Clarification on wiring a BMS?

[robt]

Hey everyone, I made an account on this forum just to ask this question because everywhere else I've been on the net hasn't had an active enough user base to help me figure this out, so I'm hoping someone here can reassure me that I'm doing this right.

Although it isn't an electric vehicle, I'm doing a cargo trailer conversion and have an electrical system planned around a 180Ah LiFePO4 battery. While looking for a BMS for the battery, I found out that I could either pay $500+ for something way overboard for a four cell battery, or go with the camp of Chinese BMSs which a lot of people on this forum seem to be familiar with.

I bought the BMS from a US store, although its quite obviously just a branded version of this one on Aliexpress:
https://www.aliexpress.com/item/Dal...lgo_pvid=54659c53-7951-4396-9752-81d0e2e7329a

Here's a photo of my electrical setup to the extent that I'm confident the BMS is wired:


And here's a photo of my BMS specifically:


Am I to understand with a common port that the P- line in my setup should be connected to the positive bus bar, the negative bus bar, and the positive post of the battery?

Also, should I assume that even though this is a common port BMS, the third connection on the charger should be ignored, and the manufacturer simply does something different on the interior to differentiate between common or charge port?

And one more question: My battery has a 1C charge/discharge rate (180Ah) and the BMS is capable of 200A, yet was manufactured with #6 AWG wires. Last I checked, 6 AWG was nowhere close to being rated for 200A, so would there be any harm in replacing the supplied wires with my own?

Thanks anyone for your help, I'm hoping this is the place that will finally help me understand if I'm wiring this correctly for my system.

 

[john61ct]

Have you got links to docs for that BMS?

There is no need for pack current to flow **through** the BMS.

Detecting volts & amps, then control through robust contactors (relays, solenoids).

For such a small setup, thorough automated protection will likely cost more than the bare cells.

Boat RV or cabin?

 

[robt]

The docs are in the Aliexpress link.

The system is going into a cargo trailer. Basically an RV system but without any charging coming from a vehicles alternator. The inverter/charger has a fully programmable computer for charging control, and I'm able to set voltage requirements for charge and discharge and stages specific to any battery.

Since my inverter/charger is handling the complicated stuff, I was looking for a passive BMS like this one, I just wasn't sure how it wired into my existing setup since I've yet to see good documentation of someone using a BMS like this in an RV system.

 

[john61ct]

There is no detailed manual in English?

The US company that sold it doesn't offer support?

 

[robt]

Unfortunately no. The only "manual" is whats on the Aliexpress site, the US seller just had that information copied verbatim. I was given this wiring diagram:



I've emailed the US seller through every address on their site and heard nothing, so I've been trying to piece together any info I can from pictures and other posts of people using a similar BMS. My next step is to e-mail the original manufacturer directly, but I was afraid I'd encounter a lot of difficultly in communication or nothing at all since I didn't buy directly from them.

 

[john61ct]

I'm learning myself here.

Am I to understand with a common port that the P- line in my setup should be connected to the positive bus bar, the negative bus bar, and the positive post of the battery?

What does "P- line" mean?

What is a "common port BMS"?

> to differentiate between common or charge port?

Are you talking about "common" as in "reference", which is sometimes Earth ground, but not in a vehicle, at best frame or chassis ground?

as opposed to the negative return of the power circuit?

> any harm in replacing the supplied wires with my own?
Bigger should always be better, long as you do it right.

Do not trust Chinese ampacity ratings, in my experience safe continuous amps is half or often less.

Use big fuses for wiring protection, in such a case I'd use smaller ones to protect the BMS.

 

[robt]

"Common" port means charge and discharge use the same line, as opposed to the charger having its own line, like this wiring diagram:


As far as I know, since the C- and P- line are combined in the BMS I have, the wiring I proposed in my first post is correct. I'm just hoping someone who's done it before can come along and say "yes that is correct", because if I wire it incorrectly I don't have many means of knowing I did until something expensive is ruined.

I'll probably use the same size wires I'm planning for the battery coming off of the BMS, and size a fuse on the low end. Although the battery can discharge 180A continuously, thats nowhere near my loads so I'll be sizing a fuse or breaker on the conservative side.

 

[hemo]

Common port Bms means charge and discharge uses same line, charge rate is same as rated discharge though not all packs set up may not like it on the charging. The line uses the same fets so charge rate is higher.

 

[hemo]

I'm learning myself here.

Am I to understand with a common port that the P- line in my setup should be connected to the positive bus bar, the negative bus bar, and the positive post of the battery?

What does "P- line" mean?

What is a "common port BMS"?

> to differentiate between common or charge port?

Are you talking about "common" as in "reference", which is sometimes Earth ground, but not in a vehicle, at best frame or chassis ground?

P line is pack discharge via Bms .
as opposed to the negative return of the power circuit?

> any harm in replacing the supplied wires with my own?
Bigger should always be better, long as you do it right.

Do not trust Chinese ampacity ratings, in my experience safe continuous amps is half or often less.

Use big fuses for wiring protection, in such a case I'd use smaller ones to protect the BMS.

P line is pack discharge via the Bms, in the case of this Bms it is also the pack charge port.

 

[robt]

Alright, the US seller got back to me and helped me figure out what I was getting wrong. This is the correct wiring for my particular system:



In the hope that someone like me eventually finds this post, here's a newbie's explanation to another newbie:

A BMS acts like a switch on the negative side of a battery. The positive line of the charger should go directly to the battery, because the BMS acts like a check on the negative side and will cut the flow of electricity if one of its parameters isn't met, at least, thats the case with this one, or any BMS with a "B-" line.

The "P-" line is the negative discharge, and goes back to the negative DC connection on the charger. Its called a common port because the same line coming off the discharge for the BMS goes back to the charger, as opposed to the negative DC line from the charger going to a separate connection on the BMS.

Apparently on my BMS, that third connection in the middle is for a solar charger. And the seller told me they were also skeptical about the 200A rating and the #6 AWG wires so they bench tested it. It gets warm but it works, though they said replacing those wires with something larger can't hurt.

Anyway, I hope this can help someone in the future! Thanks everyone who's posted thus far.

 

[john61ct]

I would not use a combi inverter charger.

In an overvoltage I just want to stop the charger, continue to feed loads.

In undervoltage I want to cut loads off but allow charging.

So I guess that requires a non-common port BMS?

 

[robt]

My particular inverter/charger will appropriately cut off loads in an under voltage scenario at the same time as the BMS to avoid drawing the batteries too far, and is programmed to stop charging at the same voltage as the BMS to avoid an over voltage scenario, so for me its essentially two checks on the same system and would perform the same with or without the BMS because I can program the exact voltage specifications for my inverter/charger. However I still need the BMS to insure that my four cells charge and discharge at the same rate.

But otherwise, yes you may want a BMS with a separate charging port if your inverter is separate from your charger and have more than one load. That will allow the BMS to continue charging the battery in an under voltage scenario if the separate inverter draws the voltage too low and the BMS cuts off discharge.