Toying with a cheap hoverboard BMS

Telemachus · Oct 23, 2017

I inherited some hover board battery packs, so I decided to mess with them a little bit in preparation for building some larger batteries.

The first thing I noticed is that the overall voltage of the battery was only 24V (should be 36). I took it apart, and found that 2 parallel sets were drained all the way to zero.

*** First Question: Should a BMS allow you to detect a voltage on a battery when some of the cells are all the way depleted? Isn't it supposed to cut power off?

I created a 6S1P battery using the remaining good Samsung 18650-22P cells. I then attached the BMS that came with the battery back, but I only attached the B- Lead and the first 6 charging leads. I thought I would have to bridge the other 4 wires, but the voltage was registering at 22V so I just went with it.

I wrapped the battery up, and carefully watched it while it charged up to 25.2 Volts with a 6S charger.

After full charge, I checked the voltage of each cell. Most were around 4.15 volts, but there was one at 4.3 and one at 4.22.

I figured I'd see if the BMS would do its job, and bring these down, so I let it sit overnight.

This morning, all of the cells had dropped about 0.01-0.02, but the 4.3 was still at 4.29.

*** Second Question: How quickly should it be able to drop the cells to an appropriate 4.2 volts?

Addy · Oct 23, 2017

I bought 5 of those hoverboard packs so I could salvage the cells from them. I would think that the BMS should cut off if some of your cells really did drop that low. Your BMS may be broken.

There are at least 2 different BMS boards used in the packs I got. The more common one has spots on the back where you can short the pads to set it to work with anywhere between 8-12 cells. Not sure if it would work with 6 cells.

On top of that, these BMS boards don't seem to have any balancing circuit - they're missing the large resistors that would be used to dissipate some power and drain the cells to keep them balanced. You're going to have to balance the cells yourself or use a balancing charger next time.

Telemachus · Oct 23, 2017

I'm amazed these hover boards aren't catching fire every day!

This is the third pack I have found that keeps pushing power despite several cells being drained to zero. It was already obvious to me that they don't have a LVC. It was the balancing part that I wanted to test.

So if they are plugged into a 42V charger, that means some of the cells will be pushed to dangerously high levels...

So the alarmhookup guy selling them on ebay by the hundreds... they all have this potential??

Addy · Oct 23, 2017

Telemachus said:
So the alarmhookup guy selling them on ebay by the hundreds... they all have this potential??

That's where I got my packs of Samsung ICR-18650-22P. I haven't tested the BMS on these packs since I have been tearing the packs apart to get the cells out. I don't seem to have any problems with cells being overdischarged. Here are the pack voltages I measured when I received them:
37.05 V
36.53 V
36.66 V
35.87 V
36.66 V

I've taken apart 2 packs so far (not sure which ones in that list). The first was disappointing, the cells tested at ~1800mAh each so I suspect that pack was stored improperly or it's not new. The second pack was much better, with the cells testing around 2050 mAh each, which is the minimum value for these cells.

I tested these cells with a 500mA discharge with 3.0V cutoff. Their rated capacity is with a cutoff of 2.75V and discharge current of ~430mA (0.2C * 2150mAh typical), so the 2nd pack's cells seem pretty close to the specification. Hopefully the rest of the cells are in good shape as well.

Telemachus · Oct 23, 2017

Addy said:
I've taken apart 2 packs so far (not sure which ones in that list). The first was disappointing, the cells tested at ~1800mAh each so I suspect that pack was stored improperly or it's not new.

Could you enlighten me with more details about how you test their capacity?

Please let us know what you find with the others. I didn't suspect that they would vary so much. Maybe my plans to use these for my next e-bike battery aren't so great.

Addy · Oct 23, 2017

Telemachus said:
Could you enlighten me with more details about how you test their capacity?

Since I'm building a 10s8p battery with these, I have been grouping the cells together in 8p groups:

Once the cells are grouped together like that, I charged them at 4A per group, so that should be about 500mA per cell. They get charged to 4.2V with 1/10C charge termination, which should be 50mA per cell. After that I discharge them at 4A, so again roughly 500mA per cell. The discharge cutoff for my first tests was 3.3V. Under these conditions, the first two 8p groups measured at 14,213 mAh and 14,190 mAh. For later tests I did the same but discharged down to 3.00V per cell. Under these conditions, the 3rd and 4th groups measured at 16,243 mAh and 16,309 mAh, which is much more reasonable.

One strange thing I noticed with the first batch is that their date codes didn't make sense. They are marked:
ICR18650-22P
SAMSUNG SDI
5EX5

This is a little bit different than the codes suggested by https://batterybro.com/pages/18650-date-code-lookup-tool and http://www.tomobattery.com/blog/samsung-18650-battery-manufacture-date-code/ for cells made by SDI. If you treat them as SDIEM cells, the production date would be Sun Oct 05 2014.

