Ebike Battery Pack Repair Help

[stypherfire]

Hello,

I am looking for some help diagnosing my battery pack which has been sitting around for a few years in my garage. I am looking to see if it is still usable as the bike is still basically brand new but doesn't charge. I wanna start off with the fact that my understanding is beginner hobbyist at best so please bare with me. I just want to get a second pair of eyes on the situation before i start ripping things apart:

Info:

1. Pack is 36V 10AH
2. There are 40 Cells

Equipment:

1. Klein Multi-meter
2. Soldering station (2 in 1 SMD Soldering Hot Air Rework Station + Stand 3 Nozzle 5 Tips 852d+ Iron Mark Ethan by Mark Ethan : Amazon.ca: Tools & Home Improvement)
3. LiitoKala Cell Charger (Lii-PD4)
4. Spot Welder (https://www.amazon.ca/SeeSii-Automa...mzn1.fos.b06bdbbe-20fd-4ebc-88cf-fa04f1ca0da8)
5. Hyperion eos 1420i net3 (Back from when i built my own Ebike (10 Years ago) which was sadly stolen T_T)

Problems:

When plugged into a 42V charger, the light on the charger alternates red to green non stop. I've tested the voltages of the entire pack by using a multiple to check the max voltage between the large red wire (see pack picture) on the bottom left side and the black wire on the bottom right. The voltage is consistent before and after the yellow fuse (around 29V) which i suspect means there is a dead group in the bunch. Here is where I am not too sure of how to proceed.

From my limited understanding, I am touch my multimeter probes on each side of every cell. The voltage on each cell reads 3.7V~ each except the first 4 cells on the bottom right side which read 0V each so I am assuming those 4 cells are full dead and need to be replaced.

Questions:

1: The cells are unlabeled and I can't tell the cell type to purchase new ones. Will my Liitokala tell me the cell type once it is inserted or should I rip off the paper covering on a cell to see if there is anything written on them?

2: As can be seen on the BMS image, there is a connector that connects all the battery packs to the BMS. Can I hook this up to my Hyperion Balancer to rebalance the cells or should i balance each cell individually in the Liitokala?

Thanks ahead of time and let me know if you need more info.

 

[amberwolf]

If all the cell groups except one read the same 3.7v, then that one group is just dead, probably drained by the BMS. It will have to be replaced; the cells have probably been damaged from being drained so far down, and even if they recharge (which may not be safe to do) they will never work the same again.

Was the 0V group the most negative group? The most negative group is the one with the big fat negative wire that goes to the BMS's B- pad. If so, then the group was probably just drained by powering the BMS, so there's likely nothing else wrong with the pack.

If the 0V group was anywhere else in the pack, it could be a failed cell in the group that drained the others down, or it could be a BMS balance channel that is stuck on. This can be tested after replacing the cells by monitoring voltages for a while after doing so once the pack has been rebalanced and charged. If the replaced cells stay charged and balanced with the rest, pack just sitting there, for a few days, then the BMS is not draining them. If this group drops, but not others, it's probably the BMS channel; replacing the BMS is simpler than repairing it but repair is possible if you can replace the tiny SMT transistor on that channel that connects the big shunt resistor to the cells during balancing.


I think all the LK tester can do is test the cell, I don't think there is any way for it to determine which specific cells they are.

If they are all at 3.7v, then they are Li-Ion of some type (not LFP or LiFePO4), and would probably charge to 4.2v full. Since it's a 10Ah pack, with 4P groups, they're 2.5Ah cells. So you could replace them with "any" same-capacity 3.7v nominal cells, but if you want to match them more closely for capabilities, checking the cell wrapper for markings may let you do that.

Two main options are slitting the cardboard on one of the cells you're going to remove anyway, to see under it, or lifting the spotwelded tabs off one end of the cell to then slip the cardboard tube off.

Even though you have a spot welder, I wouldn't cut the tabs if you can avoid it; leaving them intact and connected to the other cells makes installing replacements easier, since you can just sand or file the old welds off the bottom of the tabs, and reweld them to the new cells.

