18650 DIY Battery Building - Advise on BMS please?

[msebold]

Hi! I'm doing an ebike conversion of my new 26" Fuji mountain bike installing Kinaye's MXUS 3K Turbo 60amp direct drive motor. Partially because I am unable to find the battery pack I seek at a price I can afford (and partially because I just like to tinker) I am planning on building a battery pack using 3500mah 18650 cells. The pack I am planning will use multiples clusters of 18 cells in a series charged to exactly 72 volts. This should be able to pump out about 50-60 amps continuous, which is what my motor requires. After reading several informative posts on Endless Sphere regarding similar DIY battery packs I plan on constructing the pack using about thirty-six 4-cell plastic battery holders. (See https://endless-sphere.com/forums/viewtopic.php?f=14&t=72138&hilit=plastic+battery+holders). I am not addressing here the task of constructing the battery pack, but its battery management system (BMS).

As I'm not particularly fond of fires or explosions in my garage I seek information on the BMS I will need with these 18650 cells. As I understand it, and as I hope one or more of you good and knowledgeable people can confirm, managing my DIY 18650 battery pack will require JUST THE FOLLOWING:

1. Overcharge protection (overcharging harms the cells); and
2. Undervoltage protection (draining the battery too low harms the cells)

(As this pack will use 18650 cells and NOT LifePO4 batteries, as my present ebike uses, I understand that there is no need for cell balancing.)

To take care of issue number 1 (above) I plan on purchasing Grin Technologies "Cycle Satiator), which will properly charge my battery exactly to the desired 72 volts.

I need some help with issue number 2, low-voltage cutoff. I need to ensure my planned 18-series cell battery does not discharge below 51 or 52 volts (I am still trying to determine the best recommended cutoff voltage).

I seek here anyone's recommendation for an add-on device that contains a programmable low-voltage cutoff switch.

I also seek advise on anything else I might need or which I'm missing regarding battery management of my DIY 18650 battery pack.

Thanks in advance!

 

[Ykick]

I’ve become convinced high power handling BMS are not only expensive but often troublesome.

What I’ve taken to doing with my RC Lipo similar to many 18650 lawn tool battery pack manufacturers is to merely use a smaller BMS for charging only.

10A BMS is plenty for most charging and way cheaper and compact than 30-100A BMS. Good cells generally remain in balance and controller LVC can serve as reasonable over discharge protection. Try to ignore BMS balance functions as they mostly suck.

A clever person can probably wire an over-discharge warning buzzer/light system using BMS circuit?

Of course you never want to deal with pack fires but it’s much more important to avoid indoor fires which may occur while charging.

For discharge, in the event controller LVC fails to catch and protect from an exothermal event, you’ll most likely be outdoors and able to put some distance between you and the bike while it burns out.

For a well balanced 18S 18650 pack I’d probably go with 52V controller LVC. Edit to add: 54-58V (3-2.25V/cell) wouldn’t sacrifice that much, if any practical capacity. There’s very little energy left below 3.5V/cell resting and from there rapidly dives over the cliff - often in less than a city block.

The less time (and heat) spent discharging from low state of charge the happier most cells will be.

I’ve gone down the BMS route and wasted a lot of time & money over many years. Charge only BMS is about the best and most practical compromise I’ve adopted.

 

[msebold]

Thank you Ykick. However, some of what you write confuses me.

I am a newbie who plans to shell out big bucks to purchase a whole bunch of NCR18650GA 3500mah cells to build a battery pack to power a 60 amp motor. The voltage of my pack will be unconventional, approximately 67 volts nominal. I want to take whatever steps are necessary to care for my (expensive) batteries, and not do anything stupid to ruin them. This is all I mean by a "battery management system" or "BMS". I'm not talking about this or that company's "BMS" currently on the market. I just want to properly care for my batteries.

That being said. I understand that to properly manage, or care for my batteries, I basically need only the following:
1. Proper charging (including overcharge protection); and
2. Over-discharge protection (or low-voltage cutoff).

Question 1: IS THERE ANYTHING ELSE I NEED TO MANAGE THESE BATTERIES?
* I understand I DON'T need cell balancing, as these are not LIFEPO4 cells.

I believe that I have the first need met and I will have no problem with proper charging (using Grin Technologies Cycle Satiator). This brings me to the second need --

Question 2: Does anyone know where I can get a low-voltage cutoff switch programmable to about 51 volts that I can incorporate into the wiring of my ebike battery?

I have not yet received my new ebike motor, but I assume that whatever low-voltage cutoff switch the controller might have is not programmable. Anybody know where I might find a programmable low-voltage cutoff switch?

Thanks in advance.
Mike S.

 

[DAND214]

Thank you Ykick. However, some of what you write confuses me.

