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Old 12-31-2014, 01:20 PM   #1
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Series Parallel Battery Wiring - Which is correct?

I've seen a couple different schematics for wiring 6v batteries in series-parallel for 12v applications (like my Airstream DC) and wondered if both were "correct" and if there was any advantage of one over the other. Thoughts?

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I got the information from these websites:
  1. AtBatt
  2. BatteryStuff
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Old 12-31-2014, 01:33 PM   #2
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Two 6v batteries need to be wired in series to produce 12 volts— negative post of first battery connects to positive post of second battery. Add voltages together, but don't add amps-hours together. Two 6v 160-amp-hour batteries in series produce 12v 160 amp-hours.

Two 12v batteries have to be wired in parallel to produce 12 volts— negative post of first battery connects to negative post of second battery. Add amp-hours together, but don't add voltages. Two 12v 80-amp-hour batteries in parallel produce 12v 160 amp-hours.

If you've got four batteries, first hook up two 6v batteries in series. Then hook up the other pair in series. That makes them the same as two 12v batteries. Then hook up the sets in parallel. Simpler to understand that way.
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Old 12-31-2014, 01:34 PM   #3
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I think what you are doing amounts to the same thing. You have to have two batteries in series to get 12V then put two of those sets in parallel to double the current. You could also hook two in parallel and then put those sets in series. So whatever works best as far as your physical layout. You want to minimize the length of the final output cables or for that matter all the cables. It does not take much length of wire to loose voltage at 6V.

I assume you are running an inverter or something that needs the extra current? You want the inverter to be really close to the batteries. A more efficient system would use 24V in which case you can run all of them in series. The higher the voltage the fewer loses you get. You can run 4 times much power through the same size wire at 24V as you can at 12 V. Power=current squared times the resistance.

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Old 12-31-2014, 03:18 PM   #4
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What they are trying to do in the diagrams you supply is to have equal lengths of battery cables on the series and parallel portions of the 4 battery setup. Although it may seem strange and hard to believe, the very minor resistance differences of the cables and cable connections can make one set of two batteries charge and discharge more than the other if they don't have virtually identical wire connections and lengths.

I have no personal experience with this issue but have read some pretty convincing studies over the years which make a good case for hooking batteries up in ways which have identical cable runs for both sets when in a series/parallel setup.
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Old 12-31-2014, 03:24 PM   #5
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Originally Posted by idroba View Post
What they are trying to do in the diagrams you supply is to have equal lengths of battery cables on the series and parallel portions of the 4 battery setup. Although it may seem strange and hard to believe, the very minor resistance differences of the cables and cable connections can make one set of two batteries charge and discharge more than the other if they don't have virtually identical wire connections and lengths.
That's actually easy enough. Find the longest cable you need to connect the batteries to each other. Then make all the cables that same length, even if they're a bit too long for the other connections.
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Old 12-31-2014, 05:03 PM   #6
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Use 4/0 class 'K' welding cable (which I use in most of my battery set-ups) and you won't have to worry about resistance.

Option 2 is a better method, but I would have the negative connection cable on the #4 battery's negative, not #1. THAT will assure even charge and discharge of the bank, which is far more important than equal connecting cable lengths.
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Old 12-31-2014, 05:26 PM   #7
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First thing is get the batteries in there proper location then like others have said try to make all cables as short as possible and all aprox the same length . And make sure you use proper size cables .

Don
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Old 12-31-2014, 07:00 PM   #8
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Welding cable is not good for battery cables it is made for maybe 250 amps or a little more..Battery cables are able to carry 1000 amps or so when starting..I have a couple nice sets of welder cable jumper cables, took me a while to figure out they aren't t very good..Use only battery cables on batteries ask me how I know...Happy New Year and best wishes....
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Old 12-31-2014, 07:19 PM   #9
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Current capacity is purely a function of wire diameter. It does not matter what the wire is called. Also current capacity is somewhat subjective. The steady state current capacity is what is most important. Most battery cables are pretty crappy. Some are even copper coated steel. They are also for intermittent use. The resistance of the connections is also important. Most wire burns up because the connections are loose and the connectors get hot and fail. Equal length wires are a very good idea. Connectors that are soldered on will help a lot. Cleaning lead oxide off of battery posts is also a good idea. The oxide is an insulator. A lot of starting problems on cars are loose corroded cable clamps. There are plenty of tables on the internet that tell you what the current capacity of different size wire is. Figure out what current you really need. If you need 100 Amps occasionally don't wire for 300A. Those same cables can take 1000 A for a few seconds like for starting a car engine. Understand your application. Spec out what you are doing.

