livingaboard
Everett wa
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until I see some scientific testing I am going to call bs on this one. I have read it on many sites but I don't believe it matters where you connect what.
The batteries will take as many amps as they need to charge and no more. You can't push amps into a battery but you can push voltage. If your connections are good then they are all going to get the same voltage.
Dave
Everett, WA
2000 Itasca Suncruiser 35U, F53 Triton V-10
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westend
all over
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Doesn't the practice of balancing batteries and charge/discharge current relate to electron flow?
In a DC circuit the electrons would flow from negative to positive charge. It would occur to this dumb Swede that if you have a bank of batteries and have the charge current entering the batteries at opposite ends, the only resistance would be the associated battery connecting cables.
If the charging circuit is connected in the center of the batteries, the current would see the resistance of a part of the battery connecting cables and the outboard battery connected on the negative side would see the resistance of the ground to frame and the resistance of the circuit back to the positive battery post, the resistance through part of the battery connecting cables, and to the charger positive.
Can anybody confirm that this dumb Swede has his physics correct?
'03 F-250 4x4 CC
'71 Starcraft Wanderstar -- The Cowboy/Hilton
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pianotuna
Regina, SK, Canada
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Hi livingaboard,
Did you see that actual testing was done by the folks at the smartgauge site? (enjoy your lunch).
livingaboard wrote:until I see some scientific testing I am going to call bs on this one. I have read it on many sites but I don't believe it matters where you connect what.
Regards, Don
Kustom Koach Class C 28'5" 256 watts Unisolar, 875 amp hours in two battery banks 12 volt batteries, 2500 MSW watt inverter.
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Bit Bucket
Brookings, Oregon
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pianotuna wrote:Hi livingaboard,
Did you see that actual testing was done by the folks at the smartgauge site? (enjoy your lunch).
livingaboard wrote:until I see some scientific testing I am going to call bs on this one. I have read it on many sites but I don't believe it matters where you connect what.
But why can't he just throw ohms law away? Who really needs it? 
There is no question that you can unbalance your batteries (look at the worst case example of the (4) 12 volters). I don't think most are even taking the time to read the referenced site. And 100 amps of charging doesn't end up making miniscule differences either, far from it. This is easily seen by hooking up the batteries in those worst case scenarios and charging/discharging at 100 amps and checking individual battery currents with a DC clampon. If they are not the same from battery to battery (assuming all the batteries are good!) then this is an unbalanced system.
The question is, is that harmful in any way to the batteries. They are making the case that it wears out some batteries faster than the others over the long term. I agree that it is questionable whether or not it actually harms the batteries long term.
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DryCamper11
Hartford
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KendallP wrote:Folks are not reading the OP. He already IS balancing per the diagram above. He just wants to put the charging on the opposite posts as the load.
Exactly. I see no reason that my connection isn't "balanced" even though I don't connect the charger to the same location as the load.
Quote:I have 3-27s. They are not perfectly balanced. They are connected together in parallel with the positive to the coach on one of the outside batteries and the negative on the battery on the opposite side.
I ran like this for two years - 3 twelve volt "marine" or "leisure" type batts in a short case size to fit crosswise in my compartment, I don't recall the battery case number. I had diagonal connections, like you for one load, and an opposite diagonal connection for a second load. The charger was on one of the two load connections. My batteries failed prematurely, after only a few years, but I'm reasonably confident it had nothing to do with the connection.
In the Boonies!
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LScamper
Los Alamos NM
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The information on Smartgauge site about balancing is correct! However, there use of HIGH RESISTANCE interconnections make it appear much worse than it really is. If you use the correct interconnect wire size balance becomes a moot point. If you do a perfect job at balance there still is little chance that the system is really balanced. The reason is that the differences in battery internal resistance have more effect on balance than the low resistance interconnects.
Lou
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Bit Bucket
Brookings, Oregon
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DryCamper11 wrote:KendallP wrote:Folks are not reading the OP. He already IS balancing per the diagram above. He just wants to put the charging on the opposite posts as the load.
Exactly. I see no reason that my connection isn't "balanced" even though I don't connect the charger to the same location as the load.
Yes, we all read the OP and know what his question was, nobody said he wasn't balanced. Most don't think it matters where he connects the charger to and said so...
As you know, any technical post takes on a life of it's own and ventures off on side tangents, they always will. If he is not interested in the side tangent, just ignore it...
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pianotuna
Regina, SK, Canada
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Hi LScamper,
I was not aware that 0.0015 of an ohm (or was it 0.015--working from memory in a low bandwidth location) was considered a "high resistance" connection.
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LScamper
Los Alamos NM
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pianotuna wrote:
"I was not aware that 0.0015 of an ohm (or was it 0.015--working from memory in a low bandwidth location) was considered a "high resistance" connection."
Smartgauge uses 1.5 mOhms per link in their calculations. They use about 20cm or 8 inches of 35mm cable with a resistance of about .12 mOhms plus .2 mOhms for each of four termination interfaces on each link. I think that their termination resistance is reasonable. I add that up to .12 + .8 = .99 mOhms. 35mm cable is between AWG 2 and 3. Not sure how they get 1.5 mOhms to use in their calculations, the higher the resistance the worse it looks.
If you use eight inches of 2/0 cable with a resistance of about .078 mOhms and you use their termination resistance that would be about .88 mOhms per link or .88/1.5 = .59 of the resistance they use in their calculations. If you use these cables there will be much less difference with using an unbalanced connection.
High resistance is a relative term in this battery stuff. A 100AH battery has an internal resistance of around 20 mOhms. If the batteries are different by 1 or 2 mOhms that will have an equal or greater impact on balance than the interconnects.
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KendallP
Grants Pass, OR
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LScamper wrote:pianotuna wrote:
"I was not aware that 0.0015 of an ohm (or was it 0.015--working from memory in a low bandwidth location) was considered a "high resistance" connection."
Smartgauge uses 1.5 mOhms per link in their calculations. They use about 20cm or 8 inches of 35mm cable with a resistance of about .12 mOhms plus .2 mOhms for each of four termination interfaces on each link. I think that their termination resistance is reasonable. I add that up to .12 + .8 = .99 mOhms. 35mm cable is between AWG 2 and 3. Not sure how they get 1.5 mOhms to use in their calculations, the higher the resistance the worse it looks.
If you use eight inches of 2/0 cable with a resistance of about .078 mOhms and you use their termination resistance that would be about .88 mOhms per link or .88/1.5 = .59 of the resistance they use in their calculations. If you use these cables there will be much less difference with using an unbalanced connection.
High resistance is a relative term in this battery stuff. A 100AH battery has an internal resistance of around 20 mOhms. If the batteries are different by 1 or 2 mOhms that will have an equal or greater impact on balance than the interconnects.
Without taking the time to verify your calculations, I'm going to simply say that I strongly agree with the main premises of your fine post. My 3 - 27s are linked via "Smartgauge method # 2" using 9" of 2/0 cable per link. I'm not too worried about an imbalance.
I'm glad that someone worked out the math on Smartgauge's interconnects. I've stated many times that it bothered me that so little was offered on their webpage in the way of testing criteria. While 2 or 3 AWG is not uncommon for RV applications, no mention was made that, in lieu of perfect balancing, one can simply up the cable size and, for all intents and purposes, yield a very similar result without all the added wiring... especially when one's battery box is stuffed to the gills...
like mine...
* This post was
edited 07/09/12 05:22pm by KendallP *
Cheers,
Kendall
1986 Winnebago Chieftain 22RC
Our Camper (with no payments)
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