Or you can get replacement cells from some places with tabs already welded to them so you can then just solder the tabs together (or weld them together).

 

[stypherfire]

If all the cell groups except one read the same 3.7v, then that one group is just dead, probably drained by the BMS. It will have to be replaced; the cells

Hey Amber!

I took a few more readings. The image's top left cells are dead and need to be replaced. Gunna be difficult cause the paper casing is glued together pretty solidly. The Cells are 18650, so I'll find replacements with the same voltage/amp.

Questions:
1. Do I need to remove the 3.4V cells and rebalance them separately to 3.7v before returning them, or will the BMS balance do it when I charge the pack?
2. Is there any way to check the 1st set of cells that read 26V without unsoldering the main wires?

 

[amberwolf]

I don't understand where you are checking voltages if you have a set that reads 26v, but other sets read normal single-cell voltages.

To read single-cell voltages, you have to measure using the DC Volts setting of the meter, with the red meter lead on the positive of the group and the black meter lead on the negative of the group. (or vice-versa).

So...just do the same thing with that group as well, to read it's voltage.

If, in doing that, it reads 26v, then the cells are damaged and need to be replaced (though I don't see how they could charge that high, even if it is a "ghost" reading that would mean their internal resistance is so high, effectively open, that they are not going to allow the pack to pass current and drive the bike even if they can be charged correctly).


For any cell groups that read signfiicantly different votlages than the rest, you might want to just replace all of those at the same time, becuase it means they have significantly different characterstics (capacity, resistance, etc), and even if you rebalance them they are going to become unbalanced again, as they won't behave the same as other cells.

Without a cell tester and testing the individual cells (or groups) with it, it's tough to say if the rest of the cells are all ok, but if they are all identical votlages they probably are.


Since the group that reads 0.2v is not the most negative group, then it almost certainly means you have a BMS balancer channel stuck on that has drained that cell group.

It could be that one of the cells in the group failed internally instead, but it's more common for the BMS channel to fail. There are tests you can do on the BMS to find out using the resistance setting on the meter, but the BMS has to be disconnected from that channel's cells first (or current flow in the wrong places could damage your meter and won't show the right readings). It might have to be disconnected from all of the cells (including hte main positive and negative).

If the BMS channel is stuck on, it means that you should be able to set the meter to read ohms, and then put the red lead on the balance wire, and the black lead on each end of the large shunt resistor (that say 241 on them) for that specific channel. It should read the same, open circuit, for a working channel that is not stuck on. If it's stuck on, you'll read very low nearly zero on one end of the resistor, and about the resistance stated on the resistor at the other.

It should read the same as all the other channels if it's not broken, but it will read different from the rest if it's stuck on.

 

[docw009]

You drew it all out and you didn't see how you can measure all of the ten groups in this ten group battery? It's there at the white BMS connector. There are 11 pins. Measure the voltage of each adjacent pair, and you will have your info as to which cells are dead and what voltages the others are at.

If it turned out that one bank got drained by the BMS, and the other nine banks are at some reasonable voltage, then it might be worth replacing those cells, You will want to assess how all the cell voltages look in the battery first.

The BMS appears to have balance circuits, those being the ten 240 ohm resistors at the bottom, Who knows, maybe it will balance.

 

[stypherfire]

You drew it all out and you didn't see how you can measure all of the ten groups in this ten group battery? It's there at the white BMS connector. There are 11 pins. Measure the voltage of each adjacent pair, and you will have your info as to which cells are dead and what voltages the others are at.

I couldn't because the probes are too big and won't fit in the connector.

If, in doing that, it reads 26v, then the cells are damaged and need to be replaced (though I don't see how they could charge that high, even if it is a "ghost" reading that would mean their internal resistance is so high, effectively open, that they are not going to allow the pack to pass current and drive the bike even if they can be charged correctly).

Okay, I measured everything again. The whole pack reads 31.47V. See the video for the individual cell Vs.

