Faulty charge circuit?

[Mantraguy]

I've had my Gotway Tesla V2 (1500Wh version) for about 6 weeks now and did a ride a few days ago of 85km (just over 50mi). I charged the battery to about 80% after that, with the idea of topping up just before my next ride. When the time came, I made the mistake of plugging the charger into the EUC before plugging it into the wall, and I got some arcing from the batteries supplying power to the charger. I immediate unplugged it from the EUC, plugged the charger into the wall, and tried again but the EUC is not taking any charge. I can still measure voltage (about 78V) at the charge pins on the EUC, and I've tried a few chargers (all output to 84v without load). So there is power at the charging pins of the EUC (as well as the chargers), but there seems to be a problem with the charge circuit inside the wheel.

If I power up the charger and plug into the wheel, I only get green LEDs on the charger. If I leave it connected to the EUC but unplug the charger from the wall, the LEDs go out. I did a short ride today and the battery is ok (shows at 70%, or about 78V), but won't accept any charge. Is there a fuse or anything else I can check in the wheel? Can anyone tell me how the EUC handles charging - that is, am I likely dealing with a problem with the main board or with the BMS etc.? I appreciate any ideas...

 

[Chriull]

10 minutes ago, Mantraguy said:

Is there a fuse or anything else I can check in the wheel?

No. Neither gotway nor any other manufacturer has fuses at the charge side.

Most have reverse polarity and short circuit protection at the charge input. 

A big uncharged capacitor at the output site of the charger could put quite some burden on a unprotected charge input of a wheel.

I don't have any idea if gptway has protection circuitry by now - with older wheels they had none.

17 minutes ago, Mantraguy said:

that is, am I likely dealing with a problem with the main board or with the BMS etc.?

The BMS - the PCB together with the li ion cells in the shrink wrap.

18 minutes ago, Mantraguy said:

I appreciate any ideas...

Visually inspect all the wirings and connectors from the charge plug to the battery - we had many reports here of very bad solder spots!

The human brain is great in finding reasons why something happened - maybe it had nothing to do with your not plugging in the charger to mains but only some wire came off inside the wheel by accident...

But it seems your wheel is still in warranty? So i'd recommend contacting your reseller for exchange/repair/refund!

Bad design should not be consumers liability. At least not in western countries which normally have great consumer protection laws!

[Mantraguy]

Ok thanks. I sent the seller a message; guess I'll start having a look as I await a reply...

[Mantraguy]

Ok so I've got everything apart. The battery packs are slightly confusing; there are two packs (one each side) that appear to each be 10s4p (I had expected two 20s2p packs) and they each have a BMS.  They are connected to each other via an XT60 connector (big yellow thing) as well as an 11 pin connector for balancing. One pack (I'll call it the master) then has another XT60 to the main board, as well as the charge lead. The other (slave) pack only has connectors to join to the master (XT60 and balance). I've tested cell voltages on the balance connectors; everything's nicely balanced. The charge port shows about 0.3v lower than the discharge lead when everything's connected; seems about right. I have continuity from the pins on the front of the EUC to the charge lead, and appropriate voltage when connected. Strangely, the slave pack shows about 39v, while the corresponding connector on the master pack shows -8.5v. When disconnected, the lead that goes to the main board shows about 30.5v. I can't write wrap my head around that yet; perhaps it has something to do with a bad BMS? 

Anyway, I wonder if i were to connect to the packs together, and charge through the main discharge XT60 (disconnected from the main board) since it won't charge through the charge lead. I'd be temporarily bypassing the charge connector on the main pack's BMS that way - would it still control the charge rates properly and allow the cells to balance? I could give it a quick try under careful supervision. It might allow me to ride a bit until I can find a replacement BMS or work out another solution with the seller. Then again, it may not charge that way either. Am I overlooking any disadvantages of trying to charge through the discharge connector? Once again, I welcome any ideas or feedback! 

 

Edited January 7, 2021 by Mantraguy

[Chriull]

11 hours ago, Mantraguy said:

Gotway Tesla V2 (1500Wh version)

 

1 hour ago, Mantraguy said:

there are two packs (one each side) that appear to each be 10s4p

Ah - these were afair the custom versions from one/some aliexpress sellers?

1 hour ago, Mantraguy said:

Strangely, the slave pack shows about 39v, while the corresponding connector on the master pack shows -8.5v.

