Had added below to my other thread but it probably got lost there. I'm trying to establish the correct parallel wiring setup for two of these batteries in parallel - since I've seen some graphics that go against what I thought was the correct configuration. I'm due to mount 2x SunSynk batteries in parallel later this week. 

The other day a family member had backup power installed - 8KW Deye and 2x Hubble AM5 5kwh batteries in parallel. 

My understanding was that for correct balance, the batteries must be installed as per the diagram below (Right hand side - part B). 

In the family member's installation (that was initially also missing the neutral earth relay/contactor bond on the backup side, and has the CT clamp installed inside the inverter cavity), the two Hubble batteries were connected as as per the schematic on the left. 

i.e. Inverter via DC fuse/isolator -> Battery 1 -> battery 2 (with equal length cables). Not the neg loop back from the second battery to the fuse/isolator. 

That installer came back and insisted that this is the correct setup (or at least, is perfectly acceptable). I'm guessing it may have something to do with modern LiFePo4 batteries and their BMS. 

Please can someone confirm that this is in fact correct (or a perfectly acceptable way, with no long-term harm or uneven degradation), or whether the connection must be as per the mockup below (and why). I've seen the FreedomWon website show the basic inverter -> battery 1 -> battery 2 setup, without the loop from batter 2 neg back to isolator. 

Are both option A and option B equally acceptable for a 2-battery LiFePo4 parallel connection? If only B, then why? (in a LiFePo4 / BMS / Inverter scenario). 

 

Only option A would be acceptable, as the total wire length to each battery is identical.

Option B would not be acceptable, as the slave battery has longer total wire lengths than the master.

Thank you @JustinSchoeman - while I also think option A, I don't quite follow your reasoning for A. I've been told by someone active in DC installations that the cables between the batteries must be of equal length, but that this doesn't apply to the loop (i.e. the pos to battery 1, and neg return loop, which in above graphic is longer). 

I'm very eager to understand if option B is also acceptable given the FW battery picture (and if yes, why? BMS related?), and the installer insisting that option B is also correct (family member's installation, with two Hubbles paired to a Deye). Especially in context of uneven workload, imbalance, longevity etc. 

PS the separate FreedomWon image below should ideally be below option B, just showing this way on the forum. 

Edited March 7, 20233 yr by Kalahari Cruiser

Also worth noting that to use 50mm² wire with 160A fuses, either:

1) it may not be in an enclosed conduit, or

2) it must be high temp silicon rubber (SANS 1574-5) cable.

2 minutes ago, Kalahari Cruiser said:

Thank you @JustinSchoeman - while I also think option A, I don't quite follow your reasoning. I've been told by someone active in DC installations that the cables between the batteries must be of equal length, but that this doesn't apply to the loop (i.e. the pos to battery 1, and neg return loop, which in above graphic is longer). 

The total wire length, inverter to battery positive + battery negative to inverter must be the same for each battery.

In option B, both positive and negative leads are extended from the master to the slave, so the total wire length is longer.

If you look carefully at option A, and trace from the fuse, back to the fuse, you will see that the total length to each battery is identical.

6 minutes ago, Kalahari Cruiser said:

PS the separate FreedomWon image below should ideally be below option B, just showing this way on the forum

The FreedomWon picture does not show A or B, as what is shown in the picture is identical for both options.

To make it option A, connect one inverter lead at the top, and the other at the bottom.  To make it option B, connect both at the top...

2 Or more batteries of same size, state, ah, age etc should ideally be put on a decent busbar and ALL cables should be same length, size and ratings.
Not necessarily sans but definitely best way to ensure optimal performance.

Edited March 7, 20233 yr by WannabeSolarSparky

Thanks @JustinSchoeman really appreciate all of your inputs. OK I'm a slow learner today or perhaps my eyes are seeing something different wrt. battery lengths. 

On the FW setup - below is another image off the net that isn't quite as cropped, to my eyes it looks like a setup similar to option B, i.e. the batteries all connected with each other, and a + and - connection in at the first battery at the top (rather than, say, the neg loop from the last battery). 

8 minutes ago, JustinSchoeman said:

The total wire length, inverter to battery positive + battery negative to inverter must be the same for each battery.

