to plate disintegration. No cure other than replacement. Or....
b) A circuit load is drawing continual current. Not easy to identify.
One way to test is to remove the negative connection from the battery for the time it usually takes to discharge then observe if the battery voltage has fallen. If it has then it is battery self-discharge. If not, then it may be a connected circuit current drain.

2) There is no way to maintain a solar charge to the starter battery using only your existing equipment.

3) A method of delivering solar charge to the starter battery could be achieved by connecting a Schottkey diode from the solar to the starter battery. This would allow current to flow to that battery but prevent any current flow the other way. This would preserve the integrity of the battery for supplying the engine only. For reliability, there is a little more to the circuit but not much. Let me know if you want assistance with this. My email address is in My Profile.

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Follow Up By: Member-George (WA) - Monday, Apr 01, 2019 at 19:07

Monday, Apr 01, 2019 at 19:07
Hi Allan, I understand your point 1

Point 2, I have tried this and while on solar power, all 3 batteries equalize and while the sun was shinning, they all went from 12.6v to 13,2v
When the sun disappeared for several hrs, all 3 batteries stabilized on 12.9v

Point 3, seems to be irrelevant with reference to point 2

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Follow Up By: Allan B (Sunshine Coast) - Monday, Apr 01, 2019 at 21:59

Monday, Apr 01, 2019 at 21:59
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In that case George, your system cannot be wired as I have understood.
I would draw it out but am away from my computer and using a tablet with no drawing capability. I will try to use text to create what your wiring to SHOULD be.

ALTERNATOR----ENG BATT----- BCDC-----AUX BATTS----SOLAR REG------SOLAR PANEL

From this you can see the direct connections. There is no direct connection between the Engine Battery and the Aux Batteries. Also there is no direct connection between the Engine Battery and the Solar Regulator outpot or the Solar Panel itself. The Redarc BCDC sits between the Engine Battery and anything else and it prevents any current flow in the reverse direction.
If this is not how your system is wired then it certainly should be.
Is there any direct connection from your Engine Battery and the Aux Batteries or the solar?

If all the batteries are maintaining the same voltage at all times then they MUST be connected directly together. And if they are, then your Engine Battery is NOT protected from being discharged by the auxilliary loads.
I have no other explanation for the circumstances you describe.

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Follow Up By: Member-George (WA) - Tuesday, Apr 02, 2019 at 17:10

Tuesday, Apr 02, 2019 at 17:10


This is a sketch of my battery set up, a bit rough but I am not a draughtsman

The Solar + and - are also connected to Aux. 2 battery

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Follow Up By: HKB Electronics - Tuesday, Apr 02, 2019 at 17:35

Tuesday, Apr 02, 2019 at 17:35
Hopefully that is not the way the auto electrician wired the DCDC, if it is wired like that you have the DCDC charger input and output connected to the start battery unless there is yet another main battery hiding somewhere? You can take it frm here Allan, I'll be interested to see where you end up:)
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Follow Up By: Allan B (Sunshine Coast) - Tuesday, Apr 02, 2019 at 18:46

Tuesday, Apr 02, 2019 at 18:46
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George,

Now it is all clear.
There is an error in that setup.

The alternator should be connected directly to the starter battery. As the vehicle was built.
Then the BCDCshould be between the starter battery and the pair of aux batteries.
This will ensure that your starter battery is isolated and protected from the drain of auxilliary loads on the aux batteries.
The solar is correct as it is.... connected to the aux batteries.
But the solar will only charge the aux batteries. It will not charge the starter battery and should not need to. The starter battery is fully charged by the alternator, as the vehicle was built.

This present setup still does not explain why the starter battery reportedly lost charge. I am still of the opinion that it may be suffering from internal self-discharge. Disconnecting it completely from all wiring whilst it is fully charged and observing if it then loses charge over a week will determine this.
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Follow Up By: Member-George (WA) - Tuesday, Apr 02, 2019 at 19:16

Tuesday, Apr 02, 2019 at 19:16
Hi Allan, first of all, thank you very much for taking the time to follow up for me. Much appreciated.
I should mention that the BDCD 1220 and aux. battery 2 were installed 4 years ago and has always worked well.
I installed the solar set up and aux. battery 3 and connected all 3 batteries in parallel only yesterday.
I will also do as you suggested and disconnect aux 2 battery and see if it will hold a full charge. Cheers, George
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Follow Up By: Frank P (NSW) - Tuesday, Apr 02, 2019 at 20:45

Tuesday, Apr 02, 2019 at 20:45
As Allan is away from any sketching software I offer this as what I think George needs. (no fuses shown, just a layout)

George, of you want to get solar to your start battery I think there will need to be a couple of extra cables and manual switches or maybe a relay.