The plan is to remove the two bad sets and 1 set at 3.4V. I'll replace the bad ones and charge the 3.4v up to 3.9v in line with the rest of the pack. After that, I will attempt to charge the pack after assembly. If that doesn't work, I will check the BMS after desoldering it via the resistors, as amber wolf mentioned.

 

[amberwolf]

I couldn't because the probes are too big and won't fit in the connector.

If you have any straight pins straight pin - Google Search

they will probably fit into the holes, or into the back of the connector to touch the contacts from that side.

Sometimes single solid strands of wire can be used for this; if you have any old networking cables laying around, they may be made of small-enough gauge of solid wire to do this.

Just have to be careful not to short any of them together, or to cover the exposed metal except for the two ends.

The plan is to remove the two bad sets and 1 set at 3.4V. I'll replace the bad ones and charge the 3.4v up to 3.9v in line with the rest of the pack.

No need to remove the low voltage group to charge it. Just don't connect the balance connector to the BMS while charging it, but otherwise you can charge it up with any single-cell charger to match the rest of the groups.

You can charge (or discharge) the new cell groups before connecting them to the pack the same way.

After that, I will attempt to charge the pack after assembly. If that doesn't work, I will check the BMS after desoldering it via the resistors, as amber wolf mentioned.

Not sure what you mean by "desoldering it via the resistors"? No soldering or desoldering of resistors should be needed to do any testing of BMS parts, just measuring at the points previously mentioned.

If the main battery positive also goes to the BMS and is soldered to it without a connector, then you might have to desolder *that*, but otherwise there won't be any current flow to interfere with anything. (it requires both positive and negative in the circuit to get that).

 

[stypherfire]

It could be that one of the cells in the group failed internally instead, but it's more common for the BMS channel to fail. There are tests you can do on the BMS to find out using the resistance setting on the meter, but the BMS has to be disconnected from that channel's cells first (or current flow in the wrong places could damage your meter and won't show the right readings). It might have to be disconnected from all of the cells (including hte main positive and negative).

If the BMS channel is stuck on, it means that you should be able to set the meter to read ohms, and then put the red lead on the balance wire, and the black lead on each end of the large shunt resistor (that say 241 on them) for that specific channel. It should read the same, open circuit, for a working channel that is not stuck on. If it's stuck on, you'll read very low nearly zero on one end of the resistor, and about the resistance stated on the resistor at the other.

It should read the same as all the other channels if it's not broken, but it will read different from the rest if it's stuck on.

Sorry Amberwolf, Bad sentence structure. I meant i would do the above instructions on testing the BMS as you outlined.

Questions:
Do i need to worry about the replacement cells C rate as long as they are higher than the existing ones?

I am however having problems with finding replacement cells that match up with the existing ones. The ones am currently spying are:
1. Lithium-Ion 18650 Cylindrical Battery (3.7V 2600mAh with flat top) (No info on brand or specs, unsure of quality)
2. 18650 Cell Lithium Ion Battery (3.7V 2600mAh) - X2 Robotics in Canada (No info on brand or specs, unsure of quality. 300 charge cycles seems pretty terrible)
3. Samsung ICR18650-26JM Battery, 3.7V/2600mAh Li-Ion Cell (This one looks good and is in the same city as me. Only problem is that it looks pretty expensive per cell. )

 

[amberwolf]

As long as the new cells are at least as good as the old ones presently are (not necessarily as good as they started out to be, since they'll have aged and degraded just sitting there), then the pack will work at least as well as the worst cell of the old ones.

Since even the old cells wont' all have identical characteristics anymore, then the usual advice of trying to make sure you get new cells that match the old ones doesnt' really apply--this pack will probably always have to be rebalanced after every charge/discharge cycle (depending on how deep it gets discharged).

 

[docw009]

That's the main reason the repair guys like to replace all the cells. If the BMS killed them, that's one thing. But if the cells just failed on their own, it's likely the start of the bathtub reliability curve for that battery, Other cells gonna die soon. The repaired battery comes back as a return.

But this is hobbyists at work. Do the replacement. Do it safely. Maybe it will work out,