The BMS lead is on top of a large SS washer - it’s not in direct contact with the bus bar. I think my point is that having SS in the middle doesn’t affect BMS readings to a large enough degree. That said I will do some testing once my battery is live again ... because I did notice differences in the DMM depending on where I measured (like the stud) ... it makes me curious what differences I’d see if the BMS was directly connected to the current carrying conductor or not. It will depend on how lazy I am ?
Best practice for connecting BMS sense leads?
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Bob B
Emperor Of Solar
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I agree that overall, a proper crimp connection is superior .... even to a proper solder connection .... but I disagree that most people will do a good job with a crimp. If you go down to Lowe's and buy terminals and a crimp tool, your odds of getting a good crimp connection are VERY low.
I have seen a lot of really crappy crimp connections that people have done with poor quality terminals and poor quality crimp tools.
When talking about a connection to a bus bar, it isn't possible to crimp directly to it .... so a terminal lug must be first connected to the wire ... and then a mechanical connection must be made to the bus bar. Adding that connection to a terminal with significant current flow can introduce problems also ...... So, there are a LOT of things that can go wrong.
I think the best way is probably to do a quality crimp to a terminal, drill a hole in the bus bar and bolt that terminal by itself.
If a person has a powerful soldering iron, and alternative would be to drill a small hole in the bus bar, insert the wire and bend over the other side .... then solder .... then secure the wire with a zip tie or better yet with large heat shrink that will accomplish strain releif.
Ampster
Renewable Energy Hobbyist
My thing is consistency, so if every BMS lead goes through the same number of SS washers then I guess there is no inconsistency introduced. Whatever the resistance of Stainless Steel it should be the same for every cell. Is there a reason you do not want to read voltage from the busbar?
i think you nailed it - consistency. I’ll be curious if the cell voltage graph is more stable if I get the leads as close to the terminal as possible.
my last design had a large diameter SS washer in an attempt to distribute the force more evenly from the bolt. I think I’m going to ditch that in my next design.
On the subject of soldering connections, how are you all connecting to your (non-wired) BMS?
I’ve seen recommendation to solder wires into the holes, but that is a solder job that goes beyond my competency.
For my first learner battery, I made a pair of large copper busbars/lugs and bolted them through the two holes on either side.
My newly-arrived 300A BMS has a pair of copper busbars with 4 through-holes on both sides of the board for both B+ and P- connections and I’m thinking of going with the same solution (but interested in how others have solved this problem).
I’ve seen recommendation to solder wires into the holes, but that is a solder job that goes beyond my competency.
For my first learner battery, I made a pair of large copper busbars/lugs and bolted them through the two holes on either side.
My newly-arrived 300A BMS has a pair of copper busbars with 4 through-holes on both sides of the board for both B+ and P- connections and I’m thinking of going with the same solution (but interested in how others have solved this problem).
My latest (couple of days ago) BMS leads crimped:
High quality tinned copper, much more likely to have the wire break than the connection. My Fluke shows zero resistance to several decimal places (same as when I short the leads). I use the same Klein strippers Will recommends, I just use the Wirefy crimper and connectors (already had them and an assortment of connectors). The glue filled heatshrink makes the mechanical connection stronger than the wire, I highly recommend. You may be correct that people have a problem crimping, but really, use the right tool and components and it is so easy and quick. It's been so long since I started crimping, I forget if it was hard to figure out. I know that everyone thinks soldering is easy and simple, but even professionals make bad solder joints on a regular basis, and a busbar is not what you want to learn on. The object is to get ACCURATE measurements (in the millivolt range) of the battery terminal. Not accurate to the middle of a busbar, but the battery terminal. Since the job of the battery cell is to transfer power to the busbar, the most accurate reading will be obtained when the ring terminal is placed on top of the busbar (don't want to interfere with the busbar). I know I'm being repetitive, but I do this (or did this) for a living. You learn a lot designing power supply test equipment and then running a few thousand a week through the production line and looking at the fallout.
I'm just trying to give accurate information and proper procedures. Yes, solder is good, but it is NOT better in this application, and has a much higher failure rate. This is REAL honest to god experience (and I made plenty of mistakes learning it).
Ampster
Renewable Energy Hobbyist
It is a trade off. Earlier or on another thread I described how I used larger diameter Nord Lock washers to get more area but they only had an outside diameter as big as the ring terminals. It may depend on the thickness of the busbars as well.
