bluetooth BMS?

themelon said:

ElectricGod said:

themelon said:

ElectricGod said:

This is what 46 7S boards looks like...arrived today.

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I could use 4 boxes like that...

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These BMS are quite small. 4 boxes would be like 2000 of them.

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Not full boxes, just the quantity contained within it. I should have been more specific. I have 84 12s and 24 6s Volt modules that I may want to put these on. That's 216 boards if I don't do any additional paralleling with them.

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My testing so far has shown that balancing works pretty well, but expect about .5 amps max on the 7S boards. This might create a problem if lots of cells are in parallel. On the 16/20S smart BMS, I've noticed that they can't keep up with charge current so I set the balance start voltage at 3.8 volts. I think this will help these 7S boards as well.

First post here just to share my successful BMS experience. I'm using the 14s 60a smart bluetooth BMS from AliExpress, link here.

I did calibrate all 14 cell groups with a meter that read to the thousands of a volt when I installed the smart BMS. If anyone needs to do this still, the PC software over the USB cable needs you to type in the voltage (in millivolts) that you measure with your meter. When you click "calibrate" it will calculate the offset needed. I still need to see if I can connect the PC software over Bluetooth as a serial device, since I sealed the battery pack, and not all settings are available with the android software.

My only issue is I need to get some experience with my creation to see if there is actually a problem with cell group 13, or if it just needs more cycles to be fully balanced. I did confirm the connection of all cells when I built the pack, and again confirmed all cells connected in group 13 and 14 when I installed the smart BMS. All 84 cells are new LG MJ1 cells, and I used them for a few cycles with a dumb BMS beforehand. Also for some reason cell group 12 seems to dip lower than the rest under load, but is fine with no load. All I'm wondering is when a battery pack gets empty, is it normal for the limiting factor to be one cell group suddenly 0.3 volts lower than the rest? At this point, I'll try watching it for a few cycles before opening the pack to test the capacity of all 6 group 12 cells. Here are pictures of my timeline:

1 - off throttle


2 - on throttle


3 - on throttle


4 - off throttle


5 - off throttle after resting for a while

first of all it's ok to see bigger differences in cell strings when they are empty than when they are fully charged. if 300mV is too much is hard to say w/o knowing the exact circumstances, but at 2.8V those cells have ZERO capacity left, they will drop to next to no voltage very easily.
as you said that all cells are new, you would NOT expect one of the strings to become out of balance this easily or have a higher resistance than the others. IF the pack is welded properly. the current distribution needs to be exactly the same for all cell strings to discharge evenly. if the serial connections are of different cross section they will discharge unevenly. maybe posts pics of the pack, and we will find some ways to improve it?!

here are two examples of a perfectly even current distribution, and one of a "not so optimal" one. guess which one is the one made by me, and which one is a replacement for bionx which i tried to improve. 8) (btw: in case someone wonders: ALL of these are brand new samsung 30q. i have to replace the heatshrink on some of them after i glued them together in the wrong order, that's why they are differently colored)

Thanks for the reply I hadn't heard about different serial connections causing uneven discharging before.

I made the pack myself with Vruzend v1.5 caps, and it is pretty unique, but I'm confident that I made all of the serial connections large enough though it was impossible for them to be all uniform. Cell groups 12-14 couldn't fit inside the bike frame, so they were wrapped with the motor controller and mounted on a rear rack, and those three have very uniform serial connections. On the plus side, those cells are much easier to access if I have to, and because I put their balance wires on a molex connector, I'm going to manually finish balancing group 13 tonight with alligator clip wires and a charger. The charge yesterday finished with group 13 only being a tenth of a volt lower than the rest, even though balancing started at 4.0 volts.

Charging:


Finished result. I'm going to put group 13 on an individual charger tonight to bring it up to 4.2 volts, and then see how it does in the future.


Cell groups 1-11. Those balance wires and the old BMS are gone now, and the new wires all run on the edge of the pack instead of crossing over bus bars.


Cell groups 12-14 and the motor controller. The 4-pin molex connector is for the balance wires, and the charger negative. I used the wrong gender, because it was 1am, and I just wanted to go for a test ride!


Most recent picture. I apologize if this is too many images for some people, but it's less annoying than clicking links.

it's hard to follow the connections, so i'm sorry to not be able to comment on it.
this by the way is what my pack looks like after charging, and almost fully discharged (at rest). it's a 15s10p 30q pack:

thundercamel said:

Thanks for the reply I hadn't heard about different serial connections causing uneven discharging before.

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Nice battery build!

