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billk avatar image
billk asked

What is the cable length tolerance in connecting dc cable to Quattros in parallel

I'm discovering just how far inverter technology has progressed since my first off-grid installation over 20 years ago. It seems that hooking two of these Quattros in parallel is going to be an adventure.

I made a lovely installation of two positive 2/0 cables with a 200 amp fuse from the positive bus bar to each of the inverters before I reread the paragraph in the manual that states "The DC connection cables to the devices must be of equal length and cross-section"

Well I've got the cross-section (all cables are 70mm2) taken care of and the longest cable is 24 inches (61 cm) but what is the tolerance in equal length? Are we talking an inch, 6 inches, a foot? I'd like to know before I make up the negative cables. Thanks.

battery charginginverter current draw
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3 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi @BillK

DC battery cable lengths (resistance) must be identical in a parallel installation.

It is common to use a large bus bar with short equal lengths to the inverters, or cut the cables equal to the farthest inverter and then loop back to the closest inverter.

There is further explanation about why this is important here:

https://www.victronenergy.com/live/ve.bus:manual_parallel_and_three_phase_systems

And the linked presentations on large system wiring at that link.

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billk avatar image billk commented ·

Thanks Guy and Elimac, That is sobering that a 16cm difference in length can have that great an impact on input amperage. I feel like a Model-T mechanic being told to work on a Tesla. I'm definitely not in Kansas anymore.

I've made up my negative cables to identical lengths. Looks like I'll be pulling the positive cables and make them identical. I'm hoping that being within a couple of cm will be acceptable?

Thanks for the help I have a sinking feeling that I will have more questions before this adventure comes to an end

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ben avatar image ben ♦ billk commented ·

It's a good idea to make them the same, but it's not going to hurt anything to try with the slight length discrepancy and see what you get, as long as you have a tool to measure and the willingness to adjust afterwards.

There are other sources of imbalance as well, including tolerances in your fuses (you do have multiple fuses, right?) whether you put the exact same torque on each connector, how good your crimps are, and so on. Ironically, the better your wiring, the greater the small differences' effects can be.

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Elimac avatar image
Elimac answered ·

I have a parallel setup of 2x 3kVA Multipluses. The DC input negative cables are final (~50 cm long) but one of the positive cables is temporary with ~66cm long instead of being also 50cm. With this small difference I see an aproximate unbalance of ~10% or so in the input current. For example, the inverter with smaller input cable has 45 Amps while the other one has 41 Amps (measued with DC clamp)

I'm curious to see how the unbalance will be after I correct and install identical cables, because the cables may not be the only cause.

My AC output cables are identical to the cm.

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Delta Victor avatar image Delta Victor commented ·
@Elimac i'm interested to know if changing the cabling did take care of the imbalance
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Elimac avatar image Elimac Delta Victor commented ·
It improved, but did not take it out completely. Bit it's low enough. It's normal, the inherent tolerances of all the materials start to show.
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raymiller avatar image
raymiller answered ·

If I could also add a critical piece of information to the issue of equal resistance to both batteries and inverters.

Not only does the cable design need to be completed taking into account all resistances but the commission should also include mico ohm measuring of every high current connection.

The ready availability of suitable low ohm measurement test equipment with the likes of the HIOKI RM3548 (resolution 0.1u ohm) (suitable for professionals) or a cheaper VICI VC480C+ (resolution 10u ohm) would add a verification check on both old and new installations.

The resistance testing quickly removes all the guesswork from the high current paths and validates the engineering design adding much-needed reliability to any installation. What's not to like about increasing the professionalism of the industry? Everyone wins.

From experience, some components are not what they seem when put under the micro ohm testing allowing the installer to quickly identify best practice terminating and connection methods and remove suspect equipment.

cheers


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