DIY 18650 Pack: Can i mix cells?

[Storx]

I am doing research on how to acquire the batteries to build my own battery pack for my ebike conversion and im debating between getting a ton of used laptop batteries and extracting all the cells and building it from those or buying all of the same battery type and build from there...

So my question is can you mix and match different brand 18650 cells when assembling the pack?

Im looking to build an 36v 20ah pack, so im guessing around 100 cells needed..

 

[dogman dan]

Yes, but ideally put the same mix in each parallel group. so say you have 3 of cell a for every 1 of cell b. Put 6 cell a's and 2 cell b's in an 8p group.

This is after you weed out the crappy, worn out, high resistance cells of course.

 

[DrkAngel]

Banks should be built of equal IR and capacity.

Equal numbers of each brand cell in each bank is a good starting or finishing point ...
but ... all cells must be evaluated and rated.

Used cells: ... new cells too.

• Test for
• Self Discharge
• IR (Internal resistance)
• Capacity

Don't make the mistake of separating all cells.
Much easier to test and build if left in oem pairs etc.

See -Homemade Battery Packs
and
Bulk Capacity Testing

 

[Storx]

thank you for that info, because i was thinking of ordering like the lot of 25 untested laptop batteries off ebay and strip them down, i have called around to so many places around me asking for laptop batteries and they all have locked bins now for disposal of them, so they cant access them once they are dropped off.... just sucks, because so many people have reported getting loads of them for free...

 

[Syonyk]

Your time is free, right?

Otherwise it's not worth shelling and testing laptop batteries. They're low C-rate batteries that aren't well suited to ebikes.

You can mix and match cells, but you're likely to endlessly fight with the pack, and you really, really should be using a BMS with per-parallel-group low voltage cutoff if you're going this route.

 

[999zip999]

First where do you live ?
Used cells why waste your time. So many good cells out there now and batteries. Do you have a spot welder and lot's of time to test and sourt, balance and test for internal resistance on each cell before assembly. Don't use a solder iron.

 

[Storx]

Well i managed to get lucky today, after calling around to all the locations i was told to find used laptop batteries from for free i got lucky without even trying.. Was dropping in to home depot to pick up some led light bulbs and i noticed by the customer service center they had a sign that said recycle your old batteries here.. so out of curiousity i asked them if there was any way i could have a few of the old recycled batteries... the manage at the counter didn't really know what to do and just said sure.. so i was asked which ones i wanted and walked to the back where they had a bin where they toss them.. to my surprise out of about 70 batteries in the bin 10 were laptop batteries and 2 were cordless drill batteries that said Lithium Ion on them, so i grabbed them.. the rest were mostly NiCd and a few were just ancient batteries.. so i took the 12 home and this is what they broke down to after about an