Dave (Boog)
New Member
I find it disconcerting that there’s so much discussion/disagreement about something that should be as simple as connecting BMS leads. Reading through each of the comments just made me more confused. But I figure if crimped ring connectors are good enough for Will P. It’s good enough for me.
I soldered mine. But I am going to do something different because I am using serrated flange nuts and I recently found they were not torqued down like I left them.
I am sure it would be better to torque down the nut to the busbar. Then mount the BMS terminals with another nut, or mount/solder directly to the busbar.
ArthurEld
Solar Wizard
Some questions I ask, I get answers that contradict each other so bad I end up dropping the subject and never knowing if any answer is correct.
Most people just do a careful job crimping on the connectors. Then check each wire from end to end for resistance.
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You should probably ask Dacien about that.So, just to add to my confusion . . .. . . what is the "best practice" for the SBMS0 leads which are uncrimpable and must be soldered? And my plan is to use braided busbars?
zzyzx
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He did tell me that the leads should be soldered. I'm interested in those that have an SBMS0 on how they would do it with a braided busbar, as most have used solid busbars. Also, soldering to any busbar makes disassembly / reconfiguration more difficult.
The sense wires are a ribbon cable that mate with a 12-pin connector on the SBMS0 and are a very small gauge wire. The 12-pin connector cannot support a larger gauge ribbon cable. There won't be enough wire for a good crimp using the supplied ribbon cable.
Maast
Compulsive Tinkerer
You can take a little piece of larger wire to fill up the space inside the crimp terminals barrel, or you can fold the wire a few times to fill up the space. But if its that big of a differential you're probably better off filling the space with additional wire then soldering it - unless its aluminum wire in which case you're stuck with filling the terminal barrel and crimping. If its aluminum wire you'll also need NoAlox or the equivalent.
Ampster
Renewable Energy Hobbyist
I think "best practice" is determined by the use case. The most number of batteries in use would be EVs by manufacturers like Tesla and OEM suppliers like LG Chem and a few others. From what I have seen the cells are sonically welded to busbars and BMS wires are also welded. The Nissan Leaf pack that I disassembled 4 years ago used bolts and screws.
I have been using ring terminals on BMSs for the past seven years and occasionally have had a bad crimp but rarely found that that is easily fixed. With small wires I double the wire as others have suggested. I place the ring terminal on top of the busbar nearest to the positive terminal. I use studs, Nordlock washer and flanged non serrated nut with no issues. Proper clamping and clean connections are the most important issue to me. That has been the most frequent reason for erratic readings. My packs seem to be in a somewhat variable state so I value the flexibility of connections held together by fasteners.
I have been using ring terminals on BMSs for the past seven years and occasionally have had a bad crimp but rarely found that that is easily fixed. With small wires I double the wire as others have suggested. I place the ring terminal on top of the busbar nearest to the positive terminal. I use studs, Nordlock washer and flanged non serrated nut with no issues. Proper clamping and clean connections are the most important issue to me. That has been the most frequent reason for erratic readings. My packs seem to be in a somewhat variable state so I value the flexibility of connections held together by fasteners.
zzyzx
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Yes, the flexibility is important to me as well as I bumble my way around the build. I will try increasing the "volume" of the wire, prior to crimping ring terminals. I already have the M6 grub screws, non-serrated flanged nuts and ability to clean all connection points. I will add Nord-Lock washers and "roll" the dice. THANKS.
You can of course get solder ring terminals. It's a ribbon cable though, one end is already crimped.
zzyzx
Apprentice Neanderthal
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Since my ribbon sense cable has 28awg wires, has anyone tried these ring terminals (part # 83-19333 20-18 gauge) on small gauge wire?
Crimp Solder & Seal Ring Terminals
My thinking is that after removing enough sense wire insulation, fold over the bare wire multiple times before inserting it into the heat shrink tube (toward the ring terminal) & then fold over enough insulated wire (behind the bare end) multiple times and insert at the back end of the heat shrink tube (to add some strain relief) before heat shrinking the translucent tube .
Crimp Solder & Seal Ring Terminals
My thinking is that after removing enough sense wire insulation, fold over the bare wire multiple times before inserting it into the heat shrink tube (toward the ring terminal) & then fold over enough insulated wire (behind the bare end) multiple times and insert at the back end of the heat shrink tube (to add some strain relief) before heat shrinking the translucent tube .
If the ring terminal is close to the right size, folding over the bare wire is OK. You shouldn't have to fold over the unstripped part of the wire. The heat shrink should close down far enough onto the wire. If not, the ring terminal isn't close enough to the right size.
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