Custom E-Bike battery issues

[ebikerxx]

I think he’s duly warned. Using Micahs’ book with attention to details like safety glasses and no jewelry are top issues. I was being super careful with a disassembled pack and didn’t notice a small piece of wire on my silicon work surface. I set the bare pack down and shorted out on the wire bridging cells. Instant POW! FLASH! Trashed a half dozen PF cells.

Thankfully, I got my old battery pack disassembled from a professional.
I verified that all the old cells were fine before using them in my reconfigured battery pack. I still have the other 10 working cells stored in a safe place.

 

[ebikerxx]

I don't see a 200maH difference as major. Some theories predict a case where the 2.2AH cells go lower in voltage than the 2.4AH cell, and the bigger one will try to charge the two smaller ones. However, unless the parallel connection was awful, any voltage difference is millivolts, so only a minor charge effect. And what does Micah's book say about this? He's the guy that advocated connecting a booster battery to the main battery via the charging port. This bothers a lot of people. I tried it, and it works, but I don't see it working well if you are combining batteries to push more current. Then you need a parallel connection.

Anyway. keep reading. If you're going to use home built packs, you have to know a lot more. First, there's the safety aspect. Then there's longevity.

Yes indeed, I read about higher mah cell counter balancing the other lower mah cells when connected in parallel which gave me the confidence to try this configuration. Not sure what Michah's book says about this though. Here is a useful video that explains parellel charging:

Parellel Charging

I will keep learning for sure. There are a ton of factors which need to be considered while building a battery pack.

 

[tomjasz]

mixing of cells is his current problem

There are methods of testing and grading. Not interested but did it years ago. I’m acquainted with several successes, but I’m unwilling to risk it. Then again, In my moneyless yoot I might have done the same. Here’s where ammo cans and safe charging practices ought to be paramount.

 

[Sierratim]

Thanks all for your replies.

@Sierratim I ignored internal resistance of the cells since both the types were e-Bike batteries and the difference was just 200mah. Not sure if there is a standard resistance for electric vehicle cells? If I were to mix electric vehicle cells with laptop and power tool cells maybe then the resistance would be a major factor.

There are a number of articles discussing the importance of matching internal cell resistance in lithium cell packs. I've attached one whose conclusions are based on experimental results. batteryuniversity.com also has a number of related articles.

The main concerns with mismatched cells is that it reduces cell lifetime disproportionately (a 20% mismatch in resistance can result in a 40% reduction in cell life) and can cause uneven current sharing resulting in higher operating temps that as a minimum reduce cell life and at the extreme can cause thermal runaway.

Checking internal resistance is straight forward. I built packs some years ago and used a fairly inexpensive bench top meter for this. THIS ONE looks like it would do the job now for ~$40.

Be safe.

 

[Gionnirocket]

There are a number of articles discussing the importance of matching internal cell resistance in lithium cell packs. I've attached one whose conclusions are based on experimental results. batteryuniversity.com also has a number of related articles.

The main concerns with mismatched cells is that it reduces cell lifetime disproportionately (a 20% mismatch in resistance can result in a 40% reduction in cell life) and can cause uneven current sharing resulting in higher operating temps that as a minimum reduce cell life and at the extreme can cause thermal runaway.

Checking internal resistance is straight forward. I built packs some years ago and used a fairly inexpensive bench top meter for this. THIS ONE looks like it would do the job now for ~$40.

Be safe.

I concur

 

[ebikerxx]

Checking internal resistance is straight forward. I built packs some years ago and used a fairly inexpensive bench top meter for this. THIS ONE looks like it would do the job now for ~$40.

@Sierratim much appreciated! I was about to settle for an inexpensive but complex method of buying a 1 ohm resistor and calculate the internal resistance manually by recording the voltage drop. This tool is just what I've been looking for!

 

[Gionnirocket]

@Sierratim much appreciated! I was about to settle for an inexpensive but complex method of buying a 1 ohm resistor and calculate the internal resistance manually by recording the voltage drop. This tool is just what I've been looking for!

If you think you'll continue building batteries... A hobby charger may be a good tool as it can measure internal resistance as well as charge/discharge cells to equal levels for assembly.
SkyRC B6AC

 

[ebikerxx]

If you think you'll continue building batteries... A hobby charger may be a good tool as it can measure internal resistance as well as charge/discharge cells to equal levels for assembly.
SkyRC B6AC

Thanks that's even better for an affordable price too! The hobby charger models I checked initially were more expensive for some reason. I got thrown off by the 3s/4s/6s ratings of some hobby chargers.

If the current battery fails too soon, then probably my next battery pack will be one with no BMS and I will go for this charger.

 

[Sierratim]

Thanks that's even better for an affordable price too! The hobby charger models I checked initially were more expensive for some reason. I got thrown off by the 3s/4s/6s ratings of some hobby chargers.

If the current battery fails too soon, then probably my next battery pack will be one with no BMS and I will go for this charger.

Why no BMS? It's there to protect the battery, and you.

 

[ebikerxx]

My e-bike got phased out in a couple of years. There are many better models available today for the same price. My only bragging rights are magnesium alloy wheels and it's a foldable 26er. My vintage e-Bike has a voltmeter as a charge indicator, an awkward looking large controller and a 36 volt battery.

My 250 watt hub motor e-Bike has had a rough ride. I have been driving the bike like a moped, off-roading, and has been battered by a few storms as well. My bicycle has survived, but I had to replace the controller, throttle and now battery!