I was shocked when I learned that my 50A rated oven did not have wire over #12 inside it. The pigtail was the largest wire on the thing.

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Old 12-31-2014, 09:42 PM   #10
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Originally Posted by tjdonahoe View Post
Welding cable is not good for battery cables it is made for maybe 250 amps or a little more..Battery cables are able to carry 1000 amps or so when starting..I have a couple nice sets of welder cable jumper cables, took me a while to figure out they aren't t very good..Use only battery cables on batteries ask me how I know...Happy New Year and best wishes....
So..........how DO you know???????

It is an interesting concept, but unfortunately could not be further from fact! Perry has pretty much hit the nail on the head.

Class 'K' welding cable simply refers to a family of wire manufactured to a specific set of standards including the use of super flexible polyurethane or EPDM insulation, along with hundreds or thousands of strands of very fine conductive copper wire, either 30 AWG or .010.

The 4/0 class 'K' cable that I use contains over 2000 strands of 30AWG conductors, is super flexible (also super expensive!) and has a listed ampacity of 400 amps at 100 feet and 600 volts continuous rating. For large (2000 watt +) inverter use, it has a practical usable length of up to 10 feet. If your cables to the batteries will be over 10 feet, then a double 4/0 cable is required to carry the current properly. All cabling should be properly matched to the load and type of use by the stated ampacity of the cables ONLY!

PS: this type of cable is also used in diesel motor homes and yachts as 'battery cable' on a regular basis, and easily handles the very large transient amperage loads that engine starting require.
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Old 12-31-2014, 10:27 PM   #11
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Quote:
Originally Posted by trekerboy View Post
I've seen a couple different schematics for wiring 6v batteries in series-parallel for 12v applications (like my Airstream DC) and wondered if both were "correct" and if there was any advantage of one over the other. Thoughts?

Attachment 229569

I got the information from these websites:
  1. AtBatt
  2. BatteryStuff
Looking closely at the picture, Option #2 is drawn wrong. The negative output lead is connected to the negative terminal of 6 V battery #1 and the positive output lead is connected to the positive output terminal of the same battery #1. So, as it is drawn, there will only be 6 volts across the output leads.
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Old 12-31-2014, 10:45 PM   #12
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I think I agree, the - would need to come off of the - of battery #2?


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Old 01-01-2015, 08:24 AM   #13
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Originally Posted by lewster View Post
Use 4/0 class 'K' welding cable (which I use in most of my battery set-ups) and you won't have to worry about resistance.

Option 2 is a better method, but I would have the negative connection cable on the #4 battery's negative, not #1. THAT will assure even charge and discharge of the bank, which is far more important than equal connecting cable lengths.
As I said before, the output negative needs to be on battery #4 (it could also be on the negative of #2, but that would be counter-productive), and the positive on #1. The short cables between #1-#2 and #3-#4 are the series connections that effectively create 2 big 12VDC batteries. The long cables from + to + and - to - are the parallel connections. Positive out at #1, negative out at #4.

Another classic example of the mis-information often posted on the internet. But as we all know..........if it's on the internet........IT MUST BE TRUE!!!
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Old 01-01-2015, 10:24 AM   #14
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Quote:
Originally Posted by Nuvite-F View Post
Looking closely at the picture, Option #2 is drawn wrong. The negative output lead is connected to the negative terminal of 6 V battery #1 and the positive output lead is connected to the positive output terminal of the same battery #1. So, as it is drawn, there will only be 6 volts across the output leads.
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Originally Posted by lewster View Post
Another classic example of the mis-information often posted on the internet. But as we all know..........if it's on the internet........IT MUST BE TRUE!!!
You're right! That's my mistake, I misrepresented and inaccurately recreated the diagram on AtBatt for wiring 6v Batteries in Series-Parallel!

As @Lewster said, if it's on the Internet if MUST be right! Therefore, for the poor soul who stumbles upon my original post and tries to wire their system according to it, I offer the below corrected image (which is accurate to AtBatt's diagram and should work according to @Lewster).

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