The "same two wires" on one side of the connector show 39V, on the other side -8.5V? That would indicate a bad connector!

1 hour ago, Mantraguy said:

When disconnected, the lead that goes to the main board shows about 30.5v. I can't write wrap my head around that yet; perhaps it has something to do with a bad BMS? 

... yes, sounds a bit confusing...

1 hour ago, Mantraguy said:

Anyway, I wonder if i were to connect to the packs together, and charge through the main discharge XT60 (disconnected from the main board) since it won't charge through the charge lead. I'd be temporarily bypassing the charge connector on the main pack's BMS that way - would it still control the charge rates properly and allow the cells to balance?

It would charge with "proper" rates and balance but not protect single cell groups against overvoltage anymore! ... if all the needed internal connections are still ok..

Imho there is no reason to risk some 40 li ion cells going wild by trying this - better to solve the problem properly!

 

[Chriull]

31 minutes ago, Chriull said:

Ah - these were afair the custom versions from one/some aliexpress sellers?

Just did some history search.

When GW started with 21700 there were some serious problems - battery packs were replaced. Afair the real reasons were never stated? Just they changed the cell manufacturer?

https://forum.electricunicycle.org/topic/18494-warning-gotway-nikola-100v-1800wh-battery-fire/

Lateron in this topic msx and monster with custom battery packs from aliexpres reselleres are mentioned, which have such "split battery packs". With one battery teardown of the awfull building quality on youtube:

https://forum.electricunicycle.org/topic/18494-warning-gotway-nikola-100v-1800wh-battery-fire/?do=findComment&comment=322601

[Mantraguy]

Yes, they changed to the LG M50T batteries, which is what I have. It seems apparent to me that there's a problem with the BMS on the main pack, based on my measurements etc. I have a reasonable amount of hobby-level experience with electronics; is swapping out the BMS a project that should be reasonable easy to complete, assuming they want to send me a new one? Doesn't look like a big deal, so long as you're extra careful about not shorting anything out with tools etc.!

[RagingGrandpa]

In my experience with Gotway BMS, overcurrent on the recharging port (such as large inrush to an AC charger that is not yet connected to AC) will cause the charging input to be disabled, with a latching behavior until the charging port is open-circuit again.

Your symptoms are abnormal- it does not seem to be resetting when open-circuit.

Good that you've already checked continuity. I agree that the problem is likely inside the pack.

39 minutes ago, Mantraguy said:

is swapping out the BMS a project that should be reasonable easy to complete

No. It requires equipment for both heavy soldering and light soldering, and involves soldering live connections. Extra care must be taken, beyond typical hobby work.

If you've built a pack before, you're familiar with this already. If you haven't, EUC is not a good place to 'teach yourself' - mistakes are likely to result in sudden failures of the pack while riding, which implies crashing immediately.

Please post pics of the BMS PCB!

[Mantraguy]

3 hours ago, RagingGrandpa said:

In my experience with Gotway BMS, overcurrent on the recharging port (such as large inrush to an AC charger that is not yet connected to AC) will cause the charging input to be disabled, with a latching behavior until the charging port is open-circuit again.

Your symptoms are abnormal- it does not seem to be resetting when open-circuit.

Good that you've already checked continuity. I agree that the problem is likely inside the pack.

No. It requires equipment for both heavy soldering and light soldering, and involves soldering live connections. Extra care must be taken, beyond typical hobby work.

If you've built a pack before, you're familiar with this already. If you haven't, EUC is not a good place to 'teach yourself' - mistakes are likely to result in sudden failures of the pack while riding, which implies crashing immediately.

Please post pics of the BMS PCB!

I hadn't heard of that disabling / resetting feature before - thanks for the info. Yes, if that is accurate, it seems not to have happened in my case. 

I have built smaller (3s and similar) sized packs before, using A123 cells (LiFePO4) for RC models, so I have some experience with live connections. At least enough to know I'd have to be very careful! I doubt the seller will volunteer a new battery pack but I'll try at any rate. I agree that this would probably not be within the scope of most riders in terms of a DIY repair. 

I'll post photos of the BMS once I get that far. I haven't yet removed the heat shrink but am curious to see if there's any obvious (visible) damage. 

