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Best practice for connecting BMS sense leads?

fafrd

Solar Wizard
Joined
Aug 11, 2020
Messages
4,188
For the first ‘learner’ LiFePO4 battery I built, I just soldered the pre-tinned sense wires of my BMS to the middle backside of my copper busbars and everything worked out fine.

That battery is obviously one that will be complicated to disassemble if/when that ever becomes necessary and the bus bars that came with my new 280Ah cells are plated and protected by shrink tubing.

So I’m thinking for this bigger battery, I want to go with whatever solution is considered best practice for good connection and easy disassembly.

I see many are using round terminals that can be screwed on by the bolt/nut on top of the budbar.

Are the lead wires being crimped to connectors or washers or is there a better way to form those connections?

I’m generally a fan of soldered connections so is there any reason soldering to a washer made of copper (or whatever metal would be best) and bolting that on top of the busbar would be a bad idea?

Appreciate anyone’s suggestions as to the best way to connect tinned sense leads of a BMS to the cell terminals...

And also on a related note, the stick BMS lead harness had no problem spanning 8 90Ah cells in my small 8S learner battery build, but I doubt the harness will be long enough to span 8 280Ah cells and so will probably need to extend the sense wires.

Is there any source of longer stock 8S harnesses and if not, are there any recommendations on the best way to extend a harness to make it longer (other than extending all leads by the same amount, soldered/connected in the same way, and using the same wire)?
 
I wasn't able to find longer hardnesses so I just extended the wires I needed. I doubt they all need to be the same length unless you are doing something strange it shouldn't matter. Voltage won't be impacted unless you run a bunch of current.

I crimped mine on ring terminals and just put them over the top of any current carrying wires. Working great, no problems. I doubt there is any piece of these setups that need to be soldered.

Some of mine have four connectors on top of the bus bar. BMS, active balancing, parallel and power to the inverter. The rest I was able to spread out as you can put some on one side of the bus bar and some on the other.
 
I agree about using crimps. My wires are thin so I bent the end 1/4" of the wire back on itself to get double wire thickness.
I checked the resistance and they were all good.
I worried about that too an put a short filler piece of 16ga to fill the ferrule with the balance wire. Crimps came out nice.
 
I agree about using crimps. My wires are thin so I bent the end 1/4" of the wire back on itself to get double wire thickness.
I checked the resistance and they were all good.

I did the same.

It's hard to find a ring terminal that fits both the wire and the terminal bolt. When I get around to making my own bus bars, I think I'm going to drill and tap small holes in the bus bar to fit an appropriately sized ring terminal. That way the ring terminal doesn't have to go on the cell terminal bolt.
 
I did the same.

It's hard to find a ring terminal that fits both the wire and the terminal bolt. When I get around to making my own bus bars, I think I'm going to drill and tap small holes in the bus bar to fit an appropriately sized ring terminal. That way the ring terminal doesn't have to go on the cell terminal bolt.
I’m thinking of going with these M6 lug plate solder rings: https://www.amazon.com/X-Dr-Solder-...bbd-a222-11e9-8d7c-4cedfbbbda4e/dp/B07ZB68SYF

I was thinking about going that way anyway and after reading about the struggle to get the thin lead wires properly crimped in an M6 crimp terminal, a solder ring just seems easier (especially since the sense wire ends have already been tinned...).
 
I’m thinking of going with these M6 lug plate solder rings: https://www.amazon.com/X-Dr-Solder-...bbd-a222-11e9-8d7c-4cedfbbbda4e/dp/B07ZB68SYF

I was thinking about going that way anyway and after reading about the struggle to get the thin lead wires properly crimped in an M6 crimp terminal, a solder ring just seems easier (especially since the sense wire ends have already been tinned...).
These have worked very well for me:
Ring Terminals 1/4" Red 22-16 AWG - Heat Shrink Wire Connector

Crimping actually gives a great connection, just ohm them out to make sure, then give a strong tug on the connector to make sure it is crimped well. If you don't have a crimp tool, I can see trying to solder, but really, you'll get a better and longer lasting connection by using the right tool (crimp tool and correctly sized connector) for the right job. Just my 40 years of experience, feel free to do what you want.
 