Let things work out a few more charge cycles. It may be that the weak group is not really weak. It may be that you actually have a weak cell in there too. Smart BMS allow you to have so much better awareness of your pack status than dumb BMS. It is possible your dumb BMS was NOT doing it's job and you just didn't know it and now you do. Maybe you've had a problem with a weak set of cells all along? give things a few more charge/discharge cycles and see how it goes.


There's 2 things to think about in any pack. They have what I call a recovery voltage and a load voltage.

You will commonly see your cells sag to some lower voltage as you load them. I call this load voltage and it's the real voltage of the pack. Then take away the load and you will pack voltage rebound upwards several volts, this is recovery voltage and should be ignored as it isn't anything you can realistically use. Ideally, you want a battery pack that does NOT sag at all, or sags very little, like 1-2% at most. However, towards the bottom of your charge, you will see that sagging gets worse and worse. LEts say your pack fully charged is 48 volts and 36 volts when discharged. At 48 volts, you may see sagging to 47.6 volts. At 37 volts, you may see that under the same load that the pack sags to like 33 volts. This is pretty typical.

I'm sure you have looked at the last post by Izeman. That's what your pack ought to look like fully charged and fully discharged. Notice how similar his cell voltages are. This is what you want.

IMHO, A pack with a cell that runs down too quickly is only going to get worse and worse. IT is already weak and you need to load it just as much as the other cells around it. It won't hold up long term most likely. You have significantly reduced capacity becasue of a weak link in the chain. To extend the life of those weak cells, the entire pack has to be set up based on those weak cells. It can be used this way, but if you can fix it and eliminate the problem, that's the better solution.

There's a couple of things you can do to help yourself out, if you have weak cells.
1. set the start balance voltage lower. mine are set to 3.9 volts. This gives me more time to balance out the pack.
2. Set the LVC cut-off higher. You know some cells seem weaker than others. Set the BMS with those cells in mind.
3. If the other cells are something like 16Ah and the weak ones are 12ah, then realistically the pack can do 12Ah. Set the BMS to 12Ah.
4. Realistically you can go as low as 2.5 volts per cell on LION. I do NOT recommend this. In fact, I'd set LVC to 3 volts.

Thanks guys. Those are some really uniform voltages! Good to know that it's possible.

You're right, I'm so glad I switched to a smart BMS. I was surprised how much the voltage sagged under load, and this might be attributed to the fact that I'm pushing the Vruzend 1.5 caps to 4 amps per cell, once the efficiency of the motor controller is taken into account. I popped a 20 amp fuse when I first tested the bike, but have never blown a 25 amp fuse yet. The pack is 14s6p; 15s10p is huge!

I tried topping off the charge of group 13 last night, but the Zanflare C4 charger considers 4.15 volts to be full, which I can't blame them. My friend is bringing over his "highly configurable" charger tonight, so we can match the rest at 4.20v. I also was able to adjust my Vruzend charger from 58.8v down to 57.7v, so I need to make a post about that in the appropriate cycle life thread. Future charges should top out at 4.12 volts per cell, unless I'm planning to do a long trip ahead of time.

If there actually turns out to be a problem, I will be fixing the pack rather than using it in a degraded state.

Most of your issues can be fixed if you stop using that vruzend crap and just buy a cheap spot welder a few meters of nickel strip and some cell holders.

Possibly, but spot welding seemed too permanent to me, even if the nickel strips can be torn/drilled off and the cell ends filed down. If they actually prove to be a problem, I'll consider a more compact staggered spot welded arrangement, that might fit all 84 cells in the frame. So far, the performance has been great!

Didn't want to start this vruze end cap discussion but if you use new high quality cells spot welding is the best and most effective way to do it.
It's more rigid, smaller and can handle more current.

AND you DON'T want to solder to the batteries. The seal at the positive end will probably get overheated and damaged. At first you won't know there's a problem, but a few months down the road when the contents of the cells started evaporating away....then you will!

Spot welding 18650's is the best long term option that won't risk damage to the cells.

Those Vruzend caps can only handle 5A, so with 6P that pack can only handle 30A.

What sort of current are you pulling?

If some of the caps are not seated as well as others the the resistance may be higher, coupled with pulling more than 5A per cell this could result in uneven discharge.

Maybe set the balance voltage lower too so the BMS balances the pack before terminal voltage.

lionman said:

If some of the caps are not seated as well as others the the resistance may be higher, coupled with pulling more than 5A per cell this could result in uneven discharge.