[alcatraz]

I just read the first post but it sounds to me the charger possibly isn't working. The light turns green when the output voltage reaches 84V which it shouldn't do at 80%.

If you can measure the 80% voltage on the terminal it sounds like the wheel could be ok. They are supposed to have a kind of shortcircuit protection. I wonder if that's been triggered and somehow become stuck. In that case, should the terminal really have the voltage?

[Mantraguy]

The chargers are definitely fine. I now seem to think it's a problem with how the BMS is reading the battery voltage. 

I charged the battery once, at 1.5A, through the discharge port (XT60 connector that usually goes to the main board), it charged and balanced as it should. Obviously this was intended as a short term solution so I could still ride occasionally. I took the wheel fully apart, including the battery heat shrink, and didn't see any obvious issues with the BMS, but it's covered with a black rubbery compound (some sort of insulator I suppose) so I couldn't see much. I tested voltages of each cell and they were all perfect (3.94v, which corresponded to the overall pack voltage; I had done a ride so they weren't fully charged at this time).

I reassembled everything and plugged in the charger, and was surprised to see that it was taking a charge now! It charged to close to 84 volts, but the charger went instantly green from 1.5A so there was no CV phase and subsequent gradual reduction in charge current (I have a charge doctor and can see the charge graphs). Battery voltage now showed about 83.7V.

Before I opened up the wheel, the batteries would not charge through the charge port, and the charge doctor showed full voltage (84.2 usually) when connected to both the wheel and charger. Normally it would show battery voltage in this case, so I figured the BMS was somehow reading input voltage instead of actual battery voltage, and therefore not taking any current. 

After I disassembled and reassembled the wheel, for some reason the wheel would now accept a charge, but it seems that the BMS reads the voltage slightly higher than what it actually is. When the wheel reached 84v, it shut charging off completely and did not take the last bit of balancing current. I can't see how I could have changed anything; I didn't mess with the battery pack much besides removing heat shrink, having a careful look at things and testing cell voltages. I'm still stumped! The BMS is really small (roughly 1.5" wide x 2.5" long) so replacing it would be tricky; the balance leads are super close together. The one BMS handles both packs - I don't know if that's normal or not. I may be able to do it but I'd have to disconnect the balance leads from each cell tab first (in case I shorted at the BMS), then re-solder at the tabs once the leads were attached to the BMS. That would take a while! 

Edited January 12, 2021 by Mantraguy

[RagingGrandpa]

On 1/6/2021 at 4:09 PM, Mantraguy said:

Gotway Tesla V2 (1500Wh version)

On 9/10/2020 at 12:22 PM, Mantraguy said:

Oh, that's the 'wheel riders store' custom battery model, not a Gotway-produced battery pack like Tesla V3...

The BMS is this one:

On 10/25/2020 at 6:52 AM, smallexis said:

 

Next I would recommend charging thru the recharging port until the BMS interrupts charging again (near 84V as you said), and then manually measuring all 20 cell voltages as quickly as possible.

If there are no cells higher than 4.21V, I think your BMS is defective and should be replaced.
(The BMS does not care about the total pack voltage- it's monitoring individual cells only)

Like you said, replacing it would be very delicate, because you have to solder 21 new balancing leads while they are live. Even if you mask the nearby pads with tape, just a small mistake will cause a short and burn wires (or worse). A better way might be to desolder the leads from the cells, then connect the board, then resolder to cells...
 

2 hours ago, Mantraguy said:

I have a charge doctor and can see the charge graphs

You're much better equipped to troubleshoot this than the average owner!

Please keep us posted...

Edited January 12, 2021 by RagingGrandpa

[Mantraguy]

Thanks @RagingGrandpa - yes, I had thought that if I do have to de-solder the leads to the BMS and replace them all, that would be my plan - to first desolder the leads from the cells, then replace them after finishing up the leads and the BMS end. That would minimize the chances of shorting anything, and I'd be able to double-check for shorts with an ohm-meter before reconnecting the cells. Thankfully, because each pack is actually a 10S4P, it means I'd only have to desolder 10 cells since the other pack is connected with an 11-pin connector. So, if that's disconnected, those leads are obviously no longer live.

