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Old 04-20-2021, 06:45 PM   #1
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Wiring in 1,2, Both Switch

Hello I am planning on wiring a switch for my dual battery system. I have a blue sea 1, 2, both to use wiring them in parallel. However, when running 2 batteries I typically run the negative to 1 battery and the positive to the other battery to help balance the load. If I put one of these switches on will it still be balanced? I would assume that it would matter which post on the switch goes to which battery, does anyone have a wiring diagram for this application?Click image for larger version

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Old 04-20-2021, 10:54 PM   #2
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When wiring your batteries in parallel the neg will go to both batteries as well as the positive. I'm not sure what you are getting at about balancing the load.

The 1, 2, both switch you have has a single positive output and 2 positive inputs. The switch allows you to choose which battery to connect to your system for the load/charge. The battery negatives always remain connected.

Blue Sea has excellent documentation on thier website.
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Old 04-20-2021, 11:28 PM   #3
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When the switch is in the 1+2 position, the load (and charging) will be balanced between the two batteries. Here is the wiring diagram:
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Old 04-20-2021, 11:45 PM   #4
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Hello I am planning on wiring a switch for my dual battery system. I have a blue sea 1, 2, both to use wiring them in parallel. However, when running 2 batteries I typically run the negative to 1 battery and the positive to the other battery to help balance the load. If I put one of these switches on will it still be balanced? I would assume that it would matter which post on the switch goes to which battery, does anyone have a wiring diagram for this application?Attachment 252543
Using the opposite corner connection method is done to balance currents between two batteries. But if you switch a battery off, there's nothing left to balance, as you only have one battery left.

Probably what you want is an on-off switch for both batteries as a pair, and switch them both off at the same time. So pick one set of switch contacts (e.g. bat 1 and common) and put it inline between your battery plus cable to your fuse panel. Then just ignore the other switch position.

Or, get a simpler SPST switch.
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Old 04-21-2021, 12:00 AM   #5
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Second look at the problem!

The answer to your question is yes, using the bat1-off-bat2 switch the way it is designed can unbalance the two-corner method of battery balancing, but it can also be rebalanced by modifying the negative leads to ground.

The idea is you want equal resistance for each battery pathway, so you need equal length cables. If your bat1 and bat2 cables to your switch are matched in length, you need to do the same on the negative side as well.

So instead of cross connecting your negative posts and taking the ground connection off one corner, take both negative posts to ground using equal length cables.

I think that's the answer you're looking for.
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Old 04-21-2021, 08:38 AM   #6
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The answer to your question is yes, using the bat1-off-bat2 switch the way it is designed can unbalance the two-corner method of battery balancing, but it can also be rebalanced by modifying the negative leads to ground.

The idea is you want equal resistance for each battery pathway, so you need equal length cables. If your bat1 and bat2 cables to your switch are matched in length, you need to do the same on the negative side as well.

So instead of cross connecting your negative posts and taking the ground connection off one corner, take both negative posts to ground using equal length cables.

I think that's the answer you're looking for.
Yes exactly what I'm trying to figure out. By using the combined 1/2 setting it wouldn't be the same as putting positive on the opposite battery as the negative. Unless there is a dedicated post for battery 1 and battery 2 with intention to go "corner to corner". If that's not the case then needing to use 2 equal length negative leads to match the 2 positive leads makes sense as the only way to balance them.
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Old 04-21-2021, 09:58 AM   #7
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Old 04-21-2021, 09:59 AM   #8
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If your positive cables are not matched in length (because the switch is somewhere other than the middle), and say for example, one is 12" and the other is 24", then do the opposite with the negative cables -- make one 24" and the other 12". Then both sides will add up to 36" and you will balance the currents in both batteries.

What the opposite corner connection was doing, was balancing the resistance of the cross connect cable by creating two identical pathways for each battery. By adding the b1 b2 switch, you recognized the corner connection was no longer doing that. That's a very astute observation most would miss.

You are indeed... "not lost"!


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Old 04-21-2021, 06:57 PM   #9
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If your positive cables are not matched in length (because the switch is somewhere other than the middle), and say for example, one is 12" and the other is 24", then do the opposite with the negative cables -- make one 24" and the other 12". Then both sides will add up to 36" and you will balance the currents in both batteries.

What the opposite corner connection was doing, was balancing the resistance of the cross connect cable by creating two identical pathways for each battery. By adding the b1 b2 switch, you recognized the corner connection was no longer doing that. That's a very astute observation most would miss.

You are indeed... "not lost"!


.
Got it. Also assuming all the same wire guage of course. Parts list ends up being 2 x 12" positive cables connected from battery to the camper positive via the switch and 2 x 12" negative cables connected from battery to the camper negative. I'm just surprised as this must be a very common use for this switch that it doesn't have a built in way to accomplish this. Or maybe this is the only way, and in clearly states that in the directions that I definitely lost!
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Old 04-21-2021, 10:49 PM   #10
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Most applications for that switch are not switching batteries within a balanced group, they are switching banks of batteries or just not not balancing them.

There's one more option if you want to leave your negative leads the way they are. You can offset the resistance by altering the wire gauge between two positive leads to compensate for the difference in the negative leads.

