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Sailboat LIFEPO4 parallel wiring for 6 batteries

jc1409

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Dec 4, 2021
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After studying lots of info on parallel wiring and best balancing, my main takeaways are:
- use large gauge wire
- try to keep battery connection cabling to similar lengths to support best balancing
- Exact lengths not super critical

With this in mind, I've come up with the following wiring layout for planned 6 LIFEPO4 batteries - my engine compartment 'separates' the battery storage area, so have a fixed spacing issue that doesn't let me connect each battery with same size cables (easily). Was wondering if I could get feedback from any experts on this? thank you

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Looks pretty good. There are quite a few ways to do this. The only tricky bit looks to be the Y (T?) connection on each side but that’s just mechanical.
 
I would treat the 2X 3P banks separately.

Connect the two banks separately to the bus bar. You may have trouble keeping the two banks balanced, but the batteries within each bank should be good. Balance between the two groups can be improved if your total (+)+(-) cable length is the same to the bus bar.

Balance across 6 with the two long interconnects will be poor.
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Here is another idea that may be simpler. Get two Victron lynx power in or distributors. One on port side - one on starboard. Each battery runs to the bus bar (lynx distributor) with its own wires. Then connect the wires together.

For me, I would choose the distributor. This way I would have a ____amp (say 200a) Mega fuse on each battery. Those Mega fuses are not class T fuses- so they may not have the current interruption high enough to really protect everything, but what they will do for you is - if you need to take a battery in/out of service or a bit - they give you a nice safe place to break the circuit. Without having a power wire dangling about.
 
Thanks Rocketman!

With the separate fuses on each battery, would I avoid the need for a 'main' battery fuse for the combined load on the battery side, or would you recommend an additional main fuse for the combined battery load (I'm thinking probably need that too, but not sure). For convenience, I would move the start battery to the other side with this setup as follows:

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I would have a class T fuse as your main fuse.
The mega fuses if there was a big problem may not interrupt the current arc from a lithium battery (There was a large discussion on this site not to long ago).

If it were me, I would put one class T fuse next to the shunt (main boat fuse) and another between the two lynx distributors. (If you ever thought that cable could short - you could add a third one - so the cable connecting the distributors has a class T on both ends - but that may be overkill).

The little led lights that indicate a blown fuse on the distributors will not work(no power) but there is a hack on YouTube that shows how to get them to work. (adding a small usb transformer to deliver the correct voltage- or something like that).

Another thought… the 4pk of batteries can easily have the exact same lengths of battery wire. You could also (if you thought it wise and worth it) lengthen the 2pk battery wires so they were the same length as the 4pk ( battery wire length + the length between the two distributors) I don’t know if that would cause more problems (having a extra long battery coil of wire) than it solves.
 
Thanks again - I spent some time at the boat today measuring and examining, and then spent some more time cursing at how little space I have to do all the modifications and updates I plan!

Here's my latest iteration - I'll have to figure out the cost-benefit of making the wires on the 2-pack the same length as the wire length for the 4-pack. I think I would end up with lots of coiled up wire - would you expect large balancing benefits from this?

Regarding the wires for each battery in the 4-pack - is it correct that the positive and negative wires do not need to be the same length, and that the key is that the wires for each battery are the same as the wires for the other batteries? I.e., each battery's positive wire will be longer than the negative wire (x 4).

thanks

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My understanding is you want the total length of wire1 (pos + neg) = the total length of wire2(pos +neg)… etc. so 10” + 20” is fine with another battery of 15” + 15”.

If don’t know what difference having the 2pk batteries a shorter wire than the 4pk. Probably just a little - but will be small enough for not much practical effect (that is my guess - worth much less than you paid for it). You can test it and over time log each battery voltage- see what difference there is. You may need to balance the batteries manually just a little on an annual basis.

Also, because this is a boat - could you rewrite for 24v? - three 24v batteries? (Probably not, but if it’s not too much that could be worth thinking about.

Good luck
 
What does the Lynx do?

I would move the start battery to the right and have 3 LFP batteries on each side. Use a single bus to tie all batteries in one spot instead of the 2x Lynx with a jumper. Or piggy back the Lynx to connect in one place.

Depending on the total amps I would probably put 80 amp MEGA fuse on each battery and have a main Class T at 250 amps.
 
Thanks Time2Roll - appreciate all the input from you and everyone!

Here is a diagram of how I would implement what you describe on my setup, I think:

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Depending on current and distance you probably don't need 4/0 coming off the batteries when they all have separate feeders.
Maybe #4 would be easier to work with.

Page 4 of the manual says there is a shunt-fuse lynx module (Lynx Shunt) to put between the Lynx Power In and the Lynx Distributer. This eliminates some of the the cabling and connections shown in the diagram.

https://www.victronenergy.com/upload/documents/Lynx_Power_In/Lynx_Power_In-en.pdf
 
I am a little late to the thread, but have some more to add.

First, comparing the basic strategies above. With the batteries in a string like the earlier posts, the positive and negative need to come off of opposite ends. When you do this, it helps keep the load equal among the batteries. Also, the length of wires does not matter at all. This is because if you add up the total (positive and negative) lengths, for each battery will be exactly the same as all the other batteries. This is why the positive and negative need to be opposite ends. However, the center battery is still not going to be exactly loaded as the outside batteries. With only a few batteries not a big deal, but as you add more it becomes significant. Note that you need a fuse at each battery. If the lynx distributor is remote, you still need a fuse at the terminals. If 2 or 3 batteries are right next to each other, one fuse will work for both. As you have them arranged then, you would need 2 fuses.

The other technique will do the best job at keeping all the batteries the same, however, in this case the cables do need to all be the same length. An inch or few is no big deal, but with 6 batteries in 2 locations it is easy to have a several foot difference, and that is not ok. ABYC requires fusing close to the batteries (within a few inches), so with this method, the lynx distributor needs to be close to the batteries. Or forgo the distributor and use those fuses that mount on the battery terminals. You will need a total of 6 fuses, one for each battery.

There is a third, way to connect them, that uses less wire, less connections to the buss bar, but is more complex and harder to explain. First, connect 2 batteries together like the first way, with positive coming off one battery, and negative off the other. Then do the same for the other 2 pairs of batteries. Now, you only have a total of 3 batteries with the longer wire runs to the lynx distributor. These long wires need to all be the same length. You still need fuses at the batteries, but only 3 of them instead of 6. And you only need one lynx distributor, and less wire. This method works every bit as well as the second way, and better that the first way.
 
Thank you wholybee! I've revised the schematic and pasted below with your third option, which makes sense to me.

Regarding fuses and wire sizing - Each 100ah battery allows for 30 seconds of 200ah surge, and 1/2 second of surges above 200ah. My estimated 'maximum' load on this entire bank at any time is ~320 Ah under heaviest load, although regular load would be much lower than this. I am not planning on running my starter or bow thruster or other high-draw items from this bank.

Should my battery fusing assume relatively equal draw from the batteries in the bank, or should I plan for possible high Ah demands from individual battery sets? It seems like overkill to use 4/0 cable and 400ah mega fuses for each 2-battery set.

I plan a 400ah for the Class T fuse.
Jeff

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Using larger than required wire is good because it results in lower voltage drop.
You can assume the current from each battery will be approximately equal. So 3 200A fuses will be good for about 600A total.
 
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