Yes, the cells seem to be fully balanced both at capacity as well as a slight state of discharge. I can actually get the pack to charge at CV (and therefore seems to balance, at least as far as I can measure by looking at individual cell voltages after the fact) if I charge through the discharge lead instead of the charge lead, so I think that 'part' of the BMS is working ok. It's possible that the cells are all just charging nicely at the same rate anyway though, even if the BMS isn't balancing. I don't think there's any way to know 100% if the BMS is actually balancing cells - does that make sense?

The photo you included of the BMS looks very similar to mine... but not quite identical. On mine, the leads to the cells on that pack are on one side, while the leads to the other pack are at the narrow end of the BMS. So, looks to be a slight design change but very similar otherwise. It does have a black rubbery compound 'painted' on that I'd have to carefully remove (it comes off one bit at a time with my fingernail) and then re-cover with something afterwards (maybe silicone conformal spray or even hot-glue).

For now, I'm waiting to see if Wheel Riders Store will send me a replacement pack and/or BMS. They've been very helpful so far, but I'll reserve judgement until this is all take care of.

 

 

[Chriull]

25 minutes ago, Mantraguy said:

For now, I'm waiting to see if Wheel Riders Store will send me a replacement pack and/or BMS. They've been very helpful so far, but I'll reserve judgement until this is all take care of.

Be carefull and inspect the batteries well - at least these 3rd party batteries were extremely dangerous 

 

[Mantraguy]

So this saga continues... The seller agreed with my diagnosis that the BMS was faulty and have sent a replacement (which took 53 days to arrive - argh)! In the meantime, a couple of cell groups in the battery have gone out of balance - one group so badly that the battery will not charge. Most cell groups (18 of the 20) read between 4.11 and 4.13v so they're good and reasonably well balanced. One cell group is dead (0.5v) so I'll be replacing that group (they are sending replacement cells), and a second group is at 3.94v. Still good but not balanced with the other groups in the pack.

Before I replace the bad cells, I plan to charge them each to 4.12v so they are balanced with the rest of the groups. Should I do anything with the cells in the 3.94v group before I connect the new BMS? I have read that the BMS works best as a maintenance system to keep groups in balance, but is not necessarily designed to provide initial balancing when first connected. Any suggestions? I don't want to have to disconnect the 3.94v group from the pack, charge to 4.12v, then reconnect if there's a better way... 

Also, is there a recommended order in which to attach the leads to the new BMS? ie main leads (charge / discharge) first, then individual cell groups starting at #1? I only have to connect the first 10 groups (which are already balanced); the unbalanced groups are both in the second pack, which will all connect to the BMS at the same time by way of a single 11 pin connector. There is only one BMS which handles both packs together.

Edited March 19, 2021 by Mantraguy

[Chriull]

4 hours ago, Mantraguy said:

Before I replace the bad cells, I plan to charge them each to 4.12v so they are balanced with the rest of the groups. Should I do anything with the cells in the 3.94v group before I connect the new BMS?

Yes - charge them to 4.12V, too!

4 hours ago, Mantraguy said:

I have read that the BMS works best as a maintenance system to keep groups in balance, but is not necessarily designed to provide initial balancing when first connected.

+1!

4 hours ago, Mantraguy said:

I don't want to have to disconnect the 3.94v group from the pack, charge to 4.12v, then reconnect if there's a better way... 

One does not have to disconnect cell groups from the pack for charging - with a single cell charger they can be directly charged!

4 hours ago, Mantraguy said:

So this saga continues... The seller agreed with my diagnosis that the BMS was faulty and have sent a replacement (which took 53 days to arrive - argh)!

Maybe it was just the BMS in the beginning, but imho chances are high the pack was in no good condition from the beginning! 18650 cells do not die too fast!

4 hours ago, Mantraguy said:

One cell group is dead (0.5v)

So what i get from your story is, that it is also plausibly that you got a bad and _dangerous_ pack from the beginning, and the seller refuses replacement by discussions and delays. ?A quite normal aliexpress tactics as they have no obligation and cannot be forced to abything?. Just with obvious malfunction from the beginning one has chances, if one does not fall for false promises...

Maybe you want to do same name dropping, so future battery pack buyers have some reference! Although i assume strongly that about no aliexpress seller will replace a battery after a purchase was accepted!?

[Mantraguy]

OK, the 3.94v group has been charged up to 4.12 to match the others. Now all I have to do is replace the bad group, or find someone locally who is willing to do that for me (no luck so far).

