NineBot Z10 BMS does not balance. Any solutions?

[matthijs]

Hey all! (My first post so lmk if theres something up)

Ive been driving my ninebot Z10 for almost 4k KMs now, but it seems that the BMS is most likely fried. Please look at the provided screenshot for proof. BMS#1 has a voltage difference of around ~0.5V across the cells. BMS#2 only of ~0.05V.

It seems that the BMS is correctly reading the cell voltage, and when charging it also works. It seems that purely the BMS balancing capacity is bad. I've heard people say that BMS systems that have communication with the controller are specifically a pain to replace. Is that also the case for the Z10?

However, as communication and reading and everything the BMS#1 reports seems correct, would there be a workaround available to still install a "Dumb" BMS in between the BMS#1 and battery? (assuming the BMS#1 isn't charging the battery through the balancing leads anymore). 

Id love to get expert knowledge on this, as it would be a shame to replace the complete battery whilst both batteries are still in okay shape...  

[alcatraz]

Cell group 14 with 4.274v is a problem. Groups 10, 7 and 9 are also "weak". 

Usually overcharge protection should kick in before any cell goes over 4.25v. If your pack is unable to protect itself it could be a problem to just keep riding. Even after adding a balancing board, you need to monitor the voltages regularly.

There are a few options. Either you add a separate balancing board like this.

 

Or you disconnect all the balancing leads and with like 20AWG wire piggyback the bad pack onto the good pack "sharing its bms". 

Both options come with their set of problems and it's not a fix and forget thing usually. The balancing board stops balancing at a 0.03v difference between adjacent groups. Starts at 0.1v. It means you won't end up with all cells equal but more or less in the ballpark. Which is really good but ideally you don't want to charge to 4.2v but to something a bit lower 4.1v perhaps. And that isn't the healthy bms going to like.

With the other option its hard to find a place for the -20 wire harness to cross from one pack to the other. It's important the wires don't get damaged, pulled on, or that water/contamination doesn't run along them into the packs.

[Planemo]

If it were me I would ride the pack down to about 60% then pull it out, take the casing lid off (not difficult) and charge each string to 4.2v. Replace everything, ride/charge a few times and then see how it goes. The fact that one string has gone over 4.25v is a concern and needs monitoring. If it happens again then yeah I think the BMS has failed in some way but you've got nothing to lose by doing a per-string 4.2v charge at this point just to see if everything gets pulled back to normal.

[Chriull]

2 hours ago, alcatraz said:

Usually overcharge protection should kick in before any cell goes over 4.25v. If your pack is unable to protect itself it could be a problem to just keep riding.

 

1 hour ago, Planemo said:

The fact that one string has gone over 4.25v is a concern and needs monitoring

4.25V is the threshold for many/some BMS without tolerance. Some BMS on eg aliexpress specify some 4.25V -0/+50mV.

Afair @RagingGrandpa measured ~4.28V as single cell group overvoltage threshold for one specific gotway bms.

10 hours ago, matthijs said:

but it seems that the BMS is most likely fried

One could check if the bms charge cut off works. By monitoring with a ahelly plug and EUCW or just manual monitoring?

As the Z10 has single cell group voltage monitoring balancing could be checked easily, too. Just check after 6/12/24h how much the cell groups above 4.2V got discharged.

Afair here was once a mod shown to stop the "vampire drain" of the Z10: https://forum.electricunicycle.org/topic/13737-a-one-electronic-component-solution-to-fix-the-drain-of-the-one-z-series/

1 hour ago, Planemo said:

charge each string to 4.2v.

Could be recommended! Balancing this ~0.5V difference would need some full balance cycles.

Edited April 9 by Chriull

[Planemo]

59 minutes ago, Chriull said:

Could be recommended! Balancing this ~0.5V difference would need some full balance cycles.

Not if you are doing each string?! You are effectively doing all the balancing by charging each string...!

[Chriull]

1 hour ago, Planemo said:

Not if you are doing each string?! You are effectively doing all the balancing by charging each string...!

Yes. With "balancing" i meant the normal passive balancing by the bms.

If one charges each cell group to 4.2V, as you propose the battery is perfectly balanced!

@matthijs should take care, that the second battery is then charged to an about similar level.

[Planemo]

1 hour ago, Chriull said:

@matthijs should take care, that the second battery is then charged to an about similar level.

