Batteries Batteries!!

No problem Paul! Heres the schemat for the BMS. Makes it a littler easier to explain. Ordered some 45amp Anderson Powerpoles to connect it all together with.
 

Attachments

  • 13Slogo.jpg
    13Slogo.jpg
    56.4 KB · Views: 285
Forgot a pic of the thing. Complex little piece!
 

Attachments

  • $T2eC16VHJFwFF,EY8gQ6BR,8V5M7+g~~60_3.jpg
    $T2eC16VHJFwFF,EY8gQ6BR,8V5M7+g~~60_3.jpg
    73 KB · Views: 238
Ok after an afternoon of reading it seems that cell balancing large li-ion packs is still under development. The Tesla has 7000 cells in its battery!! It would appear that the BMS above will only balance 13 individual cells(48.1v) not multiple series cells. My pack is in essence 4) 13s packs consisting of 52 cells. Seems that at our level(home builders) balancing is a myth. If I were to connect as per the schemat it will balance 13 of the 52 cells. If so the BMS is still a great asset for voltage/amp/charge/discharge management. and I wouldnt build a pack with out one.

Somebody correct me if Im wrong, hope I am!
 
So now that the BMS has arrived, I have concluded that it will balance groups of 4cells in parallel, (13) total. In order to balance each cell you would need a wire for each cell, 52 in this case, impractical. So I guess this is better than no balance, but its not balancing in the true sense as cells of the 4 cell groups can differ.
 
I know it's a stretch, but maybe you can get 13 x 4 cell BMS units cheap and make a heirarchy of battery balancing units. I don't know if it's worth it, but with electronics the simple stuff like cell voltage monitors with buzzers and simple stuff tends to be pretty cheap.
 
16v4nrbrgr said:
I know it's a stretch, but maybe you can get 13 x 4 cell BMS units cheap and make a heirarchy of battery balancing units. I don't know if it's worth it, but with electronics the simple stuff like cell voltage monitors with buzzers and simple stuff tends to be pretty cheap.
Click to expand...
That's certainly possible, not real difficult and its not out of the question $$ wise. If one was building a new pack considering the cost of the cells it might be worth while. Average cost of a BMS of the type is $40, so an additional $160 is doable. Especially since the cells would be receiving ideal charge/discharge care. Given good quality cells a system like this should yield the longest pack life. Would be a real bundle of snakes wiring wise, but carefully done is a simple operation. Probably easiest to twist the balance wires in groups of 13 (4ea) wires since each bundle is all going to the same cell group. The 4 BMS units would also easily fit in the cell pack. Not a bad Idea!

In retrospect the R/C lipo battery is probably the easiest way to go. They are now cheap enough, offer true balancing, great power, and small size all considered. In a decade of using them I have never had an "incident".
 
The battery saga finally ends! Got the BMS installed. Very Carefully!! Each group of 4 parallel cells is treated as one cell. You begin by soldering the #1 balance wire to the 3.74 cell then you continue adding wires in exact order(1-13) to the next cell going up in voltage 3.7v at a time. #2wire 7.4v, #3wire 11.1v and so on until 48v is reached. I then read each pin in the plug to see if it increased in order. All was good! The included sorta instructions indicated a board smoke if all was not in order. Finally got the nads to plug it in. No smoke!! Next thing is to regain the nads and charge the dang thing. A few pics showing the bundle of snakes wiring to contend with. The BMS is wrapped in high density foam for protection. No problem seeing where it is. Finally wrapped it in some blue duct tape to give it that factory look---didn't work. If I wind up with a usable battery I will have less than $150 in it, far less than the $500 I just paid for a similar new battery, it did come with a better than usual charger though.
 

Attachments

  • 001.JPG
    001.JPG
    83.1 KB · Views: 258
  • 003.JPG
    003.JPG
    80.8 KB · Views: 272
  • 004.JPG
    004.JPG
    76.2 KB · Views: 265
  • 005.JPG
    005.JPG
    61.1 KB · Views: 251
Looking good cannonball. I just finished a similar battery build. No bms, just 3s4p packs, as all my laptop cells were wack.
 
Thanks snellemin! I put the pack on a charger that was primarily designed for SLAS. Puts out 54+v. Thought is the BMS will regulate/protect the pack from the rather "dumb" SLA charger. Seems to be doing just that. The charger kicks off with a pack voltage of 49.8v(charger led indicates charged) The charger still indicates 54v input at the pack connector. This must indicate that the BMS has cut out the charger. This works out to 3.83v/cell, was expecting more like 4.2v/cell. Don't easily have a way to cycle the pack until the E bike is finished. Am hoping this is normal.
 
