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Old 12-06-2019, 08:43 AM   #1
jofral58
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Multiple 12V Battery Wiring Configuration

I have a 2016 3720RL with two 12V deep cycle batteries. They are currently wired as shown in image Parallel 1. Querying multiple sites indicates the configuration in image Parallel 2 is better.

Before I change over, I wanted to get broader feedback.

How are your multiple 12V batteries wired together?

Thanks for the feedback.
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File Type: png Parallel 1.png (96.7 KB, 384 views)
File Type: png Parallel 2.png (80.2 KB, 386 views)
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Old 12-06-2019, 08:51 AM   #2
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The second image is the preferred method. It balances any resistance, and thus voltage drop, in the wiring. Theoretically, in image 1 the battery on the left could supply slightly less available voltage due to some small amount of resistance in the wiring and connection points.
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Old 12-06-2019, 09:11 AM   #3
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Always go with option two. Its not just outgoing power, its also in how the batteries accept a charge. In option two all incoming and outgoing power is forced through both batteries equally.
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Old 12-09-2019, 05:13 PM   #4
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It depends on your cabling. The only difference between the two setups is the resistance in the cable between the battery negative terminals. Otherwise the two points are electrically the same. I use 1/0 cables; you would need a high dollar meter to measure any voltage drop across 12" of 1/0 cable with a nominal amperage draw.

I know, there is a lot of info out there implying that one of the batteries might not get charged correctly. But really, there is no electrical difference in those two drawings.
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Old 12-09-2019, 08:05 PM   #5
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Quote:
Originally Posted by scottz View Post
It depends on your cabling. The only difference between the two setups is the resistance in the cable between the battery negative terminals. Otherwise the two points are electrically the same. I use 1/0 cables; you would need a high dollar meter to measure any voltage drop across 12" of 1/0 cable with a nominal amperage draw.

I know, there is a lot of info out there implying that one of the batteries might not get charged correctly. But really, there is no electrical difference in those two drawings.
Ditto on what scottz said. Only NASA can measure the resistance in a 12" piece of wire that big with the amps we draw. You know the "Resistance is Futile" But don't feel like the lone ranger. Most all here fret over the tiny details. Ours was wired like the 2nd drawing. I just revamped all my wiring this weekend so now everything is pretty!
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Old 12-09-2019, 08:41 PM   #6
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Better is one way to describe but I prefer wrong. Current follows the path of least resistance. In figure 1 the first battery gets all the current. The second battery will receive and use some amount, too much math involved to determine just how much.
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Old 12-09-2019, 09:48 PM   #7
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I agree that the resistance of the wire is extremely small and insignificant. But the left battery in the top image has an extra wire between its + post and the + source. And an extra wire between its - post and the - source. Meaning two extra connection points on the positive side and two more on the negative side that the right battery does not have. And those four connection points would have more resistance than the wire. Especially so as they age. Being that any pressure connection is going to have some resistance, the left battery is always going to have slightly more line resistance than the right battery.

On the lower image both batteries have the same number of wire connection points between the + and - sources. One poor connection could still cause some increase in resistance to one battery. But if all connections were good, the resistance to each would be the same. Not so for the top image.

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Old 12-10-2019, 12:40 AM   #8
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In theory you are all correct. Now, another question. How much does my fuel mileage drop due to alternator drag when I run with my headlights on?

PS: both of those diagrams are parallel circuits.
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Old 12-10-2019, 07:32 AM   #9
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No commercial electrical apprenticeship teaches their students to connect marine or commercial batteries in parallel using the method in question.
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Old 12-10-2019, 08:49 AM   #10
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In theory you are all correct. Now, another question. How much does my fuel mileage drop due to alternator drag when I run with my headlights on?

PS: both of those diagrams are parallel circuits.
Two alternators or one? LED lighting or incandescent? Ethanol, regular, mid grade, super or diesel? Daytime driving lights only, high beams, or low beams? Raining or foggy? You didn't mention the temperature
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Old 12-10-2019, 11:51 AM   #11
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To answer the original question; you should follow the second drawing. There is no disadvantage to doing so. Sorry for getting side tracked.
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Old 12-10-2019, 04:22 PM   #12
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Research any battery manufacture, and the will tell you Image #2 is the only right way. Image #1 will cause the battery on the right to take as much as 70% of the load, and over time will reduce the number of cycles you get out of your batteries. John Kohl posted a very good article 5 or 6 years ago, and I had to redo my wiring after reading the article.
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Old 12-10-2019, 09:15 PM   #13
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Quote:
Originally Posted by jofral58 View Post
I have a 2016 3720RL with two 12V deep cycle batteries. They are currently wired as shown in image Parallel 1. Querying multiple sites indicates the configuration in image Parallel 2 is better.

Before I change over, I wanted to get broader feedback.

How are your multiple 12V batteries wired together?

Thanks for the feedback.
If these batteries were 6v AGM's how should they be wired?
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Old 12-11-2019, 06:42 AM   #14
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If these batteries were 6v AGM's how should they be wired?
6 volts need to be wired in series. Meaning the + on battery 1 to the - on battery 2. Use - on battery 1 and + on battery 2 for load.
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Old 12-15-2019, 09:00 PM   #15
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Dual battery hookup

Image 2 has been the preferred method that I was taught and still use for all my dual battery setups.
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Old 12-16-2019, 07:45 AM   #16
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FOR 6 VOLT PEOPLE:
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Old 12-16-2019, 11:22 AM   #17
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There is no difference in the 2 wiring configurations. They are electrically the same.
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Old 12-16-2019, 03:44 PM   #18
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Sorry but that is so not true. The electrical current in the first diagram must account for the resistance of the added wire and the battery itself. I really hope nobody with a sizable investment in lithium batteries uses this method to connect batteries or cells in this fashion.
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Old 12-17-2019, 12:54 PM   #19
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Originally Posted by PNW Fireguy View Post
Sorry but that is so not true. The electrical current in the first diagram must account for the resistance of the added wire and the battery itself. I really hope nobody with a sizable investment in lithium batteries uses this method to connect batteries or cells in this fashion.
That is what I've been trying get across; probably didn't explain it well. The voltage drop across the battery cables (assuming you have good connections and proper cables) is insignificant. I turned on everything I could, pulling as much current as possible, and the voltage drop across my cables (measured by my four digit meter) was .001 volts. Suppose this was like massive telecom battery plants that I have been maintaining most of my life and had 2" wide x 1/4" thick buss bars between the battery terminals? Do you think it would make a difference where you connected to that bar?

The deciding factor on how a battery charges is the internal resistance of the battery; something we cannot control. Someone mentioned one battery taking 70% more charger than another. In that case, there is something seriously wrong the the other battery or the connections.

Anyway, we have beat this to death. There is absolutely no down side to hooking up the cables as in drawing two; the op should do so and sleep well. My batteries are connected like this, but I would be comfortable with either method.
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Old 12-17-2019, 08:44 PM   #20
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My friend your constructive communication is always welcome with me. I respectfully put forward that in one figure the charge and discharge will not be balanced and in some cases you may cook the battery cell with both positive and negative leads connected trying to charge the one connected behind it. This will also be the case when discharging. The aforementioned battery will discharge to a given DOD prior to the one connected behind it. I have seen this in my career, i have read posts on this forum about this very issue. While from a far this thread may look like a dead horse at this point I just don't want to see anyone lose their investment by hooking up expensive batteries in a manner that will inherently prevent them from a balanced charge and discharge lifecycle.
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