Given all the strings are accessible once the pack is open I didn't mean to advise anything other than doing the whole lot. 

[matthijs]

Hey all, thank you for the suggestions!

@alcatraz Thank you for the suggestion. Both options are relevant here I think. The piggyback solution may be a bit more even, as it would mean that I won't have to find a place to store the somewhat bulky second BMS. I don't think crossing over is such a problem, as both batteries shore the same housing in the Z10. Ill look into this to see if this is a solution!

@Planemo Charging the whole pack back to 4.2 to equalise the differences and see from there is probably going to be my next step! I have a 10s bms somewhere to charge most of the sells simultaneously, and then do the last 4 one-by-one. 

 

@Chriull I have EUCW installed, and keep an eye on the cells that way yes. Doesnt seem they get discharged other than the vampire drain indeed. 

 

Will report back once I've found the time to do all this. Thank you for all the advice! Ride safe!

[Planemo]

3 hours ago, matthijs said:

The piggyback solution may be a bit more even

Have you thought about where to mount a piggyback BMS? I cant think where, given the space constraints of the Z10 which is nearly as dense as lead 

[matthijs]

12 hours ago, Planemo said:

Have you thought about where to mount a piggyback BMS? I cant think where, given the space constraints of the Z10 which is nearly as dense as lead 

True. I know there's some empty space on the sides and bottom, but that will be the biggest challenge yes. Will open it up and see what the possibilities are!

[woke rider]

You could try installing the alternative firmware created by @MRN76. I'm not sure if it includes a different form of bms management that enables more power output for longer, but it might.

Or, you could try to install the Z8 or Z6 firmware then ride down the battery and recharge it, then switch back to factory Z10 firmware.

Edited April 10 by woke rider
another idea

[alcatraz]

The balancing board is quite small. Like 7mm thick and like a credit card in shape.

Still it could be tricky to fit, but one needs to have a look first.

[matthijs]

Hey all! I have opened to Battery to see if there's any space for the piggy-back BMS system. Right now I am charging all cells of the pack to 3.75v, and then charging to full to see if the BMS does something or not. I don't expect it to as it seems to have not balanced for quite some cycles resulting in the massive difference seen in the screenshot posted earlier.

Therefore, I am also looking at this piggyback BMS(link), which offers both a 14s and 7s BMS version. Regarding the dimensions, I am not sure if the 14s BMS will fit. The battery has space for a 100mmX37mm BMS (14s), however, the BMS sticks so low that the thickness (only 2/3mm max. at lowest point) might become a problem (which isn't stated on the page). 

Option 2 is to install 2x 7s BMS. Their size of 26x47mm will most definitely fit the space in the Battery. Assuming that the protection circuitry of the existing BMS still works during normal use (not charging, voltage/temp/load cutoff during normal use), could a 2x7s BMS be installed? 

Edited Sunday at 02:09 PM by matthijs

[litewave]

On 4/28/2024 at 10:04 AM, matthijs said:

Hey all! I have opened to Battery to see if there's any space for the piggy-back BMS system. Right now I am charging all cells of the pack to 3.75v, and then charging to full to see if the BMS does something or not. I don't expect it to as it seems to have not balanced for quite some cycles resulting in the massive difference seen in the screenshot posted earlier.

Therefore, I am also looking at this piggyback BMS(link), which offers both a 14s and 7s BMS version. Regarding the dimensions, I am not sure if the 14s BMS will fit. The battery has space for a 100mmX37mm BMS (14s), however, the BMS sticks so low that the thickness (only 2/3mm max. at lowest point) might become a problem (which isn't stated on the page). 

Option 2 is to install 2x 7s BMS. Their size of 26x47mm will most definitely fit the space in the Battery. Assuming that the protection circuitry of the existing BMS still works during normal use (not charging, voltage/temp/load cutoff during normal use), could a 2x7s BMS be installed? 

Not sure the right forum members or engineers will see or respond to your request since the Z10 are "old".

Maybe @RagingGrandpa can help, or he can recommend someone else who will know?

[matthijs]

Hope so! Or maybe @alcatraz, @woke rider, @Planemo, or @Chriull would have an idea? 

[alcatraz]

I'm pretty sure you can't connect multiple smaller bms together and make a larger one. So if you can't fit it inside you need an external one. 

Have a look at begode bms boards. They're super thin but large. 

