BMS questions for 1000 watt 25-35 ah battery build

shr00m

shr00m

10 W
Joined
May 18, 2016
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75
Recently purchased a 1000 watt rear hub kit on ebay. I plan to build a battery pack from 18650s i have about 140 good cells about 70 percent of them are new cells but all mixed and all pulled and detabbed from laptop batteries. I purchased a 13s 30amp bms i was planning on doing groups of 10 since i have so many batteries one thing im really confused about and ive tried to find the answer myself in videos and forums posts but i cannot seem to find this out. When using a BMS what are the 2 wires coming out of the bms for the P- C- and B-.... I realize what the 14 other wires are 4... and i had assumed the p- c- and b- became the + and - of the whole 48v battery once they are linked but then looking at the wires coming out of the BMS the 14 wires... they seem too small to carry such current to power the motor... am i missing something here? is the battery supposed to be hooked up separately to the controller and not through the bms? any tips on soldering together a large pack such as this would be appreciated too. i plan to carry it on the rear rack but would love it mounted in the triangle just dont know if i can handle that for first build. thank you
 
P- goes to bike negative
C- goes to charger negative
B- goes to battery negative

Bike and charger positive do not go through the bms. They are attached directly to the battery positive.

Don't solder the cells together. Lappy batts already have high internal resistance. Soldering will make it much worse. That said since you will likely do it anyway. Use at least an 80 watt soldering iron will a huge chisel tip to make it fast and blast air on the joint right after soldering.

What makes you think those batteries are good? You need to run a charge cycle and match capacity for groups or at least check for high self discharge cells.

And lastly lappy cells are only good for 1c discharge. You are going to be over that with that motor and those cells. So possibly cell damage and even fire while riding.

Have fun!
 
Welcome new member.
A few suggestions;
•Find and read this thread. https://endless-sphere.com/forums/viewtopic.php?f=14&t=26383
Forum member DrkAngel has created and managed this thread that contains all you will ever need to know about building packs from laptop cells.
• in the future don't take the tabs off the cells. Leave enough tab to solder on. This will help keep from soldering directly to the cells.
•update your account with your location. You never know there might be another member nearby who would be willing to help.
•Follow up on your posts. If you have a problem and get advice from members we want to know how it was resolved.
 
Thanks for the replies. The batteries have all been charged and sitting to check for self discharging cells. As I said 70 percent of these cells cane from brand new laptop batteries never used. The rest are pulled from food working laptop cells Ive charged them all about a week ago with the stand alone 2 cell chargers... I have like 10 so it took a few days to get them all charged. I'm in Arkansas if anyone was near with a spot welder that would be amazing otherwise I plan to solder them... I'd mail them to someone if I could get them spot welded and pay for the trouble. Thanks for the replies.
 
Something is missing from your board and the description from grizzi. A connection from the battery positive to power the thing.
Show us the board.

I'm not sure I would read the link offered for bms advice, the author doesn't believe in them.


Pity you broke the tabs off. Solder quickly and with a cold damp cloth to hand. You could even fridge them first. Heat is a big part of things going wrong so do it outside or at least by the door or window ready to launch it all.
 
I received the 2nd bms i ordered today.... i ordered two and had given up on 2nd one.... would it be a better idea to build 2 batteries instead of one large one? also im confused why are my cells going to overheat i have seen many 1000 watt motor builds with 18650 cells and many 1000 watt 48volt batteries made from 18650s being sold for 1000 watt application... why will i have issues using 18650s im confused? fire and heat? you are saying with 1000 watt motor you cannot use 18650s?
 
I understand heat from soldering i was talking about this comment "And lastly lappy cells are only good for 1c discharge. You are going to be over that with that motor and those cells. So possibly cell damage and even fire while riding. ".....
 
Those cells are made for a laptop that sits still on the table. Your bike pulls enough watts for your bike and your weigh also. Not all 18650 cells are the same. Go buy a battery from the 99 cent store and buy a cell for your vape pipe. BIG DIFFERENCE in power. Yugo and a Ferrari are both cars. But there is a difference.
Plus one capacity may 75% and one 90% the battery only good for 75%. Only as good as your worst cell
 
Also depends on your controllers amp draw. Could be 20 to 30 amp draw. Meaning the laptop cells are light duty so need more in parallel for the controllers demands.
 
these are mostly samsung cells and some sonys... they are good cells.... would i be better off building a battery with only the new ones even if its a smaller battery? should i just not use the cells? i thought lots of people built packs from laptop battery packs? i got the cells free from work so no big deal either way... but if its not worth the trouble i dont want to do all the work i could likely sell the cells and get money to put towards a battery... i dunno i thought with like 140 batteries i could build a decent battery i realize it wouldnt be as good if they were all matched new cells but hell even if it it only worked at 15ah capacity it was free... if its useless and wont work for my motor i dont want to do it... ive read a lot and i didnt realize it wouldnt work.
 
To discharge the cells at 1C you would need a 30Ah pack to get 30A from it
 
shr00m said:
I understand heat from soldering i was talking about this comment "And lastly lappy cells are only good for 1c discharge. You are going to be over that with that motor and those cells. So possibly cell damage and even fire while riding. ".....
Click to expand...

All 18650 cells have a maximum discharge rating. The manufactures of laptops choose cells with more capacity or lower price rather than high discharge. Laptops don't need high amperage. So 1 c is a good assumption.

