Building a pack

[westernshooter]

My 36v10ah shorted in a storm and over heated. seems to be done.
without allot to spend I've always wanted to upgrade my power. I found these..
(24) K2 Energy LFP18650P LiFePO4 Rechargeable 18650 Batteries 1250mah
Specifications:
Manufacturer: K2 Energy
Model: LFP18650P
Description: Rechargeable Batteries
Battery Chemistry: Lithium Iron Phosphate (LiFePO4)
Battery Class: 18650
Nominal Capacity: 1250mah
Nominal Voltage: 3.2V
Diameter: 18.2 mm
Height: 65.2 mm
Weight (each): 40.5 grams
Quantity: Box of 24
and am considering building my own. advise is needed.
and some help identifying an appropriate bms to use enough of these (19 I'm guessing) to build a 60v 25 AH pack.
also I need some help determining an appropriate dc to dc booster for a charger current out put 20v @ 3.5 amp..
any suggestions are welcome.
any more info needed I'll be happy to provide.

 

[wesnewell]

You need 380 of them to build a 60V 25ah pack.19s20p. If yo do that, might as well, get 400 of them for 20s20p. If you want a 25ah pack , I think it would be much easier with larger cells. And it would be much more reliable. 20s2p with these or similar would only require 40 each.
http://headway-headquarters.com/headway-cells/

 

[westernshooter]

HUH? 380 as in three hundred eighty? what am I missing. did I really screw up the calculation that much?
I was thinking if each cell is 3.2v 1250 mah multiply that to equate total, hence 20 cells + 64V, 25 AH.
help me understand?

 

[DAND214]

20 calls will get you 64 volts at 1250mah, not 25Ah. You must 25000/1250=20 x20=400 cells?

Dan

 

[100volts+]

Go lipo, no bms needed.

 

[agniusm]

You will need 20 cells to get your 25AH (20*1.25=25). So paralleled 20 cells will give you 25AH and 3.3V. Then you need to multiply that to meet required voltage and i would suggest 18 cells in series. So in total you will need 18*20=360 cells or 20P18S. You will find best choice for BMS in Bestekpower or ask Dnmun. Remembet that rated capacity will be at .5C which on this pack works out ar 12.5A draw. You are looking at 15kg battery pack.
Depending on what they cost there was a talk and tests done on LG D1 cell which is 3000mah. Different chemistry, less cycles but lighter and with less in size and 60AH pack would weigh the same.

 

[NeilP]

Go lipo, no bms needed.

I'd definitely go with a BMS if you can if you use LiPo...I have not done so yet..but I will as soon as I get around to it.

 

[dnmun]

i cannot fathom what you meant about your pack shorting because of a storm. or what kind of battery you are talking about. how did you figure it was dead? do you have the capacity to spot weld those cells together?

 

[NeilP]

HUH? 380 as in three hundred eighty? what am I missing. did I really screw up the calculation that much?
I was thinking if each cell is 3.2v 1250 mah multiply that to equate total, hence 20 cells + 64V, 25 AH.
help me understand?

But yes 400 wold be better.

Since you have made such a basic mistake, I am guessing you basic knowledge is not so high in batteries/electrics etc, so to that end, i do not think anyone has explained and expanded on your mistake enough.

If you wire any cells in series, the voltage increases, as you correctly worked out, but capacity stays the same as one single cell, and only the voltage has increased.

if you put those 20 cells in parallel, you would then have your 25Ah pack, but only at the single cell voltage

So to get 64 volt AND 25 Ah, you would need to string the cells in series o get the voltage so 20 series often written as 20s...but that is only 1.25Ah
so you then need 20 strings of 20s, all wired in parallel...20p.

so the end pack woudl be a 20s20p pack

Actually would be better to make blocks of 20cells in parallel, and the series the the parallel blocks

And if these are tabbed cells, chances are you won't be able to solder them, as the tabs don't take solder well.

Best to go with the Headway cells ...they screw together and Jim can supply yo with a BMS and charger too.

Only reason I can see for anyone building a pack from cells as small as you suggest is if they have an abundance of cheap or free cells, or are a masochist.

 

[dnmun]

the battery in that picture does not work as a full 20Ah because the connections are missing. after he said a storm had destroyed his BMS i realized what was going down. no pictures or evidence of that failure to support that argument, so i am not surprised about not understanding how to wire it up, or even realize that he could not assemble it without a spot welder.

 

[dogman dan]

If you need a pack the size of 25 ah, then the worst possible choice would be a tiny 18650 cell. Sure, they are cheap, but that does not make them easy to assemble into a big pack.