You'll need the length and wire gauge of your cross connect cable. Then look up its resistance and subtract that difference from one of your positive cables to make the other positive cable.

That takes some math, so I made up a spreadsheet.

But your way is *much* simpler.
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Old 04-22-2021, 03:20 PM   #11
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Unless you are running heavy currents through your battery wires (50% or more of rated current), the difference in resistance in the wire lengths isn't going to matter. You are going to see more voltage differences caused by variations in battery construction, environment, battery connections (especially at the battery lugs), etc.

In another words, unless you are running an inverter, don't worry about it in a typical RV. I have the same battery switch as you do, but have switched from 12V to 6V GC-2s. I wire to post 2 and ignore the 1 position on the switch. Works great.

Fred W
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2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time
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Old 04-22-2021, 06:23 PM   #12
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Unless you are running heavy currents through your battery wires (50% or more of rated current), the difference in resistance in the wire lengths isn't going to matter. You are going to see more voltage differences caused by variations in battery construction, environment, battery connections (especially at the battery lugs), etc.

In another words, unless you are running an inverter, don't worry about it in a typical RV. I have the same battery switch as you do, but have switched from 12V to 6V GC-2s. I wire to post 2 and ignore the 1 position on the switch. Works great.
The purpose of current balancing is to get more life from two batteries working together, so one doesn't do all the heavy lifting and the other one just coast along. It also equalizes the charging function to get both fully charged. It's a finer point, and often overlooked by some RV builders, but both of those goals require equal resistance pathways to each battery to balance the current.

Otherwise, what you said is correct. It requires two identical batteries, same age, same SOC, same temperature. He's not pushing the limits of the cables, he's squeezing more performance out of the battery set.
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Old 04-22-2021, 07:11 PM   #13
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The purpose of current balancing is to get more life from two batteries working together, so one doesn't do all the heavy lifting and the other one just coast along. It also equalizes the charging function to get both fully charged. It's a finer point, and often overlooked by some RV builders, but both of those goals require equal resistance pathways to each battery to balance the current.

Otherwise, what you said is correct. It requires two identical batteries, same age, same SOC, same temperature. He's not pushing the limits of the cables, he's squeezing more performance out of the battery set.
The batteries work together best when they are both are at exactly the same voltage. Any difference in voltage means the higher voltage battery is carrying more (not all) of the load until it is depleted down to the level of the lower voltage battery. The smaller the voltage difference between the 2 batteries, the less this occurs. Different SOC, different specific gravity, different aging of otherwise identical batteries, different temps - all of which are not totally under your control - can all create that small voltage difference.

Wire resistance for 6 gauge wire is 0.00040 ohms/ft. Assuming a difference in parallel cable length of 1ft (likely less than that), if your RV is using 5 amps, that means a battery voltage difference of 0.001 volts between the batteries - small enough to be ignored when compared to the other factors. On the other hand, a 50 amp draw would mean a voltage difference of 0.01 volts, which is starting to get significant. Hence my comment that if you are using an inverter having parallel cables the same length matters. If you use heavier than 6 gauge wire, the parallel cable length difference means less.

FWIW, I used 4 gauge cables to parallel my 2 12V batteries. But do learn from my experience - fuse the positive parallel cable at 50-100 amps (higher if using a big inverter). If one of the parallel batteries shorts a cell, you get a 2.1V difference between the 2. That's enough to heat 4 gauge cables red hot, and cause both batteries to boil, igniting the venting hydrogen. It's a very pretty color fire, but the only way to put it out is to cut the parallel cables. Been there, done that, don't ever want to repeat.

I now use 6V GC-2 golf cart batteries in series to avoid the pitfalls of parallel batteries.

Fred W
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2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time
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Old 04-22-2021, 07:24 PM   #14
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The batteries work together best when they are both are at exactly the same voltage. Any difference in voltage means the higher voltage battery is carrying more (not all) of the load until it is depleted down to the level of the lower voltage battery. The smaller the voltage difference between the 2 batteries, the less this occurs. ...
Exactly correct, but they need the same series resistance to achieve the same exact voltage. Balancing gives them the exact same voltage, and that requires the series resistances are both balanced.

Currents divide equally only if they have the same series resistance. In this case, where tens or hundreds of amps are involved, what seems like an insignificant voltage drop is actually a significant resistance difference.

Think in terms of both voltage and current. All loop voltages add up and Kirchoff's current law applies.

https://www.electronics-tutorials.ws...its/dcp_4.html

As a side note, this is even more critical for lithium batteries due to their very low internal resistance. Lead acid have more internal resistance, and that makes them less picky about balancing.
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Old 04-22-2021, 10:10 PM   #15
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Assuming lead acid batteries, having everything balanced is nice, but not critical. The marine world proves that everyday by splitting and recombining batteries that are not equal or balanced. In the marine world, you have to have one battery capable of starting the main engine to get back home. But most situations cannot afford (space, weight, $) to set that battery aside solely for starting and recharging independently.

If you are operating in the range of 10 amps are less, the slight difference in resistance in the 4th decimal place from 8" longer wire to one battery is immaterial in the big scheme of things.

Fred W
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