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Follow Up By: Allan B (Sunshine Coast) - Tuesday, Apr 02, 2019 at 20:47

Tuesday, Apr 02, 2019 at 20:47
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No George, wasn't it the starter battery that was losing charge?

Maybe charge them all up, remove the connections from all of the batteries and look for voltage losses. It is one part of assesing battery health.

But most of all, re-arrange the BCDC to be between the engine battery and the aux batteries.

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Follow Up By: Member-George (WA) - Wednesday, Apr 03, 2019 at 02:18

Wednesday, Apr 03, 2019 at 02:18
Hi Frank, thanks for that drawing, much better than mine.
My set up is exactly as you have drawn, except, positive and negative from Aux battery 1 is also directly connected to positive and negative on Start battery.
Everything else is as you have drawn.
I will update your drawing and show in red the additional connections.
I cannot post it on the forum right now, my PC has stopped logging on and is with the technician. Should have it back Wednesday PM and will post it then.
Allan and HKB, would like your thoughts as well.
Cheers, George
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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Apr 03, 2019 at 10:41

Wednesday, Apr 03, 2019 at 10:41
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George,

I eagerly await your updated wiring sketch to see if you have fully understood.

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Follow Up By: Member-George (WA) - Wednesday, Apr 03, 2019 at 17:36

Wednesday, Apr 03, 2019 at 17:36
Hi all, PC is back and running again.
First of all, a big thanks to Frank for providing an excellent drawing of my battery set-up.
I have revised the drawing, in colour, to better explain what was there before I made the solar connection changes.

All that marked in GREEN is exactly what my set-up was before I made any changes.
That set-up was made several years ago by a qualified auto electrician.

All the rest, marked in BLACK, is exactly what I set-up a couple of days ago.

The connections, marked in RED, is what I included in addition to the black set-up.

Questions, Should I disconnect the RED POSITIVE connection when I am driving and reconnect when back on solar ?
Or, can I leave it connected whilst I am driving ?

With reference to the starter battery slowly loosing voltage over a couple of weeks. This is not of great concern to me, when I am driving it will be brought up to full charge and when I am stopped and connect solar it will once again maintain a full charge, depending on the load applied at various times.

I hope I have explained clearly and look forward to further comments. Cheers, George.
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Follow Up By: Member-George (WA) - Wednesday, Apr 03, 2019 at 17:55

Wednesday, Apr 03, 2019 at 17:55

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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Apr 03, 2019 at 18:31

Wednesday, Apr 03, 2019 at 18:31
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Aha George,

The wires you have shown in RED should NOT BE THERE. They are shorting-out your BCDC charger and effectively bypassing it.

For your system to work properly THEY MUST BE REMOVED.

Then the engine battery will be charged directly by the alternator and the aux batteries will be charged via the BCDC.
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Follow Up By: Member-George (WA) - Wednesday, Apr 03, 2019 at 18:56

Wednesday, Apr 03, 2019 at 18:56
Hi Allan, even when I am stationary and just connected to solar ?
At the moment the BCDC is flashing on float with solar connected
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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Apr 03, 2019 at 20:44

Wednesday, Apr 03, 2019 at 20:44
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George,

With your existing system, and the engine off, the solar panel will be charging all three batteries, BUT, the BCDC is doing nothing. It may well be "flashing on float" but that is meaningless BECAUSE YOU HAVE GOT IT SHORTED OUT.

And even when the engine is running, the BCDC is doing nothing BECAUSE IT IS SHORTED OUT. Why even have the $500 BCDC if you are going to short it out?

Furthermore, one of the purposes of the BCDC is to prevent the accessory loads and the acc batteries from drawing current from the engine battery and so reducing its stored charge which could prevent the engine from cranking. With it shorted out they can do just that.

