Duel Battery System Question

Submitted: Tuesday, May 13, 2008 at 17:20
ThreadID: 57574 Views:1914 Replies:8 FollowUps:2
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With a duel battery system which battery is getting charged first? is it the cranking battery then it switches over to the deep cycle battery or are they both getting charged at the same time?
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Reply By: Peter 2 - Tuesday, May 13, 2008 at 17:37

Tuesday, May 13, 2008 at 17:37
Depends on the dual battery system itself, how it is regulated, types of batteries etc.
On the simple systems the one that is discharged the most would be charged first.
AnswerID: 303619

Reply By: Kev C - Tuesday, May 13, 2008 at 17:48

Tuesday, May 13, 2008 at 17:48
Hi Mike
There are different wys to manage your batteries. The GU Patrol I use at work had a system where the main battery charged 1st & the battery that runs all the accessories charged 2nd. They were not linked except through the charging system.

I had a problem where I ran the main battery down by leaving an interior light on for a week. When I went to start it it was flat. There I was stuck at the airport with 1 fully charged battery in the car & no way to start the motor. I had to borrow jumper leads & jumped one battery from the other.

I now have a setup where I can manually link the 2 batteries to start the car when necessary.

The point of the story is that when you set up your batteries be very sure of what you want it to do 1st or you could end up with a setup that doesn't work for you.

AnswerID: 303623

Reply By: Nomadic Navara - Tuesday, May 13, 2008 at 18:15

Tuesday, May 13, 2008 at 18:15
The short answer is no battery gets charged first. The starting battery is connected to the alternator all the time and takes what charging current it needs all the time. The auxiliary battery is not connected until the voltage across the starting battery reaches a pre-set voltage. This voltage is set at different levels by different manufacturers but it happens before the alternator reaches its maximum voltage and probably before the starting battery has ceased accepting a significant charging current (ie fully charged.)

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AnswerID: 303630

Reply By: Sand Man (SA) - Tuesday, May 13, 2008 at 18:27

Tuesday, May 13, 2008 at 18:27
Peter has it pretty well summed up.

On a "smart" Isolator, the starting battery always has priority.
Once the voltage has reached a certain value, the Isolator will allow current to flow through to the auxiliary battery(s).

In practise this happens fairly quickly if the starting battery is in good condition.

The Piranha DBS-150 I have will indicate by way of an LED when to auxiliary circuit has switched in. This generally happens within one or two minutes of starting the engine.


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AnswerID: 303632

Reply By: Hairy (NT) - Tuesday, May 13, 2008 at 18:56

Tuesday, May 13, 2008 at 18:56
I have a Pirhana set up ( cant remember the model)
but Im pretty sure it said it charges your main battery first.

AnswerID: 303638

Reply By: SoloGirl - Tuesday, May 13, 2008 at 19:59

Tuesday, May 13, 2008 at 19:59
Not intending to hi-jack this thread, but I haven't taken my 2nd battery out of the truck for about 2 weeks since I last used it. Is that good or bad? I'm presuming it's being charged/topped up all the time?
AnswerID: 303650

Follow Up By: lifeisgood - Tuesday, May 13, 2008 at 20:18

Tuesday, May 13, 2008 at 20:18
No need to take it out if you have a dual battery charging system.
Only reason to remove it would be for weight reduction. Bit of a hassle though. If it doesnt get much use (the vehicle) for a while ,you can still charge it in situ.
FollowupID: 569785

Reply By: Mainey (wa) - Wednesday, May 14, 2008 at 08:09

Wednesday, May 14, 2008 at 08:09
As stated above we do need to know what brand/model/type of dual battery isolator *system* you have installed in your vehicle as most types will work differently !!

Mainey . . .
AnswerID: 303710

Reply By: neil&brenda - Thursday, May 15, 2008 at 23:54

Thursday, May 15, 2008 at 23:54
Hi Mike, Please be gentle as we are new to this forum! I built an aux battery system a couple of years ago. Little did I realise, that its circuitry was well ahead of its time. It charges the main battery first until it is at a fully charged state (13.8v), then passes on the charge from the alternator to the aux battery. If you have a depleted aux battery from use overnight, it will regulate the current to the aux battery until it has reached a voltage of 10.5v, then it will allow a fuller charge current. This is to try and alleviate a full current blast into the spare battery when it is low. A high current draw/supply to the aux battery will/does damage the plates and will shorten its life and your bank balance. Hope this makes some sort of sense.
AnswerID: 304048

Follow Up By: oldtrack123 - Friday, May 16, 2008 at 22:21

Friday, May 16, 2008 at 22:21
I liked the 1st 3sentences but then sorry if you think you can regularly discharge your battery below 10.6 volts & as long as you slow charge iit up to 10.6 v it will last along time you are very wrong.
About the worst thing you can do to a battery is regularly dicharge below about 50%[approx 11.6v]
FollowupID: 570312

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