Wiring a BMS question

[TimC]

While I wait for my bench power supply to arrive I am mocking up my LiFePo4 battery bank. I'm making my own copper anchor points for the BMS and I'm wondering if it is OK to wire as shown for the first wire (-) to the BMS.

I am not up on electrical theory so I just want to know if the (-) wire for the BMS lead must be physically between the lug and the BMS. Or will the BMS be able to read accurately wired like this? I can't find 18-22awg x 5/16" terminals locally.



If there is no difference in physical location I will be doing something similar as this for the other 4S connection's (+) BMS connections. If it is not a good way to wire I will make those connections with two 5/16" bolt holes and a 3/16" hole in the center for the BMS wires.

Thanks,
Tim

[saltydawg]

Thats perfectly fine for what your doing, not how I would do it thou. I would order the correct ring crimps and wait.

[TimC]

I posted this same question on the DIYsolar forum and so far that hasn't gone well. My fault, the picture led to some confusion. So I will attempt to explain a little more so all hell doesn't break out and maybe an experienced electronics person can help answer the original question.



This is a mock up of a 4S LiFePo4 battery bank and a BMS. The only wire from the batteries that has an exposed end is a negative wire from battery #1. I didn't, won't, would never, ever think about connecting any of the positive battery wires until I am sure the mock up is correct.

The black wire on the left (with paper in back of it to show it clearly), which is effectively the negative terminal of these cylindrical cells, is attached to a copper plate and the BMS B- post. The small wire also attached to that copper plate is wired through a second 3/16" hole. The other connections mocked up just have a copper plate laying near the (+ and -) terminals from each cell as in a "normal" 4S configuration. The C- BMS terminal is on the upper right end of the BMS and the negative lead from the battery will connect there. The red wire from battery #4 is the positive lead from the battery. These two will be attached to battery posts on the case.

Thanks, Tim

[TimC]

Thats perfectly fine for what your doing, not how I would do it thou. I would order the correct ring crimps and wait.

Thanks Salty. Posted a clarification I thought this needed while you answered that. I appreciate the help. So I am going to assume that the BMS will accurately measure voltage/current from that connection on the side.

[saltydawg]

Yes it will it fine. Still not what I would do, I would just get bigger rings and do a direct bolt each set of cells to each other with the bms lead on the bolt as well, less connections to go bad that way.

[TimC]

I'm a bit reluctant to say so but I've had a multimeter in my teardrop for years but only know basic 12v and 120v functions as well as cont. testing.

Someone on diysolar told me to check polarity of the factory wires. I thought how do I tell if red is positive or negative with the multimeter. So I checked voltage and there it is. A - 3.2v means I have the probes "backwards". And a 3.2v reading means I have polarity correct. Is it that simple?

Thanks again salty...

[saltydawg]

I'm a bit reluctant to say so but I've had a multimeter in my teardrop for years but only know basic 12v and 120v functions as well as cont. testing.

Someone on diysolar told me to check polarity of the factory wires. I thought how do I tell if red is positive or negative with the multimeter. So I checked voltage and there it is. A - 3.2v means I have the probes "backwards". And a 3.2v reading means I have polarity correct. Is it that simple?

Thanks again salty...

should be a yes, but you can confirm it with a 9 volt or a aa battery. they are marked for polarity.

[TimC]

Salty, I'm running 10awg from the battery to the kill switch and also to the solar charge controller. I plan on a 30a fuse for that line.

This breaker might be an option. I think I could use it as the battery box post. These reset themselves after cool down, correct?

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[saltydawg]

Just use a fuse and carry a spare. The chance of one blowing in next to nothing, and lets say the breaker blows and does not reset what are you going to do?

[TimC]

Thanks salty. I'll install a fuse instead. I still have to go to a hardware store and look for some brass 5/16" bolts to use for battery posts.

I started top balancing the batteries today. I'm following Lauren's suggestion to start at a target of 3.45v so I don't need to babysit the process. I expect that will take quite a while... days. That's OK. Slow and steady. I don't have plans to go camping until next week when it cools down a bit.

I read about wiring them up to the BMS 4S style and giving them a full charge with my Noco 7.2v. Then put them in parallel to slow charge them up to 3.45v then watch closely to 3.65v. I guess I will just be patient.