I charge my batteries to 4.15v. I have some pics if you don't mind me posting them in your thread.
It looks like your bms balanced all your cells too 3.83V. I think 3.83V is for lithium ion cells.
 
Last edited:
By all means post your pics, that's what forums are all about. Im thinking you are right the BMS is in control. Hope I can deal with it. I have heard that some BMS will really control your ride. Allowing good acceleration then cutting amperage to max constant slowing the pace down. Hope not.

Heres a pic of the homebuilt pack vs. the commercially produced one. The homebuilt is 48v/10.2ah the other is 48v/15ah. Looks to have the cells grouped in a linear fashion rather than bundles like mine. The produced pack also has an on off switch cutting the battery presumably through the BMS.
 

Attachments

  • 001.JPG
    001.JPG
    55.3 KB · Views: 233
BMS's are tricky. Good ones cost money and there are so many formats available. I'm saving up for a good battery with decent bms that will last me a few years. Until then I prefer to balance all my packs manually with my charger.

Here's what I've done. Enough batteries for 15s4p or 17s4p. Nominal capacity will be 7400mah. I'll limit the batteries to 25A max and they are good for 10A continuous. Just enough for my mid drive setup.




 
So you are charging independent cell groups of the same battery seperately? I could do the same by exposing the 13pin balance plug of the BMS, finding a plug to interface with it then charge the cells individually . When done plug the BMS back in and let it regulate the discharge amperage rates and voltage cut off. I suppose it wouldn't be necessary to charge/balance the cells like this but occasionally, as once balanced the BMS would probably handle the job. Maybe a balance check say after every five charges or so?
 
OK found an old balance lead to use on the BMS plug, actually only need one wire since I only have one charger. My cells are grouped into 10.2ah 3.7v units. If this were an R/C lipo I would charge @ 1C. My charger will only output 5amps. Whats your C recommendation? My charger will recognize these "units" as a single cell and charge to 4.2v, this voltage is ok, or should I limit it a bit lower?
 
You can charge them to 4.2V. 2C is what I charge my packs at. 2.5C max with the Sony and Samsung cells. So in your case 20A is fine to charge at.
 
Ok realize the error of my thinking. Cant differentiate the cells via the 13 pin plug. So there is no way for me to individually access the cells save completely open the pack and isolate the individual groups. I can however read the 13 pins individually to get each groups voltage(by virtue of V increase) to see if the charger is indeed balancing them. Will check it out.
 
Just read all the pins and did the math. The pack is out of wack as expected, but really only 2 cells. Most are in the 3.8 range with cell# 9 being 3.67 and cell# 13 being 4.2. Seems the BMS is in the balancing act. Don't know if it can handle cells with such a differential. The overall voltage of the pack has remained in the 49v+ range for 2 days, with the charger staying in the balance mode 99% of the time. Guess I will let it do its thing. Have read it can take days to balance a pack the first time. Will be patient and monitor.
 
It can take a day for sure depending how big the pack is. That is why I charge all the batteries to around the same voltage before assembling them into a pack.
 
Well as far as I have determined the BMS is faulty. Was properly and carefully wired, I am sure I have done nothing to damage it. I finally figured it wasn't working when I hooked it to the bikes system and nothing happened. I read the input to The BMS and I had 50v in the battery. I also had the same at the main connector. As soon as I hooked the battery to the system the voltage at the connector dropped to 1.5v! Still had 50v at the BMS input. Long story short I removed the BMS. The battery had plenty of power to drive the bike. So am going to run with out a BMS and monitor the charging voltage. Live and learn!
 
Bummer about the BMS.

I rode with the battery pack today pulling the kids in their trailer and after that some offroading. So it's 5 packs making a total of 15S4p. I was pulling 35A out of them and they got warm around 100F or so. Voltage drop is high under that high load, but not a problem for me really.
Between 4.15V to 4.2V is around 475mah capacity per 4p block. So not a whole lot as you can tell. I figure I have a 8000mah battery pack, but really will limit myself to 6000mah to make them last a bit longer. I charge all 5 packs in parallel at 40A.
 
Last edited:
You must log in or register to reply here.
Back
Top