Try to find something bigger, at least the 3 ah cells.

 

[NeilP]

the battery in that picture does not work as a full 20Ah because the connections are missing.

That is because it is only half a pack being stripped after damage..check out the rust on the cell heads...Customer set fire to it after his own wiring work ..think his compressed fibre board caught fire inside his battery box ...

 

[mistercrash]

four hundred 18650 cells with the hardware and wires will weigh something like 50 pounds. Can this be installed on an open frame bike? His avatar shows what looks like a mountain bike with the battery in a bag on a rear rack. If that's the bike used, that rack won't hold for long with a 50 pound battery. Not to mention how the bike will handle. Also a finished battery made of four hundred 18650 cells is something like a block that's 14'' long, 7.5'' high and 8'' wide.

 

[agniusm]

..35 pounds to be exact.

 

[mistercrash]

..35 pounds to be exact.

Are you sure about that? Ok I admit that 50 pounds is exaggerated, but are you calculating the weight of 400 cells according to the weight of each cell displayed on a data sheet? What about all the material needed to make a nice sturdy battery like the metal strips to join all the cells and silicon beads to glue the cells together, wires and stuff to wrap the battery with, a couple layers of heat shrink or maybe even a metal box to keep it in. All the bits and pieces needed ads up quickly to the total weight. My battery made of 400 18650 cells once all done weighs much more than 35 pounds. 50 pounds is exaggerated and 35 pounds is not exact at all. How about something in the middle like 42.5 pounds?

 

[dnmun]

the risk of shorting the paralleling straps with so many cells is gonna be the defining risk imo. no insulating shields mentioned here at all.

 

[dogman dan]

It's just no easy task to assemble a pack of that capacity using 18650's. Lots of potential for a tiny mistake resulting in another shorted pack.

I'm assuming your previous one got shorted out when it got water soaked. So you need a dryer way to carry that thing too.

 

[Toorbough ULL-Zeveigh]

If you need a pack the size of 25 ah, then the worst possible choice would be a tiny 18650 cell

Only reason I can see for anyone building a pack from cells as small as you suggest is if they have an abundance of cheap or free cells, or are a masochist

try telling that to elon & see if you don't get bitch-slapped repeatedly. :lol:

 

[NeilP]

He did it from choice? buying full price cells? Yikes..definite masochist

 

[westernshooter]

i cannot fathom what you meant about your pack shorting because of a storm. or what kind of battery you are talking about. how did you figure it was dead? do you have the capacity to spot weld those cells together?

The bike was on the front of a bus when the storm hit. Something shorted engaging the motor, when I got off and went to remove the bike, the battery smelled burnt, was very hot, enough to melt the plastic case. When I got it home, let everything cool and dry out. dissembled everything and tried the charger, the red light came on suggesting it was charging. several hours later the charger went green as though it went through a normal charge cycle. but there was no voltage coming from it. I dissembled the case, was batteries were exposed I can see visual damage to two of the cells. ran a tester(voltage meter) on each individual cell 10 of the 12 come up with voltage between 3 and 4.3 volts, I'm not sure if the bms is good, the controller is good, or I may consider just replacing the two dead cells for now.
open to suggestions.

 

[westernshooter]

additional note, while the bike was on the bus rack, the wheel could not spin, causing the issue when the power was sent.

 

[westernshooter]

It's just no easy task to assemble a pack of that capacity using 18650's. Lots of potential for a tiny mistake resulting in another shorted pack.

I'm assuming your previous one got shorted out when it got water soaked. So you need a dryer way to carry that thing too.

Agreed dogman, I'm going at this kinda blind, would just as soon buy a built pack with bms, controller and charger. with little to spend it's difficult. in a couple months wouldn't be an issue, but I need this thing back on the road ASAP. even if a repair would get it going, I'd by a bigger pack later. how would I ensure the controller is still good? and the bms? could I just replace the two suspicious cells for now?

 

[dnmun]

why did you say the other two cells were dead? there is no voltage measurement.

there is no reason to think that lightning in the vicinity or even striking the bus would damage the BMS.

sounds like the controller was left on and the throttle activated while in the chocks on the bus.

if you decide you wanna test the BMS we can show you how to do that.

 

[westernshooter]

two of the cells come up voltage zero.
yes I would like to test the controller/mosfet

Crystalyte Analog System Controller -
Dual Speed Motor Controller 24V,36V,48V,72V; Amps : 20A

 

[dnmun]

the BMS should have cut off and prevented them from draining to zero V. those two channels may have the balancing shunt transistor shorted so it drains them down to zero V.