16 hours ago, Chriull said:

Maybe it was just the BMS in the beginning, but imho chances are high the pack was in no good condition from the beginning! 18650 cells do not die too fast!

They're actually 21700 cells but I get your point. Something definitely changed with the BMS and the way it reported voltage when I made my mistake with the charger; I could also see it in the charge graphs so I knew something wasn't working the way it should have. I've put about 1360 km (850 miles) and 29 rides (partial charge/discharge cycles) on the wheel since that event, so I can't be altogether surprised that a group went badly out of balance. I'm actually a bit surprised that more groups didn't go bad. Maybe most of the BMS balance circuits were ok? I don't know enough about how they work to know if that's possible.

When the packs are 'fixed' and I get everything wrapped up again, I may devise a way to be able to quickly and easily check each group voltage so I can make sure the BMS is doing its job and also that the groups are reasonably balanced even in a discharged state. Not sure what that may look like yet... I've also seen Smart BMSes available for around US$50 or so; not sure if that's a worthwhile upgrade but it's another option for later. Then I could see group voltages in real time via bluetooth. :-o

16 hours ago, Chriull said:

So what i get from your story is, that it is also plausibly that you got a bad and _dangerous_ pack from the beginning, and the seller refuses replacement by discussions and delays. ?A quite normal aliexpress tactics as they have no obligation and cannot be forced to abything?. Just with obvious malfunction from the beginning one has chances, if one does not fall for false promises...

Maybe you want to do same name dropping, so future battery pack buyers have some reference! Although i assume strongly that about no aliexpress seller will replace a battery after a purchase was accepted!?

That may be a bit harsh, all things considered. I bought the wheel from Wheel Riders Store on AliExpress. They did offer repair for free, but shipping the packs back was cost (and time) prohibitive. It's not legal to ship damaged li-ion batteries by air, and surface would take 2-3 months each way. No thanks! In reality, I paid around US$1100 for the wheel, which was at least US$600 less than a similar wheel (in terms of range and specs) locally. Given my budget at the time, it wasn't a difficult choice. (FWIW, if I had to buy a replacement battery, it would be about US$550). I went into it knowing that I may not get much support if anything went wrong, and I don't regret the purchase. In the end, the seller did ship a replacement BMS and 12 new cells (worth over $100) so I have nothing to complain about with their support. They were willing to help the whole way through, and given the price difference, I wouldn't hesitate to buy from them again. My only real issue is how long it took to reach a solution, but most of that is from the time difference (they're just getting to work in the morning around the time I go to bed. Their day is over around the time I wake up). I'm sure at least part of the failure was my own fault anyway; had I stopped using the wheel when I noticed the BMS issues, I probably wouldn't have damaged any cells.

It has been, and continues to be, a learning experience. I'm surprised at the number of battery shops here and in neighbouring cities that won't do work on li-ion packs. Eventually, with the massive number of e-bikes hitting the streets, someone will have to start offering pack repairs! I've had to order a mini spot-welder and it looks like I'll be replacing the bad cells myself. I'm under no illusions about the dangers involved, but I work slowly, deliberately and carefully and think I can tackle this myself (if I have to, I guess)! I'll definitely have a fire extinguisher handy and will be working close to the garage door in case anything (or anyone) needs a rapid exit. Will be happy to write about it later if anyone's interested.

[Mantraguy]

I also want to publicly thank @Chriull for all the help he's given me in this and other threads! I really appreciate it!

[alcatraz]

Inspect the old cells for damage/corrosion.

Attach balance leads to all the cell groups of the repaired pack. You will likely need them later as replacing with new cells does not produce a pack with cell groups of equal characteristics. It might still go slightly out of balance.

Be careful working with the parts. Keep the terminals protected. It's easy to short something out and destroy it in an instant.

[Chriull]

On 3/20/2021 at 2:22 AM, Mantraguy said:

I also want to publicly thank @Chriull for all the help he's given me in this and other threads! I really appreciate it!

You're welcome!

And thanks for posting your proceedings in detail!

The Tesla V2 1500Wh is not a fully original GW but with 3rd party battery?! Originally they have 1020? Edit: was already discussed...

The charger plug in procedure is "known" quite from the beginning, but i could not remember any report of a dead BMS by the wrong sequence. Just sparking and afair a/?some? dead chargers...

