March 13, 20224 yr On 2022/03/10 at 2:52 PM, MdF said: A picture of the new cables. Well done. But the impact of different length of cables is not so important, provided they have at least 25mm² section (from welding supply shop). The difference only comes into effect with very high discharging currents. These are exceptional and short events. But the difference of internal resistance of the batteries may be more important and you cannot change that. That's my experience with my 4 identical battery packs purchased at different times. When it charges to full SOC I noticed that they will equalize themself. The one with lower internal (or external) R will reach 100% SOC first but with its higher internal voltage will then draw less current, allowing the others to catch up.
March 16, 20224 yr On 2022/03/13 at 3:16 PM, Beat said: Well done. But the impact of different length of cables is not so important, provided they have at least 25mm² section (from welding supply shop). The difference only comes into effect with very high discharging currents. These are exceptional and short events. But the difference of internal resistance of the batteries may be more important and you cannot change that. That's my experience with my 4 identical battery packs purchased at different times. When it charges to full SOC I noticed that they will equalize themself. The one with lower internal (or external) R will reach 100% SOC first but with its higher internal voltage will then draw less current, allowing the others to catch up. You are only talking about discharging rates, but also remember: typically people charge their batteries with at least 50-80A or whatever their solar produces, having mismatched cables will cause a mismatched SOC, the battery that charges faster will also discharge first when you put a load on it. It will degrade faster and in the long this is not ideal. Skimping on R300 cable when you spend R40K on batteries makes no sense.
March 16, 20224 yr 1 hour ago, iiznh said: You are only talking about discharging rates, but also remember: typically people charge their batteries with at least 50-80A or whatever their solar produces, having mismatched cables will cause a mismatched SOC, the battery that charges faster will also discharge first when you put a load on it. It will degrade faster and in the long this is not ideal. Skimping on R300 cable when you spend R40K on batteries makes no sense. Basically you are right. But as I stated, the difference in internal resistance (Ri) of different battery packs is likely to be much greater than the resistance of 50cm 25mm² cable. I.E. the last pack I installed is connected at the end of the line, thus has the longest leads. Yet its Ri is so much lover than the others that it recharges and discharges quit faster than the other 3. The reason might be the fact that it is new while the others have already endured 400 and over 500 cycles. It could also be due to manufacturing inaccuracy or improvement. Perhaps its Ri might increase with age.
March 16, 20224 yr 4 hours ago, Beat said: Basically you are right. But as I stated, the difference in internal resistance (Ri) of different battery packs is likely to be much greater than the resistance of 50cm 25mm² cable. I.E. the last pack I installed is connected at the end of the line, thus has the longest leads. Yet its Ri is so much lover than the others that it recharges and discharges quit faster than the other 3. The reason might be the fact that it is new while the others have already endured 400 and over 500 cycles. It could also be due to manufacturing inaccuracy or improvement. Perhaps its Ri might increase with age. For some comparative numbers, 50cm of 25mm^2 wire will have a resistance of 0.32mOhm. My battery has a rated internal resistance of 0.5mOhm. When you look at battery to battery variations, you would expect them to be withing a couple of percent of each other, so you would be looking at variations of around 0.05mOhm. So wire resistance is quite significant relative to the expected variation of Ri.
March 16, 20224 yr 1 hour ago, JustinSchoeman said: So wire resistance is quite significant relative to the expected variation of Ri. Theoretically you might be right. However it contradicts my observation and experience. If I observe 0.5V drop due to a current of 20A: Ohms law: R = U/I: 0.5V/20A = 0.025Ω = 25mΩ. That is about 50 times more than the specs of your battery claims.
March 17, 20224 yr 8 hours ago, JustinSchoeman said: My battery has a rated internal resistance of 0.5mOhm. What battery is that, as it seems quite low? That seems like a typical figure for a 200Ah cell, but if your battery has 16 of them in series, the internal resistance will be 16x0.5mOhm.
March 17, 20224 yr 15 hours ago, iiznh said: It will degrade faster and in the long this is not ideal. Skimping on R300 cable when you spend R40K on batteries makes no sense. AFAIK if lithiums are kept or used between 20 to 80% SOC there seems to be no difference in rate that they will degrade so if longer cables cause 1 to be in a 5% lower SOC it is not bad for the battery. It will only come into play when discharges below 15% SOC or do I have it wrong?
March 17, 20224 yr 9 hours ago, Beat said: If I observe 0.5V drop due to a current of 20A: Ohms law: R = U/I: 0.5V/20A = 0.025Ω = 25mΩ. That is about 50 times more than the specs of your battery claims. How did you measure that? Internal resistance is the instantaneous response to changing current (not the long term chemical response of the battery). 3 hours ago, P1000 said: What battery is that, as it seems quite low? That seems like a typical figure for a 200Ah cell, but if your battery has 16 of them in series, the internal resistance will be 16x0.5mOhm. 120Ah prismatic cells - but you are correct - it should be 16x that resistance. But even then, the actual difference between batteries remains very small, and definitely comparable to the wire resistance. 3 hours ago, Scorp007 said: AFAIK if lithiums are kept or used between 20 to 80% SOC there seems to be no difference in rate that they will degrade so if longer cables cause 1 to be in a 5% lower SOC it is not bad for the battery. It will only come into play when discharges below 15% SOC or do I have it wrong? The problem is not degradation due to the SOC of the battery. The problem is that during periods of high current draw, the battery with the lowest resistance will deliver the most power (discharge the most). Once the current draw reduces, the batteries will equalize, and this battery will now charge back up from the other batteries. So with each change in current draw, this battery will use up additional partial charge/discharge cycles. It is a tiny portion of a charge/discharge cycle, but it happens on every change in load condition. And this will eventually degrade battery life.