The balancing board pictured above has a problem that it only balances between two adjacent cells. It means that if the voltage difference of adjacent cells isn't large enough (below 0.03v) it won't balance.

However unlikely although not impossible, you might end up with cell group 1 at 4.4v and cell group 16 at 4.0v and the balancing board wouldn't balance that because they're less than 0.03v inbetween. 4.40 - 4.37 - 4.34 - 4.31 - 4.28 - 4.25 - 4.22 - 4.19 - 4.16 - 4.13 - 4.10 - 4.07 - 4.04 - 4.01 - 3.98

That's why if you go that route it's best to not charge fully but to maybe 4.1v ~ 90%, and that won't jive well with the other bms.

So I'd recommend you to try: 

1. repairing the original bms

2. replacing the original bms

3. piggybacking one pack onto the other with balance wires.

[alcatraz]

If you get a begode bms it won't mate well with the other pack bms. They need to shut off charging together or you'll lose overvoltage protection = fire.

You'd need two of them. One for each pack. 

Edited Tuesday at 09:52 AM by alcatraz

[Chriull]

1 hour ago, matthijs said:

Hope so! Or maybe @alcatraz, @woke rider, @Planemo, or @Chriull would have an idea? 

I'd still check first if the original bms is ok and just cells a bit "strangely charged" first...

 

On 4/9/2024 at 9:22 AM, Chriull said:

As the Z10 has single cell group voltage monitoring balancing could be checked easily, too. Just check after 6/12/24h how much the cell groups above 4.2V got discharged.

 

[matthijs]

So, I balanced the cells and charged it to 100%. Happy to say that it doesn't over-charge anymore. However, I'm not convinced yet that the BMS is working. It did in fact look like it was somewhat balanced, but this could also be a fluke right? Especially after 24h, the voltage differences between the cells should've been minimal. 

Regarding the 2x7s BMS @alcatraz, wouldn't it be possible to connect 1 BMS to cells 1-7, and 1 BMS to cells 8-14? That way the BMS thinks it's just charging a 7s instead of a 14s. I'd prefer not to extract the BMS as it is completely glued and stuck, and I do not want to risk damaging it even further. I sadly haven't been able to find a replacement BMS for the Z10. Connecting it to the other BMS is a very realistic option in that case. The 2nd battery still provides power even if its balance leads are not connected anymore? Or would it be better to connect it to both BMS's.

[alcatraz]

Nice chart. After 24h you've got a delta of 0.07v which isn't bad. I'm surprised no cell went past 4.15v. Is the charger dialed down? The other pack's bms might not like it.

If we exclude groups 1,8,11 you have a delta of only ~0.04v. So three groups charge slower and need help. Or their bleed resistors are stuck in ON position.

It's possible the bms is causing the imbalance too. Like a malfunctioning bms can maybe activate bleed resistors at the wrong time. Disconnecting it could reduce the imbalance. Repairing it would be ideal. Like you ride on the healthy pack (slowly) for a few weeks while a repair shop has a look at the other pack.

7 hours ago, matthijs said:

The 2nd battery still provides power even if its balance leads are not connected anymore? Or would it be better to connect it to both BMS's.

For euc use the bms usually can't disable output. It would cause unnecessary faceplants. Braking is negative current. An output regulating circuit might get damaged by braking, so maybe best to avoid it.

Here's where you need to be careful. Two packs with outputs connected together need to communicate to not overcharge. That's why it isn't easy to diy ONE of the bms in a two pack system. It means you could overcharge one of them through the output. They both need to be able to disable charging as a group, individual charge disable doesn't work. (The healthy pack will overcharge the pack signaling charge cutoff.)

If you definitely don't want to mess around with bms you have to connect the packs together with thicker balance wires. Snip all the existing tiny balance wires on the bad pack. Leave the output+charging.

I'm not an electrical engineer. I'm just an enthusiast. But, in my mind this will restore normal operation.

Edited yesterday at 01:38 AM by alcatraz

[alcatraz]

If repair is out of the question, then...

To me it makes sense to ride on a single bms. If that fails one day it's easier to install a third party bms (much more advanced and reliable) once the two packs are already connected together.

Just make sure you don't have visible damage on the cells or nickel strips. If there's corrosion, cell impact damage, burn marks, those cells need replacing.

Edited yesterday at 01:44 AM by alcatraz