auto electricians

Have 2 Deep Cycle Batteries for accessories fridge etc joined in line.
Can you? should you? attach the power line to accessories to positive on one battery negative on the other or is it okay to have power line to pos and neg on one battery. Also same deal with charging from alternator
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Reply By: Member - John and Val - Thursday, Jul 22, 2010 at 08:51

Thursday, Jul 22, 2010 at 08:51
Tuck,

Provided the batteries are pretty similar in type and age, they can be connected in parallel and used as if they were a single battery. By parallel I mean that the 2 positives should be connected together and the 2 negatives are also connected together. This way, you'll have a positive wire and a negative wire to connect to, and these will connect to both batteries. The wire should be heavy gauge.

It's good practice to have a fuse right at the positive terminal of each battery and connect to the batteries through the fuses.

You can charge from the alternator by using a voltage sensitive relay (VSR) to automatically connect the batteries to your main battery while the engine is running. You will need heavy gauge wire here too, especially if the deep cycle batteries are some distance away from the main battery.

There is a fair bit more to this, so get back to us if we can help further.

Cheers

John
J and V
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Follow Up By: Member - Stuart P (WA) - Thursday, Jul 22, 2010 at 21:52

Thursday, Jul 22, 2010 at 21:52
what size fuse do you suggest?
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Follow Up By: Maîneÿ . . .- Friday, Jul 23, 2010 at 00:19

Friday, Jul 23, 2010 at 00:19
John,
You say: "It's good practice to have a fuse right at the positive terminal of each battery and connect to the batteries through the fuses"

Why would you use a fuse between the 2 x batteries wired in parallel , I've never, ever seen this done on any 12v battery bank anywhere previously.

What type & size fuse would you recomend ?? (as Stewart has also asked)

Maîneÿ . . .
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Follow Up By: Member - John and Val - Friday, Jul 23, 2010 at 08:10

Friday, Jul 23, 2010 at 08:10
Stuart, Mainey,

There's a lot of energy in a battery, capable of doing damage and causing fire. It's not clear from Tuck's description whether the batteries are side by side, close together or maybe well separated. If they are sitting side by side as a battery bank, they could certainly be wired directly in parallel and a single fuse be used for the pair. If they are separated though, I'd fit a fuse to each battery as Peter has shown in his diagram.

What size fuse? Assuming these batteries are used only for "domestic" purposes, like running lights and a fridge, I'd use 20A or 30A, big enough to handle the load easily, small enough to prevent damage.

John
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Follow Up By: Member - Stuart P (WA) - Friday, Jul 23, 2010 at 22:53

Friday, Jul 23, 2010 at 22:53
you said in your comment to put a fuse between batteries, you quoted 20-30 amp fuses, when your batteries go flat and are in need of a charge from the alternator, the alternatormay have the capacity to charge at 140 amps what happens to the flat battery?
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Follow Up By: Member - John and Val - Saturday, Jul 24, 2010 at 08:11

Saturday, Jul 24, 2010 at 08:11
Stuart,

Have you ever seen a 140A alternator pump 140A into a deep cycle battery??? The internal resistance of the battery, coupled with the wiring and switching resistance will ensure that it never happens. The alternator voltage is determined by feedback from the cranking battery, so will never rise high enough to force that sort of current into a deep cycle battery. (Of course, if it did, you'd need the fuse to protect the battery!)

I've used 20A or 30A fuses in that position for years. I also monitor charging current continuously. I've never seen more than 20A flow into my batteries.

Do you have a constructive comment to make? What would you recommend?

John
J and V
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Follow Up By: Maîneÿ . . .- Saturday, Jul 24, 2010 at 09:36

Saturday, Jul 24, 2010 at 09:36
John,
I've this *question* for you, - What’s the value of having a 140 Amp alternator ???

You've said above: "I've used 20A or 30A fuses in that position for years. I also monitor charging current continuously. I've never seen more than 20A flow into my batteries"

You believe the maximum amperage produced by your 140 Amp alternator is just 20 Amps, this is obviously because it would 'blow' your 20 Amp fuse?

So according to your information, to have an alternator that can produce anything above 20 Amps is a complete waste of time, however I don’t believe Toyota & Nissan are aware of that information ?

******************************************************

John, then you say: “The alternator voltage is determined by feedback from the *cranking* battery, so will never rise high enough to force that sort of current into a deep cycle battery”

What absolute rubbish, you obviously don’t understand the Deep Cycle battery is wired in parallel to the Cranking battery when it’s being charged.

Effectively these two batteries are in fact just one large single battery with the exact same voltage.

The alternator voltage is then determined by feedback from this one large battery, (Cranking and also Deep Cycle battery combination) not from just the Cranking battery at all.

Please keep information factual, I hope you will consider this as 'constructive comment' :-)

Maîneÿ . . .
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Follow Up By: Member - John and Val - Saturday, Jul 24, 2010 at 10:40

Saturday, Jul 24, 2010 at 10:40
Mainey,

I'm fed up with your ignorance and abuse. If you can't play nicely, go and play elsewhere.

I will not in future respond to your needling.

John
J and V
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Reply By: Member - Christopher P (NSW) - Thursday, Jul 22, 2010 at 08:51

Thursday, Jul 22, 2010 at 08:51
have you done a search on this site plenty of info regarding dual batteries.

battery stuff

i hope this helps???
AnswerID: 424851

Reply By: Battery Value Pty Ltd - Thursday, Jul 22, 2010 at 08:52

Thursday, Jul 22, 2010 at 08:52
tuck,

it's not ideal, but the resulting charge/discharge imbalances are small enough to be safely ignored.
That's as long as we're talking milli-Volts, i.e. the difference in cable length not much more than a couple of metres, at sufficient wire gauge of #6 or 8.
Don't forget the fuses need to be of the same rating, otherwise these will contribute to the imbalance (or compensate for it ;)
Take a look at the sketch below.
The difference between the length of the green and yellow paths shouldn't exceed a couple of metres or so.
Ideally, the electrical resistances of the two current paths should be identical.

Image Could Not Be Found

If that can't be achieved, the one battery closest to the load/charging source will see a slightly deeper cyclic load pattern than the other one.
That's the one battery in the shorter current path.

Best regards, Peter
AnswerID: 424852

Reply By: Maîneÿ . . .- Thursday, Jul 22, 2010 at 11:49

Thursday, Jul 22, 2010 at 11:49
tuck,
In theory, attach your (+) power cable to the positive terminal on one battery, and your (-) negative cable to the (-) terminal on the other battery to form a completed electrical circuit discharging through both batteries.
In practice you should have a fused 12v power distribution box attached to the (+) battery terminal to accept all of the (+) cables used in your power system so they are safe and compact and are fused without taking up space on the (+) battery terminal.

As Peter has said, use a similar length of suitably sized cable and a fuse placed adjacent to the (+) battery terminal.Image Could Not Be FoundMaîneÿ . . .
AnswerID: 424878

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