Craig Chamberlain avatar image
Craig Chamberlain asked

Battery Isolation Best Practice


I have an ESS system consisting of two Pylontech US5000 battery modules and a Victron Multiplus-II 48/5000/70-50. It's all single phase and based in the UK. Initially I used a pair of basic 150A bus bars but have recently upgraded my wiring in anticipation of adding two 250/60 MPPTs and a 6.8kWp solar array.

Since my US5000 battery modules can in theory discharge at 100A continuous (80A recommended) each, and since I expect to add a third battery at some point, I decided to run separate Pylontech battery cables for each module rather than daisy chaining them.

So I have my batteries feeding into a Lynx Power In module, modified to take fuses, via a 125A mega fuse each. I then have the Multiplus-II feeding from the same Lynx Power In via a 200A mega fuse and a 275A Victron DC isolator. This is on 70mm2 fine strand cable as recommended. My MPPTs will connect to the Power In via 35mm2 fine strand wire and 80A, 70V mega fuses. At this point I will be adding a second Lynx Power In as I need the extra connections.

My question is whether I should have DC isolators on each incoming battery cable or whether it's fine to rely on the Pylontech modules to power down when commanded via the red button and rocker switches, plus removal of the positive amphenol connectors for final isolation? I would prefer not to cut the manufacturer fitted M8 lugs off the Pylontech cables and add M10 ones to work with the DC isolators and I don't feel that being able to isolate the battery modules individually is ideal and might cause problems in itself.

Another option would be to install the second Lynx Power In module and place an isolator between the two Lynx modules. But that would require the 8mm holes in the bus bar to be drilled out to 10mm to accommodate the isolator and even then, the isolator bolts are not long enough to accommodate the spring washer and nut. They are long enough to accommodate the nut alone but then I'd have to use thread lock or locking wire which isn't great. Or maybe use a different isolator. Or I'd have to use little short pigtail cables which adds resistance, complexity and points of failure.

So, my question again is, is it ok to run the standard 2m Pylontech cables in a ventilated conduit from the batteries to the Lynx Power In and simply have the inverter with a 275A DC isolator and each MPPT will have a solar PV DC isolator. Then when I want to power down the whole system I would isolate everything in sequence before commanding the primary battery to shut the whole battery array down before toggling off the rocker switches and finally removing the positive leads for guaranteed isolation. Before that last step I would observe the Cerbo GX and MPPTs were powered off to ensure no load was present. This would be a much more elegant solution with fewer DC connections but I want a sanity check on the safety and compliance aspects please.

Many thanks.

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1 Answer
anthony avatar image
anthony answered ·

Having complete numerous installations with various multi module lithium battery brands including Pylon, I normally do it as you have suggest in paragraph 3 and not use DC isolators on each battery module. The goal is to reduce the number of DC connection points to a minimum whilst maintaining circuit protection.

It is essential to use a suitable cross section of cable which you appear to be doing.

I would have concerns as to how you modified the Lynx Pwr In. Ensure there is adequate and clean contact area between the applied fuse - fixings - busbar and that appropriate torque it used. The choice of spring and flat washer is also important if you added additional.

I would advise against putting and isolator between the two Lynx Pwr In units.

For the inverter and MPPT to battery main DC bus fusing, I generally deploy a fused disconnect (such as NH disconnects) as it halves the number of connections compared to a Mega fuse holder and separate isolator. To be clear a fused disconnect for the MPll and a separate fused disconnect for each of the MPPTs. This result in an electrically cleaner overall system in terms of the DC electrical circuit (Volt drop across connections). From a visual perspective it presents neater with a better a layout of fewer components and a more efficient used of space.

Your last paragraph is sound with the exception of above fused disconnect point. And if I don't need the full 2m of supplied battery module cable I don't make loops of the excess to get rid of it, instead cut as required and re-lug. Anthony

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Craig Chamberlain avatar image Craig Chamberlain commented ·
Hi Anthony, thanks for taking time to reply - much appreciated.

Yes I think I'm using appropriate cable cross section everywhere although the Pylontech cables themselves are only 21.1mm2 (4AWG) which is a little on the thin side but they are rated to 105C temperature and seem generally to be of good quality. I certainly wasn't comfortable daisy chaining multiple US5000 battery modules on these cables although they might be okay for a couple of US3000 or US2000 modules. My 70mm2 and 35mm2 cables used elsewhere are as recommended by Victron.

My modifications to the Lynx Power In are simply to add four 30mm stainless steel M8 set screws (ie. fully threaded bolts) along with the same stainless washers and spring washers and nuts as used in the Lynx Distributor. The Power In is exactly the same as a Distributor with the PCB removed and these extra four bolts etc removed. The only thing different on mine is that I've used stainless steel spring washers instead of copper ones but I intend to source some copper spring washers soon. As for torque, I have used the same 10NM for the nuts under the mega fuse and 14NM for the ones above the fuse as per Victron's manual. All metal faces were cleaned with IPA during assembly. I also checked during assembly that the mega fuse was sitting absolutely flat by virtue of the two connections points being co-planar and they were.

Understood re the use of fused disconnects, I'll certainly give that some consideration.

Lastly on potentially shortening the battery cables - in my setup I wouldn't be able to shorten them by much and I'm keen to retain the factory fitted lugs since the cable is 4 AWG and I only have a metric hydraulic crimp tool and crimps. I have crimped this wire previously and used 25mm lugs and initially a 25mm crimp die but had to drop down to a 16mm die to provide a bit of extra "squish" to get a good connection. I'm very confident that it was a good connection and certainly could not be pulled out but I feel the benefits of losing maybe 200mm from the cables might be lost by the non-factory lug attachment.

Thanks again for taking time to respond.


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