These have worked very well for me:
Ring Terminals 1/4" Red 22-16 AWG - Heat Shrink Wire Connector

Crimping actually gives a great connection, just ohm them out to make sure, then give a strong tug on the connector to make sure it is crimped well. If you don't have a crimp tool, I can see trying to solder, but really, you'll get a better and longer lasting connection by using the right tool (crimp tool and correctly sized connector) for the right job. Just my 40 years of experience, feel free to do what you want.
My reasons for preferring a solder ring to a crimp ring are two-fold:

1/ solder is a reliable connection electrically - no need to ‘ohm them out’ or screwing around if they don’t match.

2/ for a longer-lasting connection when wires will be getting moved around, I agree crimp is superior to solder, but that’s really not the use case for these sense wires.

So if there is any downside to solder other than that the connection won’t last as long if the wires move or get tugged-on (and than many consider crimping easier/more convenient than soldering), I’d go for a crimped terminal, but it it’s only that, I prefer solder...
 
These have worked very well for me:
Ring Terminals 1/4" Red 22-16 AWG - Heat Shrink Wire Connector

Crimping actually gives a great connection, just ohm them out to make sure, then give a strong tug on the connector to make sure it is crimped well. If you don't have a crimp tool, I can see trying to solder, but really, you'll get a better and longer lasting connection by using the right tool (crimp tool and correctly sized connector) for the right job. Just my 40 years of experience, feel free to do what you want.
P.s. looking at those connectors, I can also see the appeal of forming a waterproof connection, but with y to he sense leads housed within my battery housing, don’t see that adding much value in my DIY LiFePO4 build...
 
As a general rule - a proper crimp is better than a soldered joint. In extremely critical life-supporting aircraft systems - it is 100% crimped.

With that said, there are a TON of crappy crimp connectors and tools as well as improper techniques. Take some time to consume some knowledge from the high-end manufacturers like TE and/or educational resources like High Performance academy. Most likely, you don't need to treat your battery like a super sonic jet fighter, but knowing the principles will give you the best chance of your crimps lasting longer than your batteries.

I did not like using the large rings on the battery studs and ended up machining bus bars with 4-40 threads for BMS connections. That allowed me to re-configure BMS leads without disturbing the buss bars. Not exactly imperative, but a detail I wanted to address on my system.

To get the length and routing I wanted, I just purchased a spool of 22x7ga wire and made my own. I got exactly what I wanted for only a little $ and time.
 
I did not like using the large rings on the battery studs and ended up machining bus bars with 4-40 threads for BMS connections. That allowed me to re-configure BMS leads without disturbing the buss bars. Not exactly imperative, but a detail I wanted to address on my system.

That's my plan, but I haven't starting putting together the details. Do you have pictures or suggested supplies? Are your ring terminals sized to fit 4-40 screws well?
 
As a general rule - a proper crimp is better than a soldered joint. In extremely critical life-supporting aircraft systems - it is 100% crimped.

With that said, there are a TON of crappy crimp connectors and tools as well as improper techniques. Take some time to consume some knowledge from the high-end manufacturers like TE and/or educational resources like High Performance academy. Most likely, you don't need to treat your battery like a super sonic jet fighter, but knowing the principles will give you the best chance of your crimps lasting longer than your batteries.

I did not like using the large rings on the battery studs and ended up machining bus bars with 4-40 threads for BMS connections. That allowed me to re-configure BMS leads without disturbing the buss bars. Not exactly imperative, but a detail I wanted to address on my system.

To get the length and routing I wanted, I just purchased a spool of 22x7ga wire and made my own. I got exactly what I wanted for only a little $ and time.

I believe I understand why crimp connections are superior in general - just don’t see much added value for this application and I’ve had to screw around with poorly-done crimp connections as much as the next guy.

Perhaps I’m just incompetent, but I’ve never had to screw around fixing a poorly-completed solder connection (though I have had to resolder connections that didn’t last ;).

I was thinking about drilling a hole into my busbars to affix the sense leads near the center, but these busbars are plated and heat-shrinked, so I’m not sure it’s worth it.
 
