Discharging third battery

Submitted: Thursday, May 20, 2004 at 13:49
ThreadID: 13027 Views:1322 Replies:4 FollowUps:10
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After receiving helpful advice from this forum I have decided to add a third battery to my current toyota dual battery set up.
For the majority of the time the third battery will be hooked up to the output on the alternator and when away camping this third battery will be isolated to prevent draw on other two batteries.
This may be a stupid question, but do I need to hook up any device to the third battery so it is being used and not overcharged when it is not isolated. Is it sufficient draw if it is hooked up to the outpout on the alternator which means it will be powering (along with the other two batteries) the alarm etc... when the car is not being driven.
I assume by connecting the third battery to the output on the alternator that it will be charged and discharged with all other batteries (when not isolated) so no need for any devices to be hooked up.
Thanks heaps in advance.
Lachlan
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Reply By: Member - Liz- Thursday, May 20, 2004 at 17:47

Thursday, May 20, 2004 at 17:47
Lachlan,
It sounds to me that you do not have a dual batery set up. I would suggest you give this serious consideration. This will preserve your cranking battery and leave the other two for, well, whatever. There are heaps on the market just get a good one.

If you have all batteries in parallel then they will equalize and you will or should discharge from all three.

There is no danger of over charging as the cars regulator will take care of that for you. Not too sure about load on alternator with three in parallel. Should be OK.

If you do not want to spend the cash on an auto dual battery system (around $300. should get a good one) The I suggest you install manual isolation switches for the two backups so you can switch them as you like. To prevent the three equalising you could consider installing diodes in series with the + leads (make sure you have them the right way otherwise the battery will not charge) to prevent current flowing from one back to the other. You will need to get the suitable current rating. If you take this option you will need to connect the accesories you want to run directly to the battery you to run them on. Getting complicated!!!

Anyway just my thoughts. Think about a proper auto set up though it takes the worry out and you will always have a good cranking battery!

Good Luck!
AnswerID: 59438

Follow Up By: Lachlan - Friday, May 21, 2004 at 09:19

Friday, May 21, 2004 at 09:19
Liz, thanks for the reply.
I am putting isolating swithces on the second and third battery. My concern is when the batteries are not isolated is there sufficient draw on the third battery.
The third battery will be connected on the output of the alternator, does this mean the current to the third battery is still regulated and therefore do not need to worry about overcharging?
Also, what do you mean about equalising?
Thanks in advance.
Lachlan
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FollowupID: 321157

Follow Up By: -OzyGuy- - Tuesday, May 25, 2004 at 16:00

Tuesday, May 25, 2004 at 16:00
Yep I concur...

however don't understand why you (appear) to have 2 cranking batteries.
I would be using one Cranking battery then two IDENTICAL batteries wired in Parallel and seperated from the Cranking battery by a decent Electronic Isolator.
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FollowupID: 321673

Follow Up By: -OzyGuy- - Tuesday, May 25, 2004 at 16:00

Tuesday, May 25, 2004 at 16:00
Yep I concur...

however don't understand why you (appear) to have 2 cranking batteries.
I would be using one Cranking battery then two IDENTICAL batteries wired in Parallel and seperated from the Cranking battery by a decent Electronic Isolator.
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FollowupID: 321674

Follow Up By: -OzyGuy- - Tuesday, May 25, 2004 at 16:00

Tuesday, May 25, 2004 at 16:00
Yep I concur...

however don't understand why you (appear) to have 2 cranking batteries.
I would be using one Cranking battery then two IDENTICAL batteries wired in Parallel and seperated from the Cranking battery by a decent Electronic Isolator.
0
FollowupID: 321675

Follow Up By: -OzyGuy- - Tuesday, May 25, 2004 at 16:00

Tuesday, May 25, 2004 at 16:00
Yep I concur...

however don't understand why you (appear) to have 2 cranking batteries.
I would be using one Cranking battery then two IDENTICAL batteries wired in Parallel and seperated from the Cranking battery by a decent Electronic Isolator.
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FollowupID: 321676

