I could do with a definitive opinion on positive and negative cable lengths connecting from inverter to Lynx distributor. The responses I have had when searching for an answer is confusing. I have 2x 50mm2 pos and neg to be wired to the lynx. What I have gleamed, is that both the positive cables need to be same length but does not matter what the neg lengths are. A similar response had the rider that the sum of the cable lengths should be the same. Having something definitive would help with the layout. I could not find the answer in wiring unlimited unless I was looking in the wrong place.
The 2 positive cables need to be the same length as each other.
The 2 negative cables need to be the same length as each other.
The positive and negative cables can be different lengths.
Thank you for a very clear unambiguous answer.
@pwfarnell can you explain why its not critical that the positive and negative be the same length on an inverter to the Lynx but is necessary for the battery bank? Why don’t the same principles apply here?
On an inverter the positive lead will often include an isolation switch and a fuse and the negative lead a shunt, hence even if the cables are the same length they will have different resistances.
I do not believe that the positive and negative leads need to be the same length on a battery. Why do you think that they need to be the same length.
I guess I was thinking more along the lines of parallel and series connections being equal. It makes sense that from the battery bank to the buss bars is not as critical.
What does matter is running the negative and positive together as much as possible.
The more I am deducing it matters less for the inverter cables to match than the battery cables particularly if there is a bank of batteries in parallel. With the inverter supply the worst that can happen, is that there will be a temperature difference between the cables and loss of some efficiency but the result at the bus bar will probably be the same as if the cables were all the same. In the case of battery bank a continued difference of many charging discharging cycles will result in the batteries “aging” and performing differently as a result of them not all experiencing the same conditions. The importance of treating all the batteries in the bank in the same way is emphasised by several. I have 3 batteries that will connect to my system and thankfully they have come with their own factory made cables. I have not seen any quantifiable measurements to support some of what is claimed on the subject and I do not know how small a resistance difference has an effect. I measured out of interest a 3m length of 50mm2 cable using a low resistance measurement setting on a multitester and got as would expected 0.00 ohms. The same measurement on the 15M SWA connecting the outbuilding housing the energy system to the house earth was 0.02 ohms. I suspect it needs a battery scientist to really explain what the result is of mismatched cable lengths is.
I asked the expert that will be remotely commission my system next week the question, and was told they have to be the same size i.e. both plus and neg as a serious misbalance can cause DC ripple which can destroy an inverter. I did some more digging and found that confirmed and a comment that the lower the voltage the less tolerance there is on length discrepancy. So 12v worse than 24 worse than 48v. I did mine the same in the end as was not going to chance it.
If you run the cables together they will be pretty close to the same length, the difference of possibly 10cm if there is a fuse may add resistance on one side. Which then effectively makes them not the same length (in terms of resistance). Having them 100% exact is not entirely necessary. Even if you are using the lynx system there is a always a small difference in length. I have yet to see a negative impact on an installation from that.
The main aggravating factors for dc ripple is voltage drop and inductance. Both of which can be sorted by bundling them together and using a decent gauge cable which is as short as possible.
Thanks for the extra information. (Everyday a learning day) I have kept both pos and neg cables the same length for all the cabling. It was quite a challenge to do a neat job, I had 3 Pytes V12 batteries in quite a small space and they had 2m made up supply leads. Had to work from the furthest battery and put the Lynx to accommodate the full length but then with the other 2 batteries had to double back to loose the length as they were now close to the Lynx. Have a pair of coupled Lynx distributors the batteries accommodating one of them and the inverter and MPPT on the other. Apart from the MPPT at 35 mm2 all the others are 50mm2 and all just happen to be 1.5M each. I have made up a 5V power supply to illuminate the Lynx fuse LED’s. All ready now for a remote setup on Monday.