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Old 11-11-2020, 05:26 PM   #11
flyons78
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Default Re: LiFePO4 BMS Wiring Help

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Originally Posted by WalterM6 View Post
The size of that wire will be determined by the amp rating of your controller. A stock controller would be ok with #6. A 500A Alltrax would need a #2. One other thing and excuse me if you already noticed. The screws provided are barely long enough to get a grip with the larger connection on B+ and B- and the two you just mentioned. What I'm doing to prevent ripping the threads out is to make a stud connection. I'm going to get a longer M4 screw and cut the head off. Rethread with a nut and screw into the battery. Then I will just stack all the cables on and tighten down with a nut and won't have to worry about stripping the threads out.
Ah, I haven't yet stumbled on to the problem with the screws not being long enough, so thanks for bringing that to my attention. Creating the binding post is a great idea.

You did pique my curiosity with the 2 AWG wire requirement. I will switch it from 4 to 2AWG as you suggest, but considering this is only the connection between cell 8+ and cell 9-, how is it that those little metal plate connectors have less resistance than a 4 AWG wire? Not doubting you, just trying to understand.
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Old 11-11-2020, 07:15 PM   #12
WalterM6
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Default Re: LiFePO4 BMS Wiring Help

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I will switch it from 4 to 2 AWG as you suggest,
I have my Alltrax turned to 300A so I'll be using a #6. And now that I've switched to Lithium batteries I'm beginning to believe a lot of the Lead Acid stuff was overkill.
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Old 11-12-2020, 06:52 AM   #13
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Default Re: LiFePO4 BMS Wiring Help

What is the gauge of the B- and C- wires from the BMS?

Good connections and crimps are more important in this case since that short piece of cable will not have much resistance and 4ga is more than enough.

Any voltage lost in those connections during high current will affect the cell voltage readings.

The CALB cells have a clever design where the distance between two adjacent modules is the same regardless if you place them on the long or narrow sides.

That is not the case on your cells and I don't know if you are making your own connecting busbars or if the company sent you both short and long busbars so this suggestion may not apply.

If You do have long busbars (8 total) to connect the cells between the modules that are next to each other on the narrow sides you can wire the entire pack with busbars and not use external cables, the pack connections would be on the bottom left (+) and right (-) sides of the box.

You would need only one long busbar if the pack connections were the two posts near the box center hole.

I looked at the link for the BMS and I did not see the on/off signal switch wired to the board on their picture, how/when does that relay power off?

It mentions a timer to put the BMS to sleep (relay off, no balance, no monitor and no communications) but not how to get the BMS out of sleep mode.
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Old 11-12-2020, 09:23 AM   #14
flyons78
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Default Re: LiFePO4 BMS Wiring Help

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What is the gauge of the B- and C- wires from the BMS?
It's 2ga everywhere else. I went ahead switched it to 2ga between cell 8 and 9, just to keep it consistent, if anything else.

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Originally Posted by Sergio View Post
Good connections and crimps are more important in this case since that short piece of cable will not have much resistance and 4ga is more than enough.

Any voltage lost in those connections during high current will affect the cell voltage readings.

The CALB cells have a clever design where the distance between two adjacent modules is the same regardless if you place them on the long or narrow sides.

That is not the case on your cells and I don't know if you are making your own connecting busbars or if the company sent you both short and long busbars so this suggestion may not apply.

If You do have long busbars (8 total) to connect the cells between the modules that are next to each other on the narrow sides you can wire the entire pack with busbars and not use external cables, the pack connections would be on the bottom left (+) and right (-) sides of the box.

You would need only one long busbar if the pack connections were the two posts near the box center hole.
Flipping the cells and making a custom busbar between 8\9 would have been a better way to do it for sure. Wish I had started this conversation a little sooner. Unfortunately I'm already too far along with customizing things to its current configuration, including the bms balancing wire lengths. The 2ga connector will have to do. The company provided busbars for all 16 cells, but they are all the same length and meant for connecting in a single orientation, which is kinda dumb for 16S.


Quote:
Originally Posted by Sergio View Post
I looked at the link for the BMS and I did not see the on/off signal switch wired to the board on their picture, how/when does that relay power off?