 

Edited March 21, 2021 by Chriull

[Mantraguy]

I finally had a chance to replace the BMS today. Here are some photos for anyone interested. The painters tape is just to isolate the cell tabs and cleanly route the wires to the cell groups; that will be cleaned up when I properly tape the pack up and cover it in new heat shrink.

 

Top of the newly installed BMS:

 

I haven't yet scraped off the old silicone - not sure if it would really reveal anything anyway but I will if anyone would like to see what's under it. This is the top of the old BMS (the new one is exactly the same):

 

 

 

[Mantraguy]

Here's the bottom of the old BMS. The wires coming out of the top go to the pack that contains the BMS; the wires from underneath go to a connector that joins to the other pack.

Interestingly, it would appear that the BMS could handle up to 24S, since there are 4 'extra' pads at the end. I'm not sure why there's an unused pad in the middle; I'm sure removing the silicone from the top would probably shed some light on that.

 

[Chriull]

Just split @Fntmsgotwaymten3 V11 charging to 80% posts and answers off to 

to keep this topic "clean" - looking forward to @Mantraguy's reports!

[Mantraguy]

Last week I received the mini spot-welder I had ordered, and the nickel strips arrived a few days before a handful of replacement M50T cells I had couriered from California (I got impatient waiting for the cells to arrive from China). I spent a few hours charging each replacement cell up to 4.11v to match most of the other cells in the packs, then got to work disassembling the pack. I first checked the voltage of each cell group; with the exception of the 'dead' group, they were within 0.02v of each other (I did have to boost one group up from 3.94 to 4.12 but I had done that a few days previously).

Keep in mind that the battery overall is a 20S 4P configuration, made up of two 10S 4P packs. The pack in question has an unusual cell layout since it has to fit between the trolley handle (remember that this is a 'custom' 1500Wh battery, not the 1020Wh battery that came on most Telsa V2's). To allow for this, the manufacturers had soldered tabs from the cell groups to a printed circuit board, which had traces running around to connect different cell groups together. I had to de-solder a few cells from the board to gain access to the group in question, and then de-solder all of the tabs from 4 cell groups that were spot-welded together. The 'bad' cells were on the left side of the pack shown here:

I first had to remove the two cells on the far left (marked B9 and B10) to gain access to the larger group of cells that ran from the B3 - B6 line near the top, down to the bottom. However, when I de-soldered the tab at B7 (middle of the board), the tab fell onto the workbench - it had separated from the cells! I removed the 'bank' of cells (10 in total) to discover that it was actually only two cells of the 4 in each parallel group were bad. They were the two that were no longer connected to their parallel partners between B6 and B7 at the top of the board (due to the broken tab). I took a few minutes to wrap my head around how this could be possible, but it was good news - it meant I only had 2 bad cells, not one entire group of 4 cells that I had thought were bad.

Once the cell group was separated from the rest of the pack, I carefully cut the nickel strips holding those two cells to the rest of the group, and went to work spot-welding new strips to the replacement cells and then to the rest of the pack. It was fairly straightforward work; I made sure to cover all connections I wasn't immediately working with and kept a tidy workbench, ensuring all of the metal tools were kept well away from any battery cells! Here's a photo of the two bad cells in the group (far right cells) after it had been removed from the rest of the pack.

The spot-welding was fairly easy; I doubled up on my nickel strips since I was only using 8mm wide x 0.1mm thick strips; the stock ones were 8mm x 0.2mm. I did at least 3 welds per tab (the stock ones used only 2) and I also welded my 2 strips together between cells just to make sure the connections were solid. I added tabs as required to go up to the circuit board (including the one that had fallen off) and after everything was done, I double and triple-checked all my cell group voltages on the board as well as the balancing connector before wrapping the pack back up.

I reassembled the wheel, once again checking voltages when the two packs were connected and ensuring everything looked as it should. The wheel was re-assembled and powered up perfectly so I went out for a ride. It performed perfectly, just like new. I was happy and relieved. :-)

After a couple of rides and charges, I tool the side pods off the wheel again and re-checked the balancing of each cell group. At discharge (3.6v after a 60km ride), they were all within 0.02v of each other. When charged, I did have one group that was down slightly (4.11v while all other groups were 4.17-4.18v) which surprised me a bit.