I'm not keen on buying a new e-bike, hence I'm tinkering around to see how I can upgrade the parts cost-effectively. I upgraded the controller to a dual mode 36 volt / 48 volt water proof controller which burned a hole in my pocket. Replaced the broken thumb throttle with a half-twist. IMO thumb throttle is the dumbest idea for an e-Bike. And then the freaking battery died and a new battery was a no go. I opened the battery up to check the voltage, but the configuration was awkward to test, so decided to build a new battery pack.

My strategy was to make stuff as modular as possible keeping future upgrades in mind. After researching a lot I decided to use a solderless battery kit. I ordered 20x18650 cells and the battery assembly kit, meanwhile got my old battery pack desoldered by professional PCB welder, btw I did try to desolder them myself, but lack of tools and technique coupled with some fireworks meant that it was saner to take it to a professional. So then, surprise, it was just a faulty BMS!! And then a few weeks later this discussion thread spawned!

I have been extra careful while building the battery pack since I'm not a mechanical or an electrical engineer.

Few pics of my e-bike:

 

[Gionnirocket]

Thanks that's even better for an affordable price too! The hobby charger models I checked initially were more expensive for some reason. I got thrown off by the 3s/4s/6s ratings of some hobby chargers.

If the current battery fails too soon, then probably my next battery pack will be one with no BMS and I will go for this charger.

I agree with Sierratim... why no BMS?
They are relatively inexpensive and do provide a level of protection during charging and discharging. Additionally if it were me I would go with an intelligent one that has Bluetooth so I could monitor the pack and gather more information and learn. It would make future troubleshooting easier.

As for the hobby chargers with 3s/4s/6s... This is used with hobby LiPo batteries that have no BMS and allow for balancing control during a charge. This also allows the charger to take measurements of individual cells in a multi cell pack. The number represents the quantity of cells in series.

edit: Cool Bike!

 

[ebikerxx]

I did some research about the pros and cons of BMS, and initially decided not to use a BMS, and buy a balance charger instead, since I was building a modular pack, the option to add a BMS later was always open.

Just reading about Balance Charging, the setup and configuration made my mind so numb, and the various models to choose from didn't help either, so in a hurry decided to use a BMS as a cheaper and safer option.

Now that I'm learning more about BMS and balance charging, I'm more confident to try the other option just for the sake of learning more about batteries.

Additionally if it were me I would go with an intelligent one that has Bluetooth so I could monitor the pack and gather more information and learn. It would make future troubleshooting easier.

Yes indeed, I have considered this option, and to enhance the safety, balance wires with fuse is a good option.

The BMS didn't seem like a flexible option. Initially, I had thoughts of building a 48 volt battery pack. But now that I have a 36 volt working pack - testing still in progress, I might buy a 48 volt BMS if the need arises to reconfigure my battery pack.

 

[harryS]

The original battery looks exactly like a hoverboard pack, 16 cells on the bottom and 4 on top, When the cells are soldered on, they are pretty rugged. You can buy a 36V5AH for 50 bucks, but they're probably not as well made. I bought four in 2017 for $25 each. Add a steel ammo can for $20. Still using them.

If using a voltmeter on the bike, those can be accurate, so 40.6 is as good as 41.6V, If you are going to do battery building, a good meter is needed.

 

[ebikerxx]

I use a robust multimeter while building battery packs to measure the voltages. The voltmeter on my e-bike is fixed, and is essentially my only charge indicator while riding. I use my smartphone on a mount to measure speed, distance..which don't need an interface with the controller.

My old battery looks like a hoverboard battery, and maybe it is, but is designed to fit in a small frame bag. I have an option of installing the battery on a rear rack, but I will have to change the battery layout to a flat one, rendering my battery assembly kit useless. For now I'm fine with a frame bag for my battery pack.

 

[harryS]

Given new cells, and a good meter, 4.2 volts per cell at full charge for 42.0V. See what your charger puts out.

 

[tomjasz]

As the OP has reported there are significant numbers of kit builders and battery configs not using a BMS. Not an option for me but experienced kit builders have sorted the techniques.

 

[KLee]

From endless_sphere
View attachment 74833View attachment 74834

Five fingers of death

 

[ebikerxx]

I will redo the battery pack as a 10s2p pack with the new set of cells and check.

I suspect a contact issue, since the reading on the ebike voltmeter and my multimeter match, doesn't look like a meter issue.

My first suspect was the BMS, but now that the vendor has assured me about the cut-off voltages, seems like a contact issue.

 

[ebikerxx]

I will redo the battery pack as a 10s2p pack with the new set of cells and check.

I suspect a contact issue, since the reading on the ebike voltmeter and my multimeter match, doesn't look like a meter issue.

My first suspect was the BMS, but now that the vendor has assured me about the cut-off voltages, seems like a contact issue.

Well, now I'm really missing a balance charger. If I had one, I can confirm that the individual cells, and the battery pack are charging and discharging fine.

There are some rare cases of the throttle getting disconnected, which can lead to similar issues. I have verified the throttle and controller connections multiple times, but the wiring kind of looks messy, which can lead to intermittent issues. Now, how does someone isolate intermittent disconnection issues?

 

[ebikerxx]

Ok so after knocking off the old 10x2400mah mismatch cells, now I can't get the 10s2p configuration to work with the new cells.

After some troubleshooting seems like the BMS failed once again! Spoke to the vendor who says its in standby mode . Returning the BMS to the vendor. Please watch the video of my troubleshooting for more information.

BMS troubleshooting

I did try charging the battery pack as advised, but that didn't work.

Some noob tips to disassemble the kit:

1. You can use a tire lever(fiber) to lift the end caps and pop them out safely without the fear of shorting.

2. A heat resistant tape works fine to hold the contacts while testing the incomplete battery pack.