I'm better at crimping than soldering. Having the correct tool for crimps is critical. I have heat shrink crimps and they always turned out mangled. I finally realized I was using the wrong crimp tools and crimp dies for these crimps. I'm a sucker for new tools. The right tool makes the job a lot easier, even when the operator (me) is an idiot.
 
I'm better at crimping than soldering. Having the correct tool for crimps is critical. I have heat shrink crimps and they always turned out mangled. I finally realized I was using the wrong crimp tools and crimp dies for these crimps. I'm a sucker for new tools. The right tool makes the job a lot easier, even when the operator (me) is an idiot.
I think for the permanently-mounted sense leads , I’m going to stick to those flat little solder lugs I found, but I’m also going to need separate terminals for a 5A active balancer (which uses thicker wires) and all of you have pretty much convinced me I should go with crimp terminals for those larger wires (carrying some actual current).
 
As a general rule - a proper crimp is better than a soldered joint. In extremely critical life-supporting aircraft systems - it is 100% crimped.

With that said, there are a TON of crappy crimp connectors and tools as well as improper techniques. Take some time to consume some knowledge from the high-end manufacturers like TE and/or educational resources like High Performance academy. Most likely, you don't need to treat your battery like a super sonic jet fighter, but knowing the principles will give you the best chance of your crimps lasting longer than your batteries.

I did not like using the large rings on the battery studs and ended up machining bus bars with 4-40 threads for BMS connections. That allowed me to re-configure BMS leads without disturbing the buss bars. Not exactly imperative, but a detail I wanted to address on my system.

To get the length and routing I wanted, I just purchased a spool of 22x7ga wire and made my own. I got exactly what I wanted for only a little $ and time.
I'm IN the avionics business and I have to disagree, a heck of a lot of it is soldered. There are two reasons why something is crimped: cost and if something is going to be subject to extreme vibration - NOT for reduced resistance. Solder cracks when vibrated for a long time. There's also a tin whisker issue but that's out of scope here.

If a DIY guy is going to be doing crimps on small wires I can guarantee you're going to do it wrong. Hell - at home when I crimp small wires I KNOW I'm doing it wrong but I dont have the tools to do it right so I mitigate the situation:

A tinned wire into a tinned connector is pretty forgiving and you'll be good for pretty much forever. Most avionics is put together this way.
However a copper wire into a tinned connector is not a gas-tight connection and if its EVER going to be exposed to water or humid conditions eventually the contact will go high resistance. Might take a few years but it'll happen.

Two ways to mitigate this, one is soldering, if you solder it'll never go bad no matter what the conditions or how bad the crimp.

The other is to put a anti-oxidant grease (like NoAlox) IN the crimp by lightly coating the wire then crimping. The grease will squish out of the way of the contacts but remain around it and prevent water and oxygen from reaching and oxidizing the contact area.

At home I grease the wire and crimp. Big gauge wires I crimp with a hydraulic crimper.
 
I'm IN the avionics business and I have to disagree, a heck of a lot of it is soldered. There are two reasons why something is crimped: cost and if something is going to be subject to extreme vibration - NOT for reduced resistance. Solder cracks when vibrated for a long time. There's also a tin whisker issue but that's out of scope here.

If a DIY guy is going to be doing crimps on small wires I can guarantee you're going to do it wrong. Hell - at home when I crimp small wires I KNOW I'm doing it wrong but I dont have the tools to do it right so I mitigate the situation:

A tinned wire into a tinned connector is pretty forgiving and you'll be good for pretty much forever. Most avionics is put together this way.
However a copper wire into a tinned connector is not a gas-tight connection and if its EVER going to be exposed to water or humid conditions eventually the contact will go high resistance. Might take a few years but it'll happen.

Two ways to mitigate this, one is soldering, if you solder it'll never go bad no matter what the conditions or how bad the crimp.

The other is to put a anti-oxidant grease (like NoAlox) IN the crimp by lightly coating the wire then crimping. The grease will squish out of the way of the contacts but remain around it and prevent water and oxygen from reaching and oxidizing the contact area.

