First Batterypack build, some beginner questions

Hi all!

Im trying to make my first pack from used laptop cells.
Its gonna be 36v for a 350w ebike. Now a dumb question: how do i calculate how much cells i need in paralell for that? I=P/U? Would mean about max 13A draw depending on the current voltage? Am i calculating this right? I cant figure out what im doing wrong.

So i got this charger for testing: "AccuPower IQ328+ "
I started testing with 500mAh charging and 300mAh discharge. First it charges to 4.2V than discharges down to 2.8V and then charges to 4.2V again. Only at the end u can see the discharged and charged capacity. And somehow strangely the discharge capacity is mostly about 50-100mAh higher than the charged capacity. how can this be?
I already tested about 20 cells like this and there were only one or two where the discharge capacity was lower than the charged capacity.
I tested some with 700mAh charge and 500mAh discharge, but same resuts. is this normal? can i just ignore that?
Did i get a bad charger? And also this charger heats the cells up so much, i can only figure out a cell is bad if it is already kinda burning hot.

Anyway to speed up the whole process i started testing 2 cells in paralell in one slot from the charger as they came out from the laptop pack in a pair. But now if i wanna put them together with all the single cells i started testing with 500/300 charge/discharge i now test a pair of cells with 1000/500. which would mean each cell of a pair is tested with 500/250. Is this close enough to put them togeher in a pack with the single cells i tested with 500/300? And i thought i will just let these pairs togeher in paralell in the new pack too. so it shouldnt be a problem right?
i guess i just cant go now and test all my remaining single cells with 700/500.
500/300 is really slow..
so i should really have the same testing conditions for all cells to put them togeher in a pack?
I guess i should have just gotten 2 opus for 20 more with a cooling fan, but i didnt concider that.
I just got this charger fast local.
Oh yea and all the pairs i tested with 1000/500 had a higher charge capacity than discharge capacity. thats how it should be i guess?

And then i have another question about charging.
Lets say i have 10cells in paralell. How can i tell what will be my max charging current? I look at the specs of all cells and go with the lowest *10 ? So for example 1,6A*10 would be 16A.
I guess 1A per cell would be still fast enough and it would extend the cell life.

But now what charger do i get that can balance charge a 10s battery with at least 10A? kinda safely?
I didnt really find a 10s charger so i tought: cant i just get two 5s chargers? And like the middle of the batterypack is the Plus for one charger and the Minus for the other charger? Right? Doas it work like that?
I hope u understand what i mean i dont really know how else so describe it.

Here is what i found maybe u can recommend me something better:
Either two of these: "Turnigy Accucel-6 80W "
Or this one: "Turnigy TQ4 4x6S " alltho this olny can do 6A max.

And i dont really wanna get a BMS cause i wanna have the option to make more different sized packs without having to buy a BMS for every single pack. And also ubalance charge to 4.1V with all these hobbychargers.

Also what is the max temprature a cell should reach? 60 degrees? Wouldnt it be good to have a temprature sensor with alarm at a certain point? I found this: "MagiDeal W1401 24V Digital Thermostat Temperature Controller Switch Sensor Module "
There are just no instructions or a manual but it looks like what i would need and cheap.

Sorry for all this basic questions, i researched alot but i couldnt really find answers for all this stuff.
And i dont really know where to put this thread i got questions about everything kinda. But atm my biggest issue is how to correctly test all cells and having no concerns about putting them together in a pack.
And then how to build the pack? I tought about something like this with magnets: battery pack 18650 cells. No soldering to the cells only magnets
If i have a draw of 10A or less for 90% of the time and 10cells in paralell would mean 1A per cell. Shouldnt be too bad or? Worst case ill just lose some energy at the connection at higher currents i guess.

Well i hope this aint too much questions at once and u can help me maybe and tell me if im doing something wrong.

Thanks!

Ill try to answer to the best of my own 'limited' knowledge
Someone please correct me if I'm wrong

Firstly, regarding your charge/discharge capacity testing issues, that would be best answered by someone else as I do not use any off the shelf cell tester.

When you are testing the cells in pairs though be aware that one of the cells may be actually be problematic and the other good cell may be masking the issue to some extend. That caught me once before. Especially for your high drain use-case I'd opt for individual testing no matter how tedious it is.

Regrading charging in parallel, I believe you are correct in saying that if the lowest maximum charge rate in the pack of 10 is 1.6A then a charge rate of 16A is technically fine but yes, Id recommend charging slower to prolong cell life.

I have never thought to wire two 5s chargers to a 10s pack. While I could see it possibly working, the imbalance between the two chargers and the fact they have no communication with each other may make the first 5 cells imbalanced with the last 5.