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imho that's the main reason for the issues the OT sees. if only one or two cells is not fully seated in the end cap he will see those issues.
those endcaps are fine for small packs for low power usage. tbh i haven't tested them personally, but have some (not so good) experience with spring loaded cell holders.

thundercamel said:

Possibly, but spot welding seemed too permanent to me, even if the nickel strips can be torn/drilled off and the cell ends filed down. If they actually prove to be a problem, I'll consider a more compact staggered spot welded arrangement, that might fit all 84 cells in the frame. So far, the performance has been great!

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considering the problems visible in the bms i have to disagree about how well it is working.

the longer you drive with this the faster you are killing you cells.

izeman said:

lionman said:

If some of the caps are not seated as well as others the the resistance may be higher, coupled with pulling more than 5A per cell this could result in uneven discharge.

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imho that's the main reason for the issues the OT sees. if only one or two cells is not fully seated in the end cap he will see those issues.
those endcaps are fine for small packs for low power usage. tbh i haven't tested them personally, but have some (not so good) experience with spring loaded cell holders.

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Here's some 20S2P 18650 battery holders I built. There are 6 of them in parallel. Never had a single problem with them. I think they have something like 5000 miles of use. I made a seventh one just in case there was a problem, but they worked perfectly. Literally zero issues. If you expect those spring contacts to handle 10 amps, that's a bad idea. 3 or 4 amps and they do fine.




I've built several 18650 battery holders for different purposes and they have all served me well. This is jsut a few of them. If you are looking for the best cell packing possible, then they are not a good option. Spot welding a pack is the best option for maximizing space used. If you are looking for easily swapped out cells and no welding, they are perfect for that. Just plan ahead as always to keep within the limits of the spring contacts.

Six 6S2P sections. They can be connected together in any 6S increment up to 36S2P or as low as 6S12P.


12S2P


6S6P



Dual 6S1P

i know these holders, and sometimes work with them. and up to their limit (which varies based on quality) they work fine.
but i also saw them melt away. the contact surface is quite small. this leads to a high resistance -> a lot of heat.
those holders have the pro of giving EACH cell an individual clamping force. these vruze caps are pressed on, then screwed together. it can happen that this moves the holders a bit and loosens the spring load. this will lead to better or worse contact.
when it comes to batteries is like to follow the saying: "a lot helps a lot", or "the bigger, the better".

izeman said:

i know these holders, and sometimes work with them. and up to their limit (which varies based on quality) they work fine.
but i also saw them melt away. the contact surface is quite small. this leads to a high resistance -> a lot of heat.
those holders have the pro of giving EACH cell an individual clamping force. these vruze caps are pressed on, then screwed together. it can happen that this moves the holders a bit and loosens the spring load. this will lead to better or worse contact.
when it comes to batteries is like to follow the saying: "a lot helps a lot", or "the bigger, the better".

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I totally agree. For best reliability and current flow, battery holders are NOT the best answer. Spot welding a pack is the best answer.

I work in IT and I have access to loads of used computer equipment that gets recycled. Laptop battery packs fail fairly often and I had accumulated quite a few packs as a result. I took them apart and salvaged the good cells and recycled the bad or weak ones. I have a plastic bin with several hundred cells in it that I recovered. They are above and beyond the 240 that are in my scooter. I knew that there would be some amount of cell failure in salvaged cells so I didn't want to weld them into a pack. Battery holders was a good choice. Over that 5000 miles of use, I found I had 7 or 8 cells that failed. Swapping out a dead cell was simple. Pop out the dead one and pop in a good one. I ran for about 5000 miles on free and scavenged LION batteries. My total capacity was less than the 2600mah the cells were rated for...more like 2000mah per cell. That was pretty reasonable considering they are used cells recovered from bad laptop battery packs. I had something like 24Ah of capacity out of a battery pack that cost me less than $100 and nearly all of that was battery holder cost. Who hates free batteries?

If the cells don't have anything to keep them in the holder, they can slowly work loose and pop out. This is due to vibration from riding around. I always made sure the battery holders were pressed against something so the individual cells couldn't vibrate loose. In my EV that I used those 20S2P battery packs, they were packed together so this was not a problem. I ran them at what would amount to about 3 amps per cell or roughly 1C. That meant that each 2P pack would typically see about 6 amps. The typical maximum current draw at my watt meter rarely exceeded 50 amps and typical draw was more like 40 amps. The contacts and cells never got warm. I checked from time to time after doing hard rides with lots of current draw and it was not an issue.

Anyway, this is a smart BMS thread...so lets get back on topic. LOL

After absorbing over an amp hour, I got group 13 pretty well matched with the rest. As I've only done one cycle so far with the smart BMS, I will just be keeping an eye on everything as I go from here, to see if there is actually any problems.