Reply By: Member - Jeff M (WA) - Thursday, May 20, 2004 at 18:08

Thursday, May 20, 2004 at 18:08
I think it's a good idea to have some small load drawing of the battery, otherwise it good start dry it out. I have my stereo, gps, uhf and internal "dom" lights running of my aux battery so it is always getting a good charge while driving and is always getting some use as well. Don't knowif there is much science to it, but it also means if someone leaves the door open the car still starts in the morning! ;-)
AnswerID: 59442

Reply By: Eric Experience. - Thursday, May 20, 2004 at 21:52

Thursday, May 20, 2004 at 21:52
Lachlan.
There is no need to have a load on the battery. What ever you do dont use the diode idea, it will not alow your battery to fully charge as you loose .7volt across the diode.
Eric.
AnswerID: 59491

Follow Up By: Lachlan - Friday, May 21, 2004 at 09:24

Friday, May 21, 2004 at 09:24
Hi Eric,
If there is no load will the third battery overcharge or dry out as some say? The third battery will be connected directly to the output on the alternator (no idea if that means it is regulated).
Your comments on the diode - losing .7 volt is considerable, probably best avoided.
Thanks for your help.
Lachlan
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FollowupID: 321158

Follow Up By: Rosscoe - Friday, May 21, 2004 at 14:32

Friday, May 21, 2004 at 14:32
Lachlan,
If you connect diectly to the alternator, there is no regulation. ie that's because you are connected upstream of the vehicle's voltage regulator.
See if you can find out what is the voltage and current rating of the alternator. As mentioned before if it is over 15 Volts when you're cruising the highway at 100 kph for hour on end you may overcharge and damage the battery.

I suggest you invest $35.0 or so and buy Collyn Rivers' Caravan and Motorhome Electrics book.
www.caravanandmotorhomebooks.com.au.
I had to read it a couple of times to get a full appreciation of what was being said
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FollowupID: 321196

Follow Up By: Eric Experience. - Friday, May 21, 2004 at 22:48

Friday, May 21, 2004 at 22:48
Lachlan.
No your battery will not dry out without a load. There is no problem with regulation as rosko sugests, the output of the alternator is regulated and the extra volt is only there when the alternator is working hard, the voltage drops to about .2 above the cranking battery when it is fully charged. The extra .2 is just enough to ensure the deap cycle is fully charged. Collin Rivers book is good but he is not up to date with the modern fusable link setups. Eric.
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FollowupID: 321260

Follow Up By: Rosscoe - Monday, May 24, 2004 at 09:05

Monday, May 24, 2004 at 09:05
Lachlan, Eric,
I don't know the set up of the individual vehicle. If the voltage regultor is integral with the alternator you are not connecting "directly" to alternator "out put" and you'll be fine. The typical out put of the voltage regulator is 14.2 Volts. You won't achieve 100% charge but you won't do any damage.
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FollowupID: 321474

Follow Up By: Eric Experience. - Monday, May 24, 2004 at 21:27

Monday, May 24, 2004 at 21:27
Rosscoe.
You have missed the importance of this discusion. The referance for the voltage regulator on the modern systems is connected to the common distribution piont which is where the fusable links are commoned. That means that the voltage at that piont is constant, usualy 14.8V because of the resistance of the wire between the alternator and the common the votage at the alternator output rises with current, that is where the extra voltage comes from, it is this extra voltage that allows the deep cycle to get fully charged. I can not explian it any clearer than that, the best thing you can do is measure it for yourself. The land rover still uses the old system so you will have to look at a modern Jap vehicle. Eric.
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FollowupID: 321584

Reply By: Lachlan - Friday, May 21, 2004 at 15:15

Friday, May 21, 2004 at 15:15
Thanks so far to everyone for advice, very helpful. Can anyone suggest what size cable I should run from the alternator to the third battery in the boot of the toyota?
And from the battery to the fridge?
Thank you.
AnswerID: 59580

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