So 1c x 2.2ah (good guess at your cells capacities) x 10 parallel in the battery = 22 amps max discharge. Your controller will likely pull more than that.

You could build them into a 36v pack to get more in parallel. And then your motor wouldn't pull as many amps anyway.

Not saying don't build one. Just giving you a heads up on what to expect.

Oh and don't solder them fully charged. Voltage varies with temperature a bit. You don't want to heat the cells up when they are already topped up.
 
Do some research on the C rate of different 18650 cells. Most laptop cells are 2200 mah rated at 1c. This means that you can only discharge them a maximum of 2.2 amps. In order to build a 30 amp battery you will need to have 14 cells in parallel in each of your series packs. There are higher C rated cells available but plenty of people (myself included ) have build very dependable packs out of laptop cells. The packs end up being huge because of the large number of cells but we get incredable range.
Please do your homework before you start this project.
 
wow so much wonderful information. Im gonna put it on a 2004 specialized mountain bike... hardtail but shocks in the front. I live in Fort Smith, AR ... seems i need about 40 more cells to make this pack work. wow that is a ton of cells but if it has to be done i guess thats what i will do.
 
Yes but need 25ah battery. 10s or up to 14s, but 12p for battery life. 20ah or less may be to demanding for more cycle life. I mean hills and wet sand are very battery demanding. So is starting front a stop. Peddle first is best for a light amp batteries. Plus you need a torque arm or two on that aluminum frame. Rear motor ?
 
12p for battery life? but i have a 48v motor and controller i should do 13p right? I was thinking 10s and 13p
 
If you have a 48v controller you will need to do 13s to achieve 48v and avoid low voltage cut off issues with the controller. If you do decide to go with 10s then you will need to make modifications to the controller but this is preferred because you will have 13p strings so you'll be capable of 28.6 amps, a little bit over what the controller will draw at peak power (assuming you have a 26amp controller which is common with the ebay kits). 10s will give you less speed but you won't stress the batteries as much.
 
shr00m said:
12p for battery life? but i have a 48v motor and controller i should do 13p right? I was thinking 10s and 13p
Click to expand...

You have your S and P confused.

S=Series. One after another, like links in a chain.
P=Parallel. Side by side.

13s is 48v as it is 13 cells one after the other which adds their voltage together. 13 lots of 4.2v is 54.6v. Hay... something went wrong lol (13x3.7v nominal=48.1V)

One of these chains may produce a 2.2Ah 48v pack. This would supply 2.2A if we keep discharge to 1C. This 1C term is 1 time capacity. Capacity is 2.2Ah so 1x2.2=2.2A discharge. Some better cells are 2C which would be two time capacity. Capacity is 2.2Ah so we could drain 2.2Ah X 2C = 4.4A max discharge.

So with 13 cells of 2.2Ah capacity and a discharge rate of 1C we can build a 48v string with a capacity od 2.2Ah and a maximum discharge rate of 2.2A.

2.2A is not enough. You need quite a few of these 13s packs is Parallel. This won't add the voltages together like in series. This will add the Ah capacities together. If you put two of your 13s packs in P then you retain 48v and through summation have 4.4Ah now. Each of your two series strings can still produce 2.2A so we now have 4.4A available. So, your 13s string in a 2P (thats two of them in parallel) configuration provides 48v 4.4Ah and 4.4A max discharge.

If you made 10 of your 13s strings and configured them all in parallel you would have 13s 10p 22Ah and 22A max discharge.

Yeah?
 
Sounds good. Build parallel string of equal voltage cells first then series with insulation between series string of equal voltage. No sparks or rush of energy. These cells are alive and can smoke or worst. Alive.
Plus a gasket before welding on the positive side.
 
thanks for the great info. yea i had confused s & p. i think im going to go with the 13s 10p if i have enough good cells i will go 11. what kind of gasket for tops of cells before soldering? yes its a rear motor. this specialized bike has the twist grip gear change it seems like it will be hard to mount the thumb throttle with the twists ? what type/kind of torque arm should i get?
 
built the pack this weekend 13s 11p .... took it for a test run last night works great. doesnt have the top speed i was hoping for seemed to go around 23ish didnt use a gps to test though. the battery meter doesnt dropout at all on full throttle so thats good and it seems like it will have a really good capacity i rode it around about 15 minutes and it dropped from 54 to 53.4. i built my pack by using copper wire from an extension cord... i soldered the wire across straight banks of 11 and alternated pos and neg rows after i built the pack that way i went back and jumpered in series to get to 54 i used 5 jumpers per cell i just used the same copper wire i used for the rest to do that. i used a bms on it. have not ran the pack any more than this. ideally i will likely cut the pack in half and stack it on top of each other so it doesnt have such a large footprint. but i want to do capacity / range testing before going to the trouble of doing that. any tips?
 
wow. i was completely off on my estimates on my gps tracker on my phone i just clocked 34 mph. i rode about 18 miles total and climbed quite a few hills. the battery went from 54 to 46 volts started turning off due to low voltage i suspect. all in all for free a good pack... id love to build one with really good batteries now. but wow ebikes are freaking amazing. can someone please tell me how to mount the torque arm i got one of the universals on ebay and i cannot figure it out.
 
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