Does this not only apply for the battery lengths between each of the batteries. But the + and - cable from inverter doesn't need to be the same? (that's what I understood from what I was told previously). 

 

13 minutes ago, JustinSchoeman said:

If you look carefully at option A, and trace from the fuse, back to the fuse, you will see that the total length to each battery is identical.

I'm not 100% sure that I follow you here. To my eyes, the neg return from the slave battery is longer (and for illustrative purposes should be clearly longer) than the pos incoming, but I thought that this was moot, since only the lengths between the batteries mattered, and the initial connection and return loop didn't have to be the same length. I am probably misunderstanding you though, sorry! 

4 minutes ago, WannabeSolarSparky said:

and ALL cables should be same length, size and ratings.

Thank you @WannabeSolarSparky. What you are saying then is that this also applies to the connection between isolator and batteries (i.e. the return loop must be the same length as the + incoming to battery 1? Excuse the imperfect lingo. 

And with respect to same thickness wires, in scenario A, don't the short lengths between the two batteries carry less current than the connection to and from the inverter / isolator, hence the latter should be thicker cable? 

14 minutes ago, Kalahari Cruiser said:

Thank you @WannabeSolarSparky. What you are saying then is that this also applies to the connection between isolator and batteries (i.e. the return loop must be the same length as the + incoming to battery 1? Excuse the imperfect lingo. 

And with respect to same thickness wires, in scenario A, don't the short lengths between the two batteries carry less current than the connection to and from the inverter / isolator, hence the latter should be thicker cable? 

Cable thickness should be chosen to carry the expected current on a short circuit or fault and matched with the correct over current protection device.

Ideal Fuse or protection device rating should be lower than the cable amp rating.
You want to protect the cabling as that's more expensive to replace than the protection devices.

Edited March 7, 20233 yr by WannabeSolarSparky

35mm and 50mm should be fine provided you have the correct over current protection included.

 

15 minutes ago, Kalahari Cruiser said:

 

This is Option B, and is NOT the recommended way of paralleling batteries - the top battery has the lowest total resistance, and will work harder and wear faster than the others. It is plain to see that the total circuit length to the bottom battery is far longer than that to the top.

Although with the heavy bus bars FW uses, the actual loss in life will be small.

Thanks @WannabeSolarSparky. I was looking for that table, my fuses are 2x 160A. Not quite sure what Groups 1-3 are though. 

On the PV side - 6mm cable between PV protector box and inverter. 

6 minutes ago, WannabeSolarSparky said:

35mm and 50mm should be fine provided you have the correct over current protection included.

Just remember that 'Group 3' means mounted in a ventilated tray. So to use 35mm² it may not be in a conduit or mounted against a solid surface.

But that is really pushing it.  70mm² gets noticeably warm at 140A.  I would personally not use anything less than 70mm² at 160A.

2 minutes ago, JustinSchoeman said:

This is Option B, and is NOT the recommended way of paralleling batteries

Ok, that's exactly what I thought as well. Not sure why I'm getting slightly lost on one or two other cable length comments. The installer on family member's system is like that, and installer insists it's ok (against my advice from the outside). But I thought perhaps with modern BMS etc, it's less important maybe. 

So Option A - the correct way - involves 

(a) cable lengths between battery 1 and battery 2 (+ and -) being of equal length. 

(b) cable length from batter 2 (- neg) return back to fuse / inverter is a bit longer than the cable length from inverter / fuse to battery 1 (+pos), and is ok? 😳

1 minute ago, JustinSchoeman said:

I would personally not use anything less than 70mm² at 160A.

The DC elec I spoke to said as much - minimum 70mm - albeit I think he also felt that the 160A fuse is too light. All installations I've seen though seem to use thinner cable, like 35mm, including as specced on quotes. 

Context above is 2 batteries (2x1C) paired to a 8.8KW Deye (family) /Sunsynk (me). I guess one would also limit the discharge rate to be within a certain limit?

4 minutes ago, Kalahari Cruiser said:

(a) cable lengths between battery 1 and battery 2 (+ and -) being of equal length. 

This applies to Option A and Option B equally.

