Nice house @Neb. I am envious of the blue sky. Was that photo shopped? 

Shot Don.



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14 hours ago, Neb said:

Here's a diagram to show the resistance at each battery string, shown as a circuit with values for cable resistance as R1 and R2, I'm only using 2 different length cables. The total resistance for each string is R5

I agree that I need balancers and fuses but I'm struggling to understand why I would need busbar and equal length cable when the resistance in my circuit is all the same for each string?



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I'm not going to try to be funny, but do you have actual lab calibrated equipment to measure resistance with, because the difference in resistance we are referring to is in the milli-ohm ranges and not even a proper Fluke multimeter will be able to accurately measure that. Measured values of 1 ohm and 2 ohms is just to inaccurate in this case.

Take a simple example: Assume the batteries are all new and have the same internal resistance of 0,50 ohms per battery, that will be 2,00 ohms per string of 4 batteries. If you are charging the bank in bulk mode @ 58V and you have a resistance of 0.05 ohm in the total cabling of the first string and 0.06 ohm resistance in the total cabling of the second string, you will have a charging current of 28.29A in the first string and a charging current of 28.16A in the second string. That will result in a voltage drop of 1.41V across the cabling of the first string and a voltage drop of 1.69V across the cabling of the second string. The charging current in the second string will always be lower because of the higher cable resistance. When the total charging current drops to a point where the charger changes the charging mode to say to float, the first string will be charged more than the second string and that will happen every time the bank is charged. The same thing will happen when the bank is discharged (used), the most current will be drawn from the first string, the one with the lower resistance in the cabling. This means that the first string is worked much harder than the second string, the batteries will always be out-of-balance and might cause premature battery failure in either the first string, because the batteries are worked harder or in the second string because the batteries are never fully charged before the next discharge cycle starts.

A battery balancer might help to balance everything in all the strings, but on a larger battery bank, the balancer might not be finished with the balancing process before the bank is discharged or charged again and that will result in an unbalanced bank and premature battery failure.

This was discussed many times here on the forum.  If you have parallel battery strings you need to ensure that the cable lengths in each string is of the same length and type in order to use (discharge) and charge each string equally. The easiest method is to add busbars and the have equal length cables between the batteries and the busbars. You already have equal lengths and types of interlink cables between the batteries in each string, but the ends of the strings are not connected optimally and therefor the strings will not be utilized equally.

Another must have, besides the fuse / breaker between the battery bank and the inverter is a fuse per string - also discussed a few times on the forum.
If everything is connected as described above, you can get away with only 1 x HA-02 battery balancer.

 

Not a problem, I'm here to learn

When you talk about equal length cable referring to the method in the diagrams above, did you use lab equipment to measure the resistance?
I understand what you are saying about small differences in resistance but surely it also applies to the above circuit as much as it applies to my circuit?

I still see my circuit as balanced because each string sees the "same (without lab equipment) resistance", the initial advice was to change the way I've wired the batteries together and use bus bar and equal length cable. I.e. More cable, more resistance.

If we are going to start talking about measuring the ohms in our "equal length cable" with lab sensitive equipment then surely this applies to everyone with "equal length" cable between strings and not just me ?





Just asking ?


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8 minutes ago, Neb said:

When you talk about equal length cable referring to the method in the diagrams above, did you use lab equipment to measure the resistance?
I understand what you are saying about small differences in resistance but surely it also applies to the above circuit as much as it applies to my circuit?

If you are using cable of equal lengths (accurate to say about 10 millimeter) and of the same type, the resistance per length of cables should be very close to each other - you don't have to measure it, you can assume that the difference will be negligible.

8 minutes ago, Neb said:

I still see my circuit as balanced because each string sees the "same (without lab equipment) resistance", the initial advice was to change the way I've wired the batteries together and use bus bar and equal length cable. I.e. More cable, more resistance.

Referring to my explanation above, you can see that even something as low as 0.01 ohm has a substantial effect on the amount of current flowing through each string. In this case where you do not have equal lengths of cabling in each string and have rounded resistances of 1 ohm and 2 ohms, can you imaging what the possible effect could be? (I said could, because it is possible that you are lucky and that the resistance is actually exactly 1.000 ohm and 2.000 ohms)

By adding more cable and busbars, it will probably result in a higher resistance per string, but if the total cable length (and type) per string is exactly the same, you will have the same amount of resistance per string and thus the same current flowing through each string and each string should be much more in balance to the other strings and utilized equally. The increased resistance per string will not have such a great effect on the system performance, but unequal resistance per string will negatively affect your batteries. As always, use the thickest possible cable - the thicker the cable, the least the resistance. 

8 minutes ago, Neb said:

If we are going to start talking about measuring the ohms in our "equal length cable" with lab sensitive equipment then surely this applies to everyone with "equal length" cable between strings and not just me ?