There really is no more that I can contribute. I must get on with other things in my life.
Perhaps other contributors to this forum may take up this matter.
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Follow Up By: Member-George (WA) - Wednesday, Apr 03, 2019 at 22:42

Wednesday, Apr 03, 2019 at 22:42
Hi Allan, thank you very much for the time you have taken to follow this through, you have been very patient with me.
I think your final answer has really completed my understanding of the whole set up.
I will certainly do as you suggested and remove the cables shown in red.
As you mentioned, this may well have been a costly lesson for me.
Also thanks to all others that have had input.
I now consider the matter closed.
Cheers, George
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Reply By: HKB Electronics - Monday, Apr 01, 2019 at 10:39

Monday, Apr 01, 2019 at 10:39
George,

What is your charge voltage?
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Follow Up By: Member-George (WA) - Monday, Apr 01, 2019 at 13:50

Monday, Apr 01, 2019 at 13:50
Hi, the charge voltage from the alternator is 14.5v
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Follow Up By: HKB Electronics - Monday, Apr 01, 2019 at 14:43

Monday, Apr 01, 2019 at 14:43
No problem in that regard then, just do what you first intended and either connect you solar to the main if you want to charge it from solar too and not using a dual sense or if your using a dual sense connect solar to the aux.
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Follow Up By: Allan B (Sunshine Coast) - Monday, Apr 01, 2019 at 22:21

Monday, Apr 01, 2019 at 22:21
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HKB, have you not read above that George is using a Redarc BCDC-1220 as his isolator? There is no "dual sense" or otherwise in the equation. George also says that the solar is already connected to his paralleled aux batteries.
If then he were to connect the solar "to the main" as you suggest then he would have the solar connected to both the engine battery and the aux batteries at the same time. In other words, all the batteries would be in parallel. And this would effectively short-out the BCDC and defeat any isolation of the engine battery.

Mind you, it already sounds as though he has some such connection, either by intent or accident.
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Follow Up By: HKB Electronics - Tuesday, Apr 02, 2019 at 09:47

Tuesday, Apr 02, 2019 at 09:47
Hi Alan,

I read that after my post, when I found the OP had changed his mind as to what equipment he had installed ie a VSR to in reality having a DCDC charger I decided to drop out of the conversation.
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Follow Up By: Allan B (Sunshine Coast) - Tuesday, Apr 02, 2019 at 11:11

Tuesday, Apr 02, 2019 at 11:11
Yes, it is difficult when the information comes piecemeal.
What do you think? Has George somehow got all three batteries connected in parallel?
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Follow Up By: HKB Electronics - Tuesday, Apr 02, 2019 at 11:36

Tuesday, Apr 02, 2019 at 11:36
Hi Alan.

To me it sounds like the OP did not install the system, at least that is what I'm assuming. He has also indicated the main slowly drops charge so I'm assuming this is at a different rate to the other two. From this and as the system is working it sounds like it has been installed correctly, so we have main to DCDC and then aux and the other tow in parallel.

The spanner in the works is the OP wrote all three batteries equalized when solar was connected therefore solar appears to charge all three. Is this the case though as the OP has also asked asking how to connect the solar to maintain the main?

As you have indicated without a schematic as to how its currently wired it is impossible to provide any worthwhile input.
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Follow Up By: Allan B (Sunshine Coast) - Tuesday, Apr 02, 2019 at 12:17

Tuesday, Apr 02, 2019 at 12:17
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Nah Leigh, he said that... "I have only done the connections today".
He also says variously that "all 3 batteries are connected in parallel" and "while on solar power, all 3 batteries equalize.... going fom 12.6v to13.2v" and then "when stationary for a week the starting battery is slowly dropping voltage".
So it is difficult to deduce just how it is all connected!

When he says that "all batteries are in parallel" I don't think that means they are firmly connected in parallel.

I suspect that he has got the system properly arranged as standard with two aux batteries in parallel charged from the dc-dc and from solar. But that his starter battery is draining either from a parasitic load or from internal discharge. It is just that he is not making observations correctly. He also wishes to apply charge to the start battery from the solar.... possible, but a little difficult and not really necessary.

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Follow Up By: Glenn C5 - Wednesday, Apr 03, 2019 at 08:47

Wednesday, Apr 03, 2019 at 08:47
HKB. While we are on the subject of batteries in parallel . Do the batteries have to be of the same amperage. Thanks Glenn
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Follow Up By: HKB Electronics - Wednesday, Apr 03, 2019 at 09:43

Wednesday, Apr 03, 2019 at 09:43
It would be better if they were but as long as they are similar chemistries you can use different Ah capacities, ie they should have similar fully charged terminal voltages.
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Follow Up By: Glenn C5 - Wednesday, Apr 03, 2019 at 10:24

Wednesday, Apr 03, 2019 at 10:24
Thanks HKB
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