4 minutes ago, Kalahari Cruiser said:

(b) cable length from batter 2 (- neg) return back to fuse / inverter is a bit longer than the cable length from inverter / fuse to battery 1 (+pos), and is ok?

Correct - shorter is better, but it makes no difference if one is longer than the other here.

Thanks very much @JustinSchoeman - greatly appreciated. That's been very helpful of you. 

8 minutes ago, JustinSchoeman said:

So to use 35mm² it may not be in a conduit or mounted against a solid surface.

Just to clear up as wasn't sure about these regs:

-> Battery 1 to battery 2, 35mm cable, just run the cables exposed, without conduiting? 

-> Cables between fuses and battery 1 / battery 2 return, not quite sure what a ventilated tray is, but can a 40x40mm square PVC conduiting (with opening top) be used, and this conduiting (tray?) be mounted against the wall? 

9 minutes ago, Kalahari Cruiser said:

Context above is 2 batteries (2x1C) paired to a 8.8KW Deye (family) /Sunsynk (me). I guess one would also limit the discharge rate to be within a certain limit?

I have a similar setup. Go for 200A fuse, or you will face a lot of practical issues (overriding BMS discharge limits, for example).

3 minutes ago, Kalahari Cruiser said:

-> Battery 1 to battery 2, 35mm cable, just run the cables exposed, without conduiting? 

Correct

4 minutes ago, Kalahari Cruiser said:

> Cables between fuses and battery 1 / battery 2 return, not quite sure what a ventilated tray is, but can a 40x40mm square PVC conduiting (with opening top) be used, and this conduiting (tray?) be mounted against the wall? 

No idea - this is not contemplated in the SANS requirements, so you will have to fight with the sparky issuing the CoC about this.

12 minutes ago, JustinSchoeman said:

Go for 200A fuse

Thanks - I already have the 160 fuses in the Keto disconnector (and was a bit of a struggle to source, as most places were sold out for a long time). We are not major power users and I hope that I can get away with that, i.e. not have such high current draw. Interestingly, a few of the installs that I've seen (with similar battery setup) all had 160A fuse disconnectors. My DC elec contact had also recommended 200A though. 

 

10 minutes ago, JustinSchoeman said:

this is not contemplated in the SANS requirements

Noted, thanks. 

20 hours ago, JustinSchoeman said:

I would personally not use anything less than 70mm² at 160A

Just another one on this 🙏

The SunSynk batteries use those special battery connectors (I have 3 or 4 pairs, I think, for my two SS batteries). Mine are still in the packet but I think I've seen on this forum that they are limited to a smaller cable size (25 or 35mm?). How does one connect a 70mm cable to them on the battery side - DC fuse/inverter side is obviously no problem as uses lugs?

 

 

Fuse requirements:

I installed 125A fuse to each 5kW inverter. Each of the 4 battery packs has its own 125A breaker. Cables are 25mm². Absolutely no issue.

To understand a fuse: A 200A fuse does not blow at 201A! There is a current/time characteristics curve. The higher the over-current, the faster it blows. As a landmark value I remember the time value for 150% nominal current (300A for a 200A Fuse): approx 20 minutes to blow.

Concerning regulations: Installation regulations apply for the low voltage range, that is 50V to 999V. Below 50V is "small voltage" with almost no regulation. Installations have ±10% operational tolerance allowance. So a 48V battery installation may operate at up to 52.8V (charging voltage). Legally critical in this respect are 16 cell battery packs with nominal voltage 51.2V. Theoretically they should be installed following "low voltage" regulations.

Very interesting @Beat, thanks for that. 

On the battery connector issue, I also found this thread and diagram. Is the solution then getting an extra set of battery connectors, and running an extra cable from the second battery to the DC fuse, as well as from DC fuse to the first battery, i.e. 2x35mm? And then at the DC fuse side (Keto fuse holder) to double up on the lugs - is there sufficient space and does this work? 

 

 

Kind of like this (but for SunSynk batteries) - pic from thread above. 

 

 

Would this work - given the Sunsynk battery connector size limits? (now with new neg to first battery, and new pos return from second battery). Or just nonsensical?