As per above, if you are using cable of equal lengths (accurate to say about 10 millimeter) and of the same type, the resistance per length of the different cables should be very close to each other - you don't have to measure it, you can assume that the difference will be negligible. Without trying to measure the resistance, but by using cable of the same type and length, you should be OK.

Use the advise, don't use it. A few meters of cable is much cheaper than a battery or 4.

As per above, if you are using cable of equal lengths (accurate to say about 10 millimeter) and of the same type, the resistance per length of the different cables should be very close to each other - you don't have to measure it, you can assume that the difference will be negligible. Without trying to measure the resistance, but by using cable of the same type and length, you should be OK.

Use the advise, don't use it. A few meters of cable is much cheaper than a battery or 4.

Thanks superdiy. I agree with you on this.

R1 and R2 in my diagram are just values to indicate the different lengths of cable.

I measured my cables with lugs to be within 1-2mm difference but tried to get them exact.

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3 minutes ago, Neb said:

R1 and R2 in my diagram are just values to indicate the different lengths of cable.
I measured my cables with lugs to be within 1-2mm difference but tried to get them exact.

In your sketch you have 1R and 2R etc. and that is industry standard terminology equivalent to 1 ohm and 2 ohm (1R = 1 ohm, 1R8 = 1,8 ohm etc.) 

In your sketch you have 1R and 2R etc. and that is industry standard terminology equivalent to 1 ohm and 2 ohm (1R = 1 ohm, 1R8 = 1,8 ohm etc.) 

Sorry, that was a mistake.


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6 hours ago, Neb said:

Not a problem, I'm here to learn

When you talk about equal length cable referring to the method in the diagrams above, did you use lab equipment to measure the resistance?
I understand what you are saying about small differences in resistance but surely it also applies to the above circuit as much as it applies to my circuit?

I still see my circuit as balanced because each string sees the "same (without lab equipment) resistance", the initial advice was to change the way I've wired the batteries together and use bus bar and equal length cable. I.e. More cable, more resistance.

If we are going to start talking about measuring the ohms in our "equal length cable" with lab sensitive equipment then surely this applies to everyone with "equal length" cable between strings and not just me ?





Just asking ?


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Neb, looking at your photo again, I'm going to going to try and make it as clear as possible. 

 

From the fuses you have one black cable to the top battery bank. The 2nd black cable, going to the bottom battery bank looks longer. 

Then, between the two battery banks (top - can't see the bottom, but let's assume it's the same), there's a long red cable between the two battery banks. THAT cable should be the same lenght as the black cable going to the first bank. Well, it won't be possible, so the black cable going to the first battery bank should be the same length as the red one. 

And this is where others have suggested to use a bus bar, from the fuses, and then two identical length cables to the two battery banks. 

Those different lengths have no effect, all the batteries see that cable length


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Those different lengths have no effect, all the batteries see that cable length


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18 hours ago, Neb said:

Those different lengths have no effect, all the batteries see that cable length


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Oh, ok. Guess I was wrong then. 

Oh, ok. Guess I was wrong then. 

Why do we assume that the length of the positive lead needs to be the same as the negative when these cables are outside the circuit?


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The total V drop across all my cable is 30mv @ 50A


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@Neb I don't assume anything. I work with what science has taught me. And many year's experience. If you're happy with the volt drop, then leave it as is. 

 

@Neb I don't assume anything. I work with what science has taught me. And many year's experience. If you're happy with the volt drop, then leave it as is. 
 

That's cool. I need a scientific explanation about why my circuit is not going to work because this disagreement was about my banks being out of balance because of the method of wiring them together
I've wired my batts as the diagram shows.

There are 4 pieces of cable all within 2mm tolerance in the length labeled R2
There are 2 pieces also within 2mm tolerance in the length labeled R1

I know I may seem argumentative but that's not what this is about, I know you guys have more experience than me, I also know that you can't go wrong with the method of equal length jumpers and bus bar.

I was chatting to an electronics engineer (both my neighbors are electrical engineers) and he showed me mathematically that if all the batteries see the same resistance then it's as well balanced as the equal jumper method (maybe with even less cable)

PS yes I'm happy with the 0 Vdrop[emoji6]






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I will also change it if it turns out that the method I've used is wrong


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On 2/11/2017 at 4:27 PM, Neb said:

Why do we assume that the length of the positive lead needs to be the same as the negative when these cables are outside the circuit?

Positive and Negative leads do not have to be the same length - the length per string needs to be the same as per the image below. If you are going to measure / monitor the midpoint voltage, e.g. with a BMV702, the length of cable on each side of the midpoint, in each string, must be the same - if not you will have current flowing between the midpoints of the strings continuously and the batteries will always be out of balance.