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klandos avatar image
klandos asked

Which battery cabling to prefer?

Hey there,

I've already done a couple of installations which all runs fine. I always stick to the "Unlimited wiring" guide for best DC / AC cabling. Normally there is plenty of space to have one big battery bank with equal DC lengths (f.e. multiple Pylontech + Victron Lynx).

Now I will install a bigger bank on a boat (catamaran) and would like to know which way would be best. Both options have pros & cons, maybe there is a third option I don't have in mind.

There is a bit space in each hull and in the middle of the catamaran. Which makes me have 4 spaces for the batteries.

Each battery pack will have it's own BMS. In total the battery will consist of 144x 280Ah Lifepo cells.

Keep in mind that in the schematic are no fuses and kill switches. Surely they will be add in the installation.

The normal use during driving will most likely draw only 2-4 kW in total. But the system has to be capable of maintaining 6kW to both engines over a couple of hours if necessary.



Left Option like Page 19 at wiring unlimited "Diagonally":

Pros:

- should make equal current on all banks

- only one battery monitor (shunt)

Cons:

- horrible long cables, about 28m for each + and - cable to the bus bar. Another 9m for each engine (just 125A)

- a couple of volts drop even with 120mm2 cable


Right Option like Page 19 at wiring unlimited "Busbars":

In this case the cable itself is the busbar.

Pro:

- shorter cables

Cons:

- 4 batteries in Victron System (not a problem at all, monitoring will be through (modified) MarineMFD App on a Raymarine MFD)

- unequal usage of batteries
- batteries in hulls will be discharged faster and get less charge than center batteries!?


I think (hope) last point shouldn't be a problem when laying on anchor the banks should be equalling themself, right?

Anyone tested this?



I also had the idea to build real separate banks and use Orion 48/48 to charge the banks for the engines. But the Orions are really getting only near 90% efficiency and I think that is a bit to much inefficient as the engines will be the biggest consumers.


Schematic:

batoptions.png


Lithium Batterybattery connectionbusbarmarine mfd app
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Mike Dorsett avatar image Mike Dorsett commented ·
I don't think that using un equal cables to wire Lithium banks in parallel is an issue, PROVIDED that the system can meet the peak current demand. Lithium batteries will redistribute the charge when at idle, and the charge efficiency is 96%. This was a major issue wiht lead batteries as the charge efficiency was typically 60%.

Thus you are better off minimising the cable runs to avoid volt drop, and recognise that the banks with longer cables will see a little less through put than the others. The saving in cable losses (and weight) will more than compensate for this.


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klandos avatar image klandos Mike Dorsett commented ·

Seems that not much people have experience with such a setup.

Yesterday I started a little test just to know how a system behave.

On my home setup I've 16x Pylontech US3000C.

I took one of them and put it out of communication.

Rewired it as below:

- main busbar -> BMV -> 15m 6mm2 Cable -> smartShunt -> removed Pylontech modul

- plus cable with according fuses also 15m with 6mm2

total of 30m 6mm2 cable + and -.

In VRM you can see that during discharge the amount is not 1/15 of total Amps it's more like 1/30 due to the voltage drop.

6mm2 cable shows effect, but that's what I'm trying to figure out!

Sad you cannot see the voltage from the smartShunt in VRM. That's why I hooked up BMV & SmartShunt to check how much it is. From viewing yesterday up to 0,5 Volt between BMV & SmartShunt screencap1.png


During charge today you can see almost the same.

The single Pylontech module is charging with about 1,8A while the other 15modules take 186A (12,4A each).screen2.png

Just when the main modules are nearly full and the voltages goes up there is a little peak for the single Pylontech module:

screencap2.png


My Conclusion:

Using 4 batteries, each with it's on BMV shouldn't be a problem as every single battery bank is capable of providing the needed energy. Over time they will be equal at the low end or top end.

Only thing I need to ensure in my opinion is that the voltage drop from "bus bar cable" is less than the difference from a empty battery vs. a half full one, right? 70mm2 should do the job, opinions?

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