I have since discovered (while using my Charge Doctor and logging graphs of the charge processes) that my 'new' 5A charger gradually decreases current output when it gets close to 84.0v, down to about 2.5A. At about 2.5A, it suddenly cuts off entirely and goes straight to 0A (not even 0.02 - 0.1 A that I'd expect to see during the balancing phase). As it turns out, my 1.5A charger does a similar thing - it cuts off at 0.5A, but after resting a while the overall pack voltage is still only about 83.4V and not a full 84.0v. I'm guessing this might be the reason the pack is not 100% balanced after a complete charge. The only charger that seems to gradually drop down to 0.02A continuously (and achieve a true 84.0V charge state) is the 'cheap' 5A charger that actually only outputs 2.5 - 3.0A, and a very erratic output at that. I'll throw a screen shot of the graph up here some time soon. But for now, I charge to cut-off with the fast (5.0A) charger, then swap it to the 'cheap' 2.5A charger to do the balancing. It seems odd to me that a charger would gradually drop to 2.5A from 5.0A, then just cut off entirely and avoid the balancing phase. Is this controlled by the charger itself when it reads a battery voltage close to 84V, or is the BMS controlling the current that it's accepting from the charger?

Overall I'm just happy to have my wheel back on the road (and that I didn't burn the house down trying to fix it).

 

 

 

 

 

 

 

Edited April 5, 2021 by Mantraguy

[Chriull]

8 hours ago, Mantraguy said:

I did have to boost one group up from 3.94 to 4.12 but I had done that a few days previously).

One group dropped from 4.12V to 3.94V within some days?

That would be >~15%S(tate)o(f)C(harge) within a couple of days! (1)

Normal should be some 5% within a month! (2)

So i'd guess this group is to be changed, too.  Could easily be the reason for 

8 hours ago, Mantraguy said:

... when it gets close to 84.0v, down to about 2.5A. At about 2.5A, it suddenly cuts off entirely and goes straight to 0A ... As it turns out, my 1.5A charger does a similar thing - it cuts off at 0.5A, but after resting a while the overall pack voltage is still only about 83.4V and not a full 84.0v

That is the BMS cutting off due to single cell (group) overvoltage (3) somewhere at about 4.25V.

Normally that's caused by a weaker cell group that reaches 4.2V earlier than the others and gets overcharged even with the activated balancing resistor.

8 hours ago, Mantraguy said:

After a couple of rides and charges, I tool the side pods off the wheel again and re-checked the balancing of each cell group. At discharge (3.6v after a 60km ride), they were all within 0.02v of each other.

That's great news and as perfect as it could be!

Afaik weaker cells also drop their voltage faster and by this can get dangerously low voltages at lower pack overall voltages! Great that yours are all within 0.02V! Although that's a bit of a strange outcome with all the other symptoms!

Quote

When charged, I did have one group that was down slightly (4.11v while all other groups were 4.17-4.18v) which surprised me a bit.

This were the voltages after a full charge after the BMS cut off?

The 4.11V was the same group that deopped to 3.94V before?

Should fit all together - replace the 3.94/4.11V group! Just that all cells discharge to 3.6V+/-0.02V is a bit strange...

As you've seen with the 1.5A charger you can charge more (downto 0.5A). If you then let the pack rest a bit and charge again you'll get further a bit more... Until after some cycles the cells should be all at full!

Maybe some very small discharges (short rides) inbetween the full charges.

Once they have equal voltages after cgarging the weak group should drop to lower voltages after riding. Once it degrades by too low voltages more it should trigger the overcharge alarm again. If you do not ride your pack down too much (% wise) so this cell group stays safely above ~3V it could be still fine usable some time!

8 hours ago, Mantraguy said:

the manufacturers had soldered tabs from the cell groups to a printed circuit board,

I'm not sure if i get tge right meaning of  this sentence: They did _solder_ a tab to a cell!? Or this means that the tabs, all welded to the cells are soldered to the pcb?

Thanks for your great documentation! And all the best for the supervision and unfortionately very possible future works on this pack

 

(1) "Capacity with full saturation" Table2 from https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries)

(2) https://batteryuniversity.com/learn/article/elevating_self_discharge#:~:text=The self-discharge of all,self-discharge of all systems.

(3)