At home I grease the wire and crimp. Big gauge wires I crimp with a hydraulic crimper.
Thanks for posting (and reinforcing the direction I’m planning to go).

So if you were more proficient soldering connections than crimping them, would you use solder even for a higher-current connection (5A)? These are harness wires to a removable Active Balancer, so still very little vibration/movement (only when the balancer is attached or detached).
 
Are the lead wires being crimped to connectors or washers or is there a better way to form those connections?
I use ring terminals and crimp and shrink wrap an inch of the wire and terminal crimp. I also use the original length of the wire to have uniform resistance for all connections. I also use number labels on the wires. I am transitioning from a 2P16S pack to a 3P16S pack and using aluminum busbars which is why my preference for ring terminals.
 
I use ring terminals and crimp and shrink wrap an inch of the wire and terminal crimp. I also use the original length of the wire to have uniform resistance for all connections. I also use number labels on the wires. I am transitioning from a 2P16S pack to a 3P16S pack and using aluminum busbars which is why my preference for ring terminals.

The BMS should only take voltage readings when the balance circuit is inactive - this ensures there is no current flow and therefore the length of the wire is not important. Do they all work like this? I cannot say, but this is how it should work.

With all that said, it is not hard to keep the wire length reasonable matched. For a BMS with typical balance current of a few hundred mA, it is just not all that critical. The larger gauge the wires, the less critical it is as well.
 
I'm IN the avionics business and I have to disagree, a heck of a lot of it is soldered. There are two reasons why something is crimped: cost and if something is going to be subject to extreme vibration - NOT for reduced resistance. Solder cracks when vibrated for a long time. There's also a tin whisker issue but that's out of scope here.

If a DIY guy is going to be doing crimps on small wires I can guarantee you're going to do it wrong. Hell - at home when I crimp small wires I KNOW I'm doing it wrong but I dont have the tools to do it right so I mitigate the situation:

A tinned wire into a tinned connector is pretty forgiving and you'll be good for pretty much forever. Most avionics is put together this way.
However a copper wire into a tinned connector is not a gas-tight connection and if its EVER going to be exposed to water or humid conditions eventually the contact will go high resistance. Might take a few years but it'll happen.

Two ways to mitigate this, one is soldering, if you solder it'll never go bad no matter what the conditions or how bad the crimp.

The other is to put a anti-oxidant grease (like NoAlox) IN the crimp by lightly coating the wire then crimping. The grease will squish out of the way of the contacts but remain around it and prevent water and oxygen from reaching and oxidizing the contact area.

At home I grease the wire and crimp. Big gauge wires I crimp with a hydraulic crimper.

Curious what connections you are referring to - are you talking about terminal connections being soldered?

Tin whiskers are only an issue on PCB's - not ring terminals used on bus terminations. I am NOT an A&P, but I am a mixed discipline engineer that has worked in aerospace. Terminals were outside of my area, but I never saw one that was soldered. Of course all the control systems are soldered internally, but external wiring was not. I saw this from United Space Alliance with orbiter flight hardware to a company that makes motor systems for aerospace. Both applications were certainly in the critical, high-vibration, ultra-reliability, life-safety categories.

Mind you, I never really asked or was responsible for defining how connections were made - just my visual observations.
 
Thanks for posting (and reinforcing the direction I’m planning to go).

So if you were more proficient soldering connections than crimping them, would you use solder even for a higher-current connection (5A)? These are harness wires to a removable Active Balancer, so still very little vibration/movement (only when the balancer is attached or detached).
Soldering more than a half-dozen wires is a pain the butt so at home I crimp with my somewhat-decent set of crimpers, if all I had to crimp with was a set of pliers or needlenose I'd solder. 5A is nothing but since balance wires are dealing with millivolt level differences you want as low as a resistance as possible.

Having said that, just grease and crimp it, if you dont have a set just go to the hardware store and get yourself a $20 set of crimpers and you'll be golden.

When you crimp make sure you put the connectors split against the OUTSIDE of the crimp area, you DONT want the crimper dimple pressing down on the seam, you want the crimper dimple pressing on the solid side. Freaking nobody does this right and it drives me nuts.
 
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