You say you want to make different size packs? If your talking about different number of cells in each parallel pack to equal out the capacity of all of them then thats fine but if you go making parallel packs with different capacities then the battery will only last as long as its smallest pack

Cell temp. Do they get hot when charging or discharging. Some say the heat is just the electrolyte reforming after long periods of inactivity. Charge and discharge hot cells a few times and see does it calm down. Personally, I bin them if they go over 45 degrees Celcius after a few cycles. 60 degrees is dangerous, be careful.

Do not build the pack with magnets, definitely not with a high drain pack. Solid connections are best whether spot welding or soldering, whichever your more comfortable with but spot welding is considered better for the cells health. Fuse wire is also the way to go. Its going on a moving object so you want them solid. Imagine low-siding or crashing and one of the magnets becomes dislodged and shorts the battery. Bad day made worse

Finally, to find how many amps the 350W motor will draw, you divide the wattage by the voltage (be aware of this value for not only nominal voltage but also charged 42v and discharged 30v)

How many cells can you fit into the bike?

A 350w nominal e-bike would draw a nominal 10A current. However, it could easily ramp up to 20A draw. That said, since laptop batteries are low discharge rate batteries, I only use new power cells for e-bike packs. Unless you have a way to fit 200 cells on the bike, which would make a 10s20p battery that would easily handle 20A of current.

Also, as an added bonus to the high-amp draw of e-bikes, you almost definitely should use a BMS, even one of the $10 ones.

Also, the majority of laptop cells I've harvested have a max discharge of between 3.5A and 4.5A. You could make it a 10s10p battery if your caught for space so you'll be drawing a maximum of 2A (peak), 1A (continuous) from each cell but as thanar mentioned, 10s20p would be better if you have space for 200 cells.

However, I think you could get away with 10s10p

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thanar said:

Also, as an added bonus to the high-amp draw of e-bikes, you almost definitely should use a BMS, even one of the $10 ones.

Click to expand...

I have a question about this too. Since its a high drainuse-case that involves frequent charging, would a BMS really be required if such regular top-balancing is done and the packs are matched well? Im thinking of Jehu's BMS-lessvan

hm i thought a BMS is only for balance charging?
I dont get it why should i need a BMS? It has the same function as a hobbycharger or not? both just balance charge?
And also u can balance charge to 4.1V with all these hobbychargers. Cant do that with a BMS.

And about loading a 10s pack with two 5s chargers i could just do it like he did: How to build a safe and better lithium battery WITHOUT a BMS.
I guess u just cant let the two 5s packs connected in series while charging.

So noone has ever experienced measuring a higher discharge capacity than charge capacity of a cell?
Should i be concerned about this?
I mean normally i dont even have to know the charge capacity

And about paralell charging: i got at least 60 of these pink samsung 26c. and none of them had voltage drops or any sign of beeing faulty. highest voltage drop of a pair was 0.4V in 2 weeks.
Im feeling preety safe about paralell discharge testing them. And pretty much all pairs have about 4600-4900mAh.
I guess if a pair has less than 4200mAh ill may should take it apart and test each cell. But still even if one cell from a pair is kinda faulty, both together in paralell will always have the same capacity. So if i never seperate them and put them in my pack in paralell too i feel like it should be no problem.
If ur interested i saw this video about paralell charging: Testing 18650 batteries in parallel Opus 3100
And some others where they tested them in pairs and seperated but i cant find it right now.

My only concern is that when i test a pair with 1000/500 charge/discharge and a single cell with 500/300 can i put them togeher in a pack? Its not really the same testing conditions.
I cant go and test half my cells with 700/500 and the other half with 500/300 and make a pack out of them?
I guess there would be alot of difference.
But in my chase u could say one cell of a pair is tested with 500/250 and a single cell i test with 500/300. So only 50mA difference at dicharging current. I dont know if i should put them together like this.
Im sure i could just make a pack with only pairs.


So this 60 samsung cells would be already enough to provide the max of 20A like thanar said.
But if i was already calculating it correct, then i have the most current when my battery has the lowest possible voltage. which would be 28V i guess. 2,8V per cell at 10 in series. So I=350/28=12,5A maximum.
So how would 20A be possible?

And yeah my plan is to have about 10 cells in parallel. Alltho i could build atleast a 10s20p. But i still have so many to go through. And id rather make two 10s10p packs. Wont need a too ridiculus charger for that.
Hmm yeah and im thinkingabout getting a BMS but id need to get a good one that can do about 10A if i dont wanna sit forever with my 10p pack. And yeah it would be more comfortable charging with less wires, easy charging setup.
But i kinda like the idea of beeing abe to charge all different sized packs. And if i wanna make a 12s pack someday i dont need too buy another BMS. Just would need to get a 6s 10A Dual balance charger.

If you balance charge (charging with many wires coming off the battery) then you don't need a BMS for charging. What about discharging then? What if a parallel set drops under 2.5V? It would get damaged. The BMS won't allow this. Also, the BMS will protect against an external short. You can easily find cheap BMS for even 100A draw. Cheap BMS don't balance charge, they sit there just for the sake of it, ready to disconnect in case of under/over voltage, short or over current. Your 350W motor can easily draw more than 20A, trust me. If your cells are not capable of providing the current, don't expect too many rides off of them. I also have noticed that on most e-bikes, power drops as the battery voltage falls, so it's not constant power at 350w, rather constant current at whatever value makes up 350W nominally.