March 17, 20224 yr 2 hours ago, JustinSchoeman said: How did you measure that? Internal resistance is the instantaneous response to changing current (not the long term chemical response of the battery). The BMS measures and tells those instant values. I monitor them closely. And no - Ri is basically a constant value. It might however varie somewhat with SOC and age: lower SOC - higher Ri. 6 hours ago, P1000 said: What battery is that, as it seems quite low? That seems like a typical figure for a 200Ah cell, but if your battery has 16 of them in series, the internal resistance will be 16x0.5mOhm. Right! That brings us to 8mΩ, that's in the same range as the calculated, perhaps somewhat exaggerated value of 25mΩ. Compared to this the 0.32mΩ of cable length difference is peanuts. 2 hours ago, JustinSchoeman said: The problem is that during periods of high current draw, the battery with the lowest resistance will deliver the most power (discharge the most). Once the current draw reduces, the batteries will equalize, and this battery will now charge back up from the other batteries. So with each change in current draw, this battery will use up additional partial charge/discharge cycles. It is a tiny portion of a charge/discharge cycle, but it happens on every change in load condition. And this will eventually degrade battery life. A somewhat weird theory. But even so: The degradation of batteries has the effect of increasing the Ri. With this it will eventually reach the same Ri as the others and from then on behave like the others.
March 17, 20224 yr 2 hours ago, Beat said: Right! That brings us to 8mΩ, that's in the same range as the calculated, perhaps somewhat exaggerated value of 25mΩ. Compared to this the 0.32mΩ of cable length difference is peanuts. But again - we are talking about the difference in Ri between batteries, which is a small percentage of the absolute resistance. Unless you are using hopelessly mismatched batteries, in which case you will have the same issues regardless of wire length. 2 hours ago, Beat said: A somewhat weird theory. But even so: The degradation of batteries has the effect of increasing the Ri. With this it will eventually reach the same Ri as the others and from then on behave like the others. Not exactly 'theory' - it happens in practice, and you can demonstrate it for yourself if you want. And yes, Ri will eventually normalize - at the cost of whatever portion of your battery life span would give you the same Ri loss as the extra cable's resistance. But I am not willing to give up any portion of my battery's life for the cost of a proper set of cables.
August 26, 20223 yr Just like we all waited a lifetime for the internet to finally work at speed, so do we wait for the day where we have batteries, or storage, to which none of this matters. I'm looking forward to that day:-) Edited August 26, 20223 yr by LouisM
August 26, 20223 yr On 2022/03/01 at 3:27 PM, MdF said: Afternoon All, So, I have seen some discussions about this but would like to know if battery cable length makes a difference to the whole system over very short distances? On the initial install, Battery 1 has a cable length to the switch of 55cm. On the second battery install a few months later, Battery 2 has a cable length of 1.1m. The reason for the doubled length on Battery 2 is that the terminals are located on the right hand side of the unit and need to fold back towards the switch. Is this difference in length a problem and should I consider getting the cables redone to match the same lengths exactly, i.e. 1.1m? MdF
March 28, 20233 yr I have moved my luxpower inverter out of the house due to the noise to the garage. As a result,the distance between my inverter and the DB has changed form 1m to 20m. Since this change happened my batteries don't last beyond 90 minutes during load shedding. Could the longer cable be a reason for the batteries failing to supply electricity beyond 90minutes?
March 28, 20233 yr Hi Giyani. If you mean by moving the inverter and the batteries to garage this will have no effect on the lasting time of the batteries. The only thing that should have changed is the size cable from DB-Inverter and Inverter- DB because of the possible voltage drop that can concur. If you have more than 1 battery please ensure that they have proper communication between them self and the inverter.
May 2, 20233 yr I have 3x 48V batteries. I was thinking normal parallel wiring and then from the last battery to the DC fuses but after reading the above it looks like the better way would be diagonal (1 and 3) or would halfway be better?
May 3, 20233 yr Hi Weimaraner, welcome to the club! Read my posts on the matter further up. Edited May 4, 20233 yr by Beat
May 4, 20233 yr On 2023/05/03 at 11:42 AM, Beat said: Hi Weimaraner, welcome to the club! Read my posts on the matter further up. Thanks!
May 16, 20233 yr On 2023/05/02 at 9:37 PM, Weimaraner said: I have 3x 48V batteries. I was thinking normal parallel wiring and then from the last battery to the DC fuses but after reading the above it looks like the better way would be diagonal (1 and 3) or would halfway be better? Wired up the batteries the last weekend, used "diagonal". 70mmsq battery cable is very hard to work with! Came out good although!
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