This setup is actually my wife's bike, and after experiencing how well it works, I can't help but assemble parts to convert my own bike.
The last piece of the puzzle that I've been waiting to become available are the Vruzend 2.0 caps. Hearing ElectricGod tell me that there should be less than a volt of sag under load, flippy's opinion, and the unknown wait time has got me looking into spot welding instead. If I notice big differences between the two bikes after that, I'll be able to rebuild her pack and possible fit all cells into the frame with a staggered/honeycomb setup. Future developments with me can be found at the build thread I just created, so that we can keep this thread focused on smart battery management systems

izeman said:

IMG_0312.JPG

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This is a very dangerous - but common - method to build batteries, without any holder.
Unfortunately, road vibrations will soon cause shortcircuits and falmes in such a battery.
Always use holders!

That said... I didn't read the whole thread, but I read some other ones, and I see there are some firmwares around, for BT BMS.
Is there any opensource firmware available for this BMS?

jumpjack said:

I see there are some firmwares around, for BT BMS.
Is there any opensource firmware available for this BMS?

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It is even worse, there are same type Bms's that only work with its own tier of software. Open source would be so nice, I already miss some settings.

jumpjack said:

izeman said:

IMG_0312.JPG

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This is a very dangerous - but common - method to build batteries, without any holder.
Unfortunately, road vibrations will soon cause shortcircuits and falmes in such a battery.
Always use holders!

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you may want to look at my battery build thread. i was only showing the nickle welding, not the finished battery. https://endless-sphere.com/forums/viewtopic.php?f=14&t=92611&p=1359481#p1354420

izeman said:

jumpjack said:

izeman said:

IMG_0312.JPG

Click to expand...

This is a very dangerous - but common - method to build batteries, without any holder.
Unfortunately, road vibrations will soon cause shortcircuits and falmes in such a battery.
Always use holders!

Click to expand...

you may want to look at my battery build thread. i was only showing the nickle welding, not the finished battery. https://endless-sphere.com/forums/viewtopic.php?f=14&t=92611&p=1359481#p1354420

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in a "road application", cells should NEVER touch one to the other: only a very thin foil of plastic separates alluminum bodies of adiacent cells, and it wears out a little at each vibration, until you get a short circuit, regardless of how many cool things & wrappings you use to cover the overall pack.
You need a bunch of these:

if you want to safe cells from touching each other then there are way more adequate solutions than this rubbish cell holders which really are only good for stationary application. the interconnecting nickle strips btw are a joke as well. not able to carry the current needed.
those cell holders add real flex to the whole battery, and put stress on welding spots.
there are triangular spacers that do a better job here. or you use some hot glue that does the very same job ...
i still don't know how you think that there is the slightest form of movement inside the battery, and that this could be enough to shear off enough material from two layers of heat shrink, two layers of kapton tape and hot glue to cause a short?!

izeman said:

if you want to safe cells from touching each other then there are way more adequate solutions than this rubbish cell holders which really are only good for stationary application.

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I really don't get why you say that. Did you ever try them, or have just seen them in picture?

the interconnecting nickle strips btw are a joke as well. not able to carry the current needed.

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It of course depends on application (i.e. current flow), just do calculation. Single strips are around 10x0.3mm (=3mm 2) in section and just 40mm long. Multicell strips can of course carry much more current.

And adding tin over a nickel surface (to reduce resistance) is much easier and safe than "unwelding" a spot-welded strip and replacing it with "something bigger".

those cell holders add real flex to the whole battery, and put stress on welding spots.

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Flex? It's true exactly the opposite: without any holder, nothing prevents the pack from stretching and bending and flexing, which results in rubbing of cells surface.

there are triangular spacers that do a better job here.

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As far as I know, they are triangular just to save some space. (Indeed I used them in one battery to save some mm)

or you use some hot glue that does the very same job ...

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Hot glue on a "device" highly sensitive to high temperatures is not good.

i still don't know how you think that there is the slightest form of movement inside the battery

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Try it. The longest the battery, the easiest is to change its shape by applying pressure on one side.

, and that this could be enough to shear off enough material from two layers of heat shrink, two layers of kapton tape and hot glue to cause a short?!

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Rubbing, rubbing, rubbing, continuous rubbing for years, and years, and years.
What could ever happen to a thin plastic foil between two metal cans?

i give up. let's talk about it in 2-3 years when i replace my battery. but this discussion is completely off-topic here, and i will stay on topic here.