It mentions a timer to put the BMS to sleep (relay off, no balance, no monitor and no communications) but not how to get the BMS out of sleep mode.
This is a good question and I have no idea. Is this something I should ask them about, or plan to have a manual switch to control power?
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Old 11-12-2020, 10:41 AM   #15
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Default Re: LiFePO4 BMS Wiring Help

You just need the short buss-bar between cells 8 and 9, no other cell orientation changes are needed on your design.

I saw online they sell both length buss-bars for those types of cells, I understand it is too late for you but perhaps future members reading this thread will have the opportunity to get the two size buss-bars for a more compact setup.

I would ask the seller the question about the BMS relay coil handling, meaning how does it turn off and back on again.

Even if it is automatically based on the BMS detecting a load (which means the BMS would not be totally "sleeping") You then need to ask if they have a "soft start" circuit.

If You have a voltage converter for 12v, there will be a good size spark when that relay "closes" or the BMS may never go to sleep depending on the threshold current needed to wake up the BMS since the cart is always using current.

Those boards have a place to solder two small gauge wires to make a "on/off" signal switch, depending on their answer you may want to do that before mounting everything and then enable the switch in the software.
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Old 11-12-2020, 04:20 PM   #16
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Default Re: LiFePO4 BMS Wiring Help

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Originally Posted by Sergio View Post
You just need the short buss-bar between cells 8 and 9, no other cell orientation changes are needed on your design.

I saw online they sell both length buss-bars for those types of cells, I understand it is too late for you but perhaps future members reading this thread will have the opportunity to get the two size buss-bars for a more compact setup.

I would ask the seller the question about the BMS relay coil handling, meaning how does it turn off and back on again.

Even if it is automatically based on the BMS detecting a load (which means the BMS would not be totally "sleeping") You then need to ask if they have a "soft start" circuit.

If You have a voltage converter for 12v, there will be a good size spark when that relay "closes" or the BMS may never go to sleep depending on the threshold current needed to wake up the BMS since the cart is always using current.

Those boards have a place to solder two small gauge wires to make a "on/off" signal switch, depending on their answer you may want to do that before mounting everything and then enable the switch in the software.
I'm starting to feel like I bought the wrong BMS. I just sent them a message about the sleep and wake function. I'm afraid it will be hard to get clear answers from them. Also, I do have a 12v converter. Let's assume I don't get the answers I need, what would my path to getting this working look like? Or would buying a different BMS be an easier and better solution?
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Old 11-12-2020, 04:54 PM   #17
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Default Re: LiFePO4 BMS Wiring Help

Having a relay instead of FETs is a good idea for that design as it allows for more current handling without the heat concern, it is just a matter of finding out how they handle the relay activation as it needs to be powered OFF when not in use so it does not burn the coil.

Perhaps it is some type of "latching" relay so that is not even an issue.

WalterM6 may have more information since he got the same type of BMS before you did.
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Old 11-12-2020, 05:33 PM   #18
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Default Re: LiFePO4 BMS Wiring Help

Thanks. Yeah, I'll wait for him to chime in. I was planning to follow his setup, but his battery issue set him back a bit. Sounds like he may be back on track.
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Old 11-13-2020, 07:19 PM   #19
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Default Re: LiFePO4 BMS Wiring Help

Good news. I got my batteries installed today. The problem with the runaway battery was solved by top balancing the cells. They are all now within 10mv. Set my max voltage to 53.6V, 3.35 v/c. That is supposed to be 99% charge. The idea about the binding posts worked out good. I got all the big terminals connections secure with them. Btw I went with 4 ga for the B8-B9 connection, it was more flexible. I took it out to test drive and the performance was similar to the leaf pack. I took it up the biggest and longest hills and the neither the battery or the terminals showed any great heat increase. That was one of the things I was worried about because of the small bus bars and hardware. The only thing I have left is to calibrate the SOC reading. I am now a happy driver.
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Old 11-13-2020, 08:56 PM   #20
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Default Re: LiFePO4 BMS Wiring Help

So jealous! Congratulations!!!
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