Or how about i make two 5s packs with the same capacity and use a5s hobby charger that can put out 20A and use a paralell charging board? So each pack will get 10A i guess. And after charging i put them back together in series. This really looks like the cheapest way.

If you get the type of BMS units that can be set (or make your own), you can set the Low Voltage Cutoff, High Voltage Cutoff, High Amp Cuttoff, Temperature Cutoff, and many other settings.
The cheap ones on eBay do not have these functions, correct. But if you are willing to pay $50USD (or there abouts) or more then you can get a BMS that is configurable.

Making 2 5s packs that you connect in parallel (that would mean each of the series connections are paralleled with the other pack; ie, 6 wires connecting each pack together), then you could use a 5s balancer charger and be fine. That would make all cells be in balance with each other. It would just take twice as long to charge. But, would be safer overall.

Am I right in thinking that in any paralell configuration there could be a dead cell and the BMS wouldn't even notice as the other cells are still reading normally ?
If so 10P has an even greater chance of not really being balanced by a BMS.
I would seriously think of going down the 2x 5P route and wiring the 2 packs in series and charging in paralell, just get a Y cable made up.
I'm just about to increase my 36v 5P pack to 48v 5P. Laptop cells averaging about 2150Mah, no BMS. will check each cell first to see if any are bad, Ive charged this about 20 times so far.
( supposedly 500W motor ).
The aim of going to 48v is to see what difference it makes in torque / speed. If successful I will be buying Samsung 29E or similar from Nkon and I guess a BMS too and do it all properly.

A friend gave me a 24v Silverfish case with LiPoly inside, after an hour I got them out and amazingly 14s 4P will fit buy may have to go with 13S to get a BMS in aswell

Yes, the more cells in parallel, the harder it is for the BMS to determine here the issue is.
However, if there's 1 bad cell in a parallel bank, and the BMS is properly watching and reporting the status of all the banks in parallel, it will be easy to spot which section is out of balance.
Then, you can take that pack out of service and do some diagnostics to find the problem cell.

The problem with splitting the packs into 2x 5p as opposed to 10p, is that there is now needed to be twice the hardware for monitoring. This directly equates to twice the cost of materials for monitoring.
On a small setup, this might not be an issue. But as this scales up, it can be potentially the back breaker for the overall cost compared to staying on the grid.

thanar said:

A 350w nominal e-bike would draw a nominal 10A current. However, it could easily ramp up to 20A draw. That said, since laptop batteries are low discharge rate batteries, I only use new power cells for e-bike packs. Unless you have a way to fit 200 cells on the bike, which would make a 10s20p battery that would easily handle 20A of current.

Also, as an added bonus to the high-amp draw of e-bikes, you almost definitely should use a BMS, even one of the $10 ones.

Click to expand...

I don't use a BMS on any of my batteries, I construct my packs to be electrically two 7S8P battery pack with two main ports and two balance connectors coming off of one single battery pack. When I plug the pack into my ebike, I take the two 7S packs and connect them in series. For charging, I connect both batteries in parallel.

I would suggest OP does the same for his battery pack. Your pack is only 36v, so build two 5S battery packs and connect them in series for discharging. Then connect them back into a single larger 5S pack for charging with something like an ebay imax b6.

The reason I don't use a BMS on any of my packs is because I use salvaged power tool cells or lipo batteries. Both these types of batteries are very easy to go out of balance, and a standard BMS will not be able to balance the pack back after a high discharge cycle like an ebike.

To make sure your pack doesn't get over discharged, connect one of these to each balance port and they beep really loudly when one cell goes below 3v:

https://www.ebay.com/itm/2pcs-RC-Li...427279&hash=item4d719896e0:g:vFkAAOSwDGhazwAk

If OP wants to use laptop cells, for his load they absolutely can. Older laptop cells don't handle high current very well, but most newer laptop cells can actually discharge saftely without toomuch sagat about 1C.

I would suggest OP goes for a 36v, 20AH ebike battery if they want to go the laptop cells route. This will require 10 cells in parallel and 10 in series for a 100 cell pack. If OP assembles their packs like I do in an offset triangular pattern, it shouldn't be that big. Currently I have plans to build a 168 cell 24S8P 84v 14AH battery pack made of hoverboard or power tool cells to power a 3000w hub motor, so I need power.

I will be pulling about 50 amps form this pack continuously, so I need 18650 cells that can do 20 amps each to keep the heat down.

It has been very hard to find good cells like that, but for ebikes under 500w, laptop cells can work totally okay. For my situation where I may be asking 7-10 amps per cell, I need high drain cells.