I am a newbie who plans to shell out big bucks to purchase a whole bunch of NCR18650GA 3500mah cells to build a battery pack to power a 60 amp motor. The voltage of my pack will be unconventional, approximately 67 volts nominal. I want to take whatever steps are necessary to care for my (expensive) batteries, and not do anything stupid to ruin them. This is all I mean by a "battery management system" or "BMS". I'm not talking about this or that company's "BMS" currently on the market. I just want to properly care for my batteries.

That being said. I understand that to properly manage, or care for my batteries, I basically need only the following:
1. Proper charging (including overcharge protection); and
2. Over-discharge protection (or low-voltage cutoff).

Question 1: IS THERE ANYTHING ELSE I NEED TO MANAGE THESE BATTERIES?
* I understand I DON'T need cell balancing, as these are not LIFEPO4 cells.

I believe that I have the first need met and I will have no problem with proper charging (using Grin Technologies Cycle Satiator). This brings me to the second need --

Question 2: Does anyone know where I can get a low-voltage cutoff switch programmable to about 51 volts that I can incorporate into the wiring of my ebike battery?

I have not yet received my new ebike motor, but I assume that whatever low-voltage cutoff switch the controller might have is not programmable. Anybody know where I might find a programmable low-voltage cutoff switch?

Thanks in advance.
Mike S.

Are you sure the Cycle Satiator will charge that high? I see 24 ~ 60v. Is there a special high voltage version?

Can't help on the LVC, I just use my controller, at 3.5v per cell One is 12s and the other is 18s, programmable with a pc.

Dan

Dan

 

[amberwolf]

Are you sure the Cycle Satiator will charge that high? I see 24 ~ 60v. Is there a special high voltage version?

http://www.ebikes.ca/shop/experimental/satiator7205.html

 

[Lurkin]

I understand that there is no need for cell balancing

That's a great leap of faith and not a statement I'd agree with.

Some of the branded, genuine Samsung 18650 cells I've owned have pretty much remained in balance on their own. They still floated around a little but not drastically out.

Others, non branded, ebay crap 18650 cells definitely haven't, in fact, in the absence of proper balancing at charge I'd be surprised if I couldn't get a fireball out of them. I realise you are saying you are buying genuine, quality cells. But how do you know you haven't bought a manufacturing dud in the middle of your pack?

Really, all batteries should be balanced at least periodically. There are meticulous people on here who have certain strategies and methods to their charging which ensures they are balanced frequently - I'm lazy and prefer to set things up with a BMS or straightforward harness for balance charging.... each to their own, its just my 2c.

 

[msebold]

Thanks for your posts DAND214, amberwolf, and Lurkin.

The 72 volt Cycle Satiator comes out next month.

Regarding cell balancing perhaps then, I do need to be concerned with this issue. Again, I am a newbie. I will do some more research. Thanks again.

Regarding bad or dud cells, I do plan to keep a close eye on this issue. Of course I will test the individual cells before using them. I plan to monitor the pack, watch for any voltage variations, any and periodically test the cells. This is one reason I plan to construct my custom pack use 4-cell battery holders. Bad cells can easily be swapped out, and the whole bank can be replaced when they are depleted.
http://www.ebay.com/itm/Black-Plastic-Battery-Storage-Case-Box-Holder-For-4X-Rechargeable-18650-3-7V-DIY-/131558894255?hash=item1ea1856aaf:gCUAAOSwjVVVqQdG

Again, thank you guys for your comments. Much appreciated!

Still looking for any advice anyone might have in finding a 51-52 volt low-voltage cutoff switch.

 

[Lurkin]

Err. You'll see the ad makes no mention of current carrying capacity.

I think you'll find most of the forum members on here that have used those holders have upgraded the connections to ensure they can carry enough current without fire. Alternatively, you might considering spot welding nickel strip, although that is less convenient for single cell access. Have a search of forum member Snath 's posts - he has made safe, compression packs for 18650s which do not require spot welding.... 8)

 

[msebold]

I have learned that there are at least two other items to monitor in caring for my batteries. Overheating and short circuits (from bad cells).

I could go the route of installing battery cases that incorporate "protection circuit modules" (or PCM - Ugh! more acronyms.) See http://www.master-instruments.com.au/files/tech___protected_liion_batteries.pdf.pd

Battery cases that incorporate these PCMs can be purchased on ebay. See http://www.ebay.com/itm/1-pcs-1S4P-18650-3-7V-Holder-Case-Battery-Li-Ion-PCM-Protection-Circuit-Module-/151448639279?hash=item23430ad72f:g:cRYAAOxyfS1R0nbV

However, this would be a big expense, and I suspect that that once tripped, these PCMs must be replaced. (Not sure -- anybody know anything about these?) In other words, I'm out riding about and I suddenly find I've overdischarged my battery pack, tripping one or more PCM. I fear that the PCM is not self-resetting, but more like a fuse in that once tripped it is done for and must be replaced. Anybody have any experience with these?

Thanks in advance.

 

[msebold]

Thanks Lurkin. I'll check out Snath's posts.

Just a newbie. Still learning. :wink: