14s BMS, detection of defective cells

[Bouteille51]

Hello all,

after a lot of reading and lot of advices from the forum (https://endless-sphere.com/forums/viewtopic.php?f=14&t=81996), I decided not to buy cheap batteries from ebay or expensive batteries from other sellers but to try building my battery pack.
I'll be using Li-ion cells in a 14s5p configuration.

My current questions (I will have more for sure) are :

1. The BMS

I would like to find a 14s BMS for my pack that cuts after >35A (or >40A), does balancing, implements LVC and HVC, and possibly a temperature cutoff.
I saw that bestech seems to be a reference company but I cannot find how to order from them, do they have approved resellers ?

BTW what is the difference between all these models except the size (and one has a slightly higher bleeding current) ?
http://www.bestechpower.com/518v14spcmbmspcbforli-ionli-polymerbatterypack/BMS-D163.html
http://www.bestechpower.com/518v14spcmbmspcbforli-ionli-polymerbatterypack/PCB-D167.html
http://www.bestechpower.com/518v14spcmbmspcbforli-ionli-polymerbatterypack/PCM-D107.html
http://www.bestechpower.com/518v14spcmbmspcbforli-ionli-polymerbatterypack/PCM-D268.html


Otherwise I found these options but I am totally unaware of the quality of these products (and whether they fit to my project) :
http://www.aliexpress.com/item/58-8...charge-and-discharge-current/32700563427.html
http://www.aliexpress.com/item/Bran...-bms-polymer-lithium-battery/32700356644.html
http://www.aliexpress.com/item/14S-...continuous-60A-peak-Discharge/1741119929.html

http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2014-3F-JRYV.6NDWK
https://bmsbattery.com/bmspcm/733-10s-16s-50a-lipo-bms-system-battery-management-system-bms-pcm.html


2. Defective cells in parallel group

When a cell is damaged in a parallel pack, what are the symptoms ? How is this detected/handled by the BMS ?
My guess is that the parallel group will have a lower capacity as compared to other and will therefore reach their full SOC quicker, then this group will 'overcharge' and trigger the BMS HVC (and start bleeding) before the other groups are done charging... correct ?

As a user how do you detect that you have a defective cell somewhere in your pack ?


Thank you for your help !

 

[Bouteille51]

Just adding this one from ebay to the list :

http://www.ebay.com/itm/50-4V-51-8V...-Polymer-Battery-BMS-PCB-System-/322056033831

Do you think all these circuits have comparable quality ?
Should I just base my pick on the board dimensions (seems odd to me) ?

 

[dogman dan]

Since you are building your own pack, put a second set of plugs in so you can use a cellog 8 or similar device to monitor each cell groups voltage as it charges.

Two of the jst plugs would allow two cellogs to plug in at the same time, showing 7 cell groups voltage at each plug.

Now you'll know which cell was lowest, if you run it till the bms pops, or which cell fills first. If a cell is dead in there, it will be very obvious which group it's in.

 

[Bouteille51]

Yes I think it's a good idea, thank you for the suggestion.

I was also thinking about getting a RC charger to monitor the cells from time to time (discharging graphs for example) so these plugs will definitely be useful.

However, a 14s RC charger is pretty expensive (250$-300$) :
- Hyperion EOS 1420i NET3
- Thunder Power TP1430C
- Hyperion HP-EOS 720i DUO3 (not a 14s but a double 7s)

Any cheaper sugestions (with AC tranformer included if possible) ?
I probably don't need a powerful one as I'll be using the bulk charger most of the time (hopefully the satiator)... This would just be for charging the pack once in a while (weekends for instance).

Is that possible to use a double 7s one for charging a 14s packs ?

Has someone used their bulk charger to feed their RC charger?



Do you have any suggestions for picking the BMS ? Are they all roughly similar ?

 

[Bouteille51]

Also, 16s BMS seem much more common and cheaper.
Is that fine to get a 16s and only connect the 14 cells to it (or connect the last cell to the 3 last slots) ?

 

[Hwy89]

The forum expert on BMS was dnmun. Unfortunately, he no longer posts but if you search his posts you might find answers to your questions.
I do know that on the Bestech BMS you do not need to use all of the channels. I used a 16s BMS on a 14s pack and it worked fine.

 

[dogman dan]

You can also add another pair of wires, so that the pack could be tested, graphed as two 7 s packs. Then all you need is a good 8s charger.

That's the beauty of build your own, you can add all the extra plugs you like, for monitoring, for manually balancing, etc.

 

[flippy]

I saw that bestech seems to be a reference company but I cannot find how to order from them, do they have approved resellers ?

you can order directly from them. there is a minimum order quanity of 2. just email them, they ship directly to you with DHL.

 

[Bouteille51]

you can order directly from them. there is a minimum order quanity of 2. just email them, they ship directly to you with DHL.

That is correct thank you ! I contacted them by email and they were very helpful.
It's a good thing that I actually need 2 BMS (If my bike works well I'll build a second one for my beloved)

For information, the D107 is discontinued and has been replaced by the D167.
The D268 is a 26s... A bit too much

The only difference between the D163 and the D167 are the dimensions.
The D167 is a bit smaller but I didn't like how they cluttered everything on the board to get this smaller footprint.
For a matter of clarity (and eventually robustness) I ordered the D163.




The price they asked is fairly similar too (based on an order of two pieces):
D163 : 43.00 USD
D167 : 41.50 USD
Shipping to France by DHL : 22.50 USD

You can also add another pair of wires, so that the pack could be tested, graphed as two 7 s packs. Then all you need is a good 8s charger.
That's the beauty of build your own, you can add all the extra plugs you like, for monitoring, for manually balancing, etc.

The forum expert on BMS was dnmun. Unfortunately, he no longer posts but if you search his posts you might find answers to your questions.
I do know that on the Bestech BMS you do not need to use all of the channels. I used a 16s BMS on a 14s pack and it worked fine.

This sounds perfect ! The 16s BMS from Bestech Power have two plugs (8 cells each)... It will make it super easy to plug the 8s RC charger once in a while for checking the cells.

They confirmed by email that the unused channels must be left unplugged.
That is also why they list all >14s (up to 26s) BMS in their 14s category.