The good cells were marked:
ICR18650-22P M
SAMSUNG SDIEM
5FY2

So the production date is Mon Nov 02 2015.

Another strange thing is that the cells have different positive terminals. The good SDIEM ones have positive terminals with 3 "legs" and the SDI cells have positive terminals with 4 "legs". This might just be a difference between SDI and SDIEM factories, or it's possible they're fake cells.

Telemachus · Oct 23, 2017

The abundance of all these hover board batteries on ebay is really a mystery to me. It appears that literally hundreds of them were made 3 years ago and have been sitting somewhere in Pennsylvania. I guess at less than $1.5 a cell, it's worth it to get a few (dozen) bad ones. Even the lower capacity ones are useful.

Is your holder made out of ABS, or Pet-G? Does it tolerate the soldering process well?

amberwolf · Oct 23, 2017

Telemachus said:
I'm amazed these hover boards aren't catching fire every day!

For a while when the many clones came out, they did. IIRC a lot of them were during charging.
https://www.google.com/search?q=hoverboard+fire&ie=utf-8&oe=utf-8

My suspicion about these packs is that they are either NOS that didn't get used after a recall shutdown the original board seller/maker/etc., or they *are* the recalled packs from at least one brand of board, that were supposed to be recycled but like so many such things instead got resold and stuck on ebay/etc.

Either way, the problems you're seeing could definitely lead to fires, if you don't either replace the bad cell groups, wiring, and BMS units, or manually monitor all the cell groups to prevent overdischarge and overcharge.

Using a balancing RC charger, with a plug coming out from each pack to the charge port for all cell groups, would do that for charging. (otherwise you have to sit there and watch a bunch of voltmeters during bulk charging).

Alternately, characterize all the cell groups in all the packs, ditch all the BMS units and existing wiring (because wiring shorts and opens could lead to the same problems you see), and either use a balancing charger or a completely different BMS unit you trust, or manually check them, after you've built your larger packs with them.

Addy · Oct 23, 2017

Telemachus said:
Is your holder made out of ABS, or Pet-G? Does it tolerate the soldering process well?

I've printed these with PETG, which tolerates the soldering very well. On another pack, I was able to solder wires to nickel tabs directly on the PETG holder, and it was barely affected.

The model for the holders is available here: http://endless-sphere.com/forums/viewtopic.php?f=3&t=90058&start=30

It's parametric, so you can customize it to fit different cell configurations and sizes

Tommy L · Oct 23, 2017

I just sent my test PACKS back to EBAY.

after 1ah of use....... they fade.

I built a 72v nominal 8AH pack..... 1/8th into the capacity and started fading badly.

It started off being very powerful and promising....... then right about 1ah to 1.5ah into the capacity it
faded so bad.

What appears to be an amazing deal is not good for ebike. Maybe good to run LED lights in an emergency for hours.

fechter · Oct 23, 2017

Post a picture of the BMS if you get a chance. Both sides. We can probably tell what functions it has just by looking.

x.l.r.8 · Oct 23, 2017

I have a few I tore off the packs when i got them, there are a few different types and I've not come across any issues with the cells them selves but I have only just finished balancing the cells to be able to group them.

amberwolf · Oct 23, 2017

This image from here
https://endless-sphere.com/forums/viewtopic.php?f=14&t=89757&hilit=hoverboard+bms

shows one side. I couldn't find a pic of the other side in a quick search.

But the pics on ebay of the packs show a similar but not identical BMS sandwiched between cells in a way that makes me suspect there's no room for much, if anything, on the back side.
https://www.ebay.com/itm/201920561877

Telemachus · Oct 24, 2017

I'm yet to see a bms sandwiched like that in about 5 packs. They are taped to the side with kapton. Pics to follow tomorrow.

amberwolf · Oct 24, 2017

Be interesting to see the version you have. All the ebay auctions I've seen for them (including those posted about here on ES) show that setup, even if they claim different cells or capacities, etc.

Telemachus · Oct 24, 2017

I have heard from people (here i believe) that Alarmhookup ebay auction delivers packs with the BMS on the side.

Now the ones I share here are NOT from an eBay auction but from packs I have pulled from hover boards purchased locally in SoCal (offerup et al)

Telemachus · Oct 24, 2017

In the link that you referenced above, he said that the BMS was shutting off on his daughter's hoverboard.

In my experience, these BMS don't shut off at all. It may be that I am dealing with a subset of bad BMS, and that's why the defective batteries got into my hands. But in each of the three I have taken apart, the overall voltage is much lower than any reasonable LVC, and the BMS allows them to keep pumping power. In one battery, 6 parallel sets had all been drained to 0, with only 4 sets still over 3V.

Of note, the hoverboard batteries are set up to charge and discharge through the same wire, not sure if that matters.

Also, it doesn't appear that they balance at all. The combination, not shutting down and allowing some cells to go to zero, and then allowing a full 42V charge is an absolute recipe for disaster. There is a potential that some of these cells could get much over 5V charge.

I do have two "working" hoverboard batteries that I am willing to test, as these results have freaked me out significantly.

timmy66 · Oct 24, 2017

I've bought some of these packs, too, same as photoed above.

One of the problems in these is that there is a single 8mmx0.15mm (or less?) strip of nickel carrying the series current in several links along the 10s strings.

Many others have posted that the most current this would be suitable for is around 9 amps. These BMSes are definitely allowing 20A to go through them, I've tested 23A on a specific motor (my motor controllers limit).

So that thin strip could easily be melted apart, which perhaps under certain circumstances could cause some problems.

At best, there are the double 5mm (perhaps x 0.15mm?) strips, that are doing the series connections on some of the sequences.

fechter · Oct 24, 2017

The nickel strips in the pic look like they are discolored from heat. I bet they get hot enough to melt the cell wrappers.

The BMSs in the pics above both look like they should do balancing. The first one has two ports, the second one is a single port. Doesn't make much difference, but a single port may be harder to reset once it gets tripped.

Just because the BMS has the parts to balance and protect the cells from overdischarge doesn't mean they will work as intended. 1001 ways they can screw up, both in design and in construction. Both boards look like typical chinese BMS units. They should have under voltage, over voltage, over current protection and balancing functions. The one in the top pic may not have overcurrent protection, as I don't see a shunt, but the lower one has it.

Addy · Oct 24, 2017

Here are the 2 types of BMS boards I've found on my packs from alarmhookup. These were all on the sides of the packs.

Telemachus · Oct 24, 2017

timmy66 said:
I've bought some of these packs, too, same as photoed above.

One of the problems in these is that there is a single 8mmx0.15mm (or less?) strip of nickel carrying the series current in several links along the 10s strings.

The way they are constructed, those two nickel strips fold on each other so that they abut in the middle of the pack. While there are balance wires soldered there that creates some space, technically it becomes a double strip of nickel...

:roll:

Yet despite all these shortcomings, there are literally thousands of these things out there being used everyday. I have 3 hover boards in my home, used everyday...

docw009 · Oct 24, 2017

I have bought several of those hoverbomb batteries, and the last pair I received is different. The prior ones had balance wires with a BMS on the side. These has a large BMS sandwiched in the middle. Circuitry on both ends, and apparently nothing underneath the 4 cells on top. There are no balance wires. It seems the 11 battery junctions are soldered to the main board. Cells are SDIEM 5FX4.

I'm showing the side without the 4 MOSFETs. Look like any balance circuits in there? It looks like a nifty package design. Would be a shame if it doesn't balance. If it doesn't, I do have some 10S balance boards I could perhaps swap in. Somewhat messy with the board soldered to the cells though.

This battery has not been charged yet. It came in at 36.2 volts. I checked the balance pads. I get 3.62 volts on every pair. Like the others, it has a single discharge/charge port.

The prior units all delivered about 4 AH in ebike usage. I used them to power my Q100H motor with 20" wheels. Satisfied with the performance. My wattmeters gives wacky WH numbers, so I have been looking at the AH usage. They can deliver about 4AH. A hour of riding uses about 2.5A, covers 13 miles, and takes me down to 36 volts. If I want more range, I put two in parallel. One pack can deliver 20A in burst mode.

I bought this last pair planning to take 6 cells out of one and graft them to the other to make a 13S-2P with a new BMS. I can still do that, but I'll use the earlier ones.

fechter · Oct 25, 2017

There seems to be a wide variety of BMS styles. Using the circuit board to eliminate the balance wires is a good concept.
One of the units in Addy's picture seems to not have the balancing parts installed, so I assume it does not have a balancing function.

docw009 · Oct 25, 2017

I opened up one of the first batteries I bought back in July. It's had about 10 charge cycles from 42V down to 36V. I had last charged it last week.

1. 4.12
2. 4.12
3. 4.12
4. 4.12
5. 4.12
6. 4.12
7. 4.00
8. 4.12
9. 4.10
10. 4.12

I gave it a few hours on the charger. All but #7 went to 4.22 V. That stayed at 4.0. Weak cell group. Well, this was the weakest one of my batteries. This will be the battery I will break up then to get 6 cells for my 13S-2P. My 13S BMS from ebay also arrived today. Sure is small, compared to my 10S boards (bottom). It's only 25A though,

fechter · Oct 26, 2017

If you manually charge the low cell, it might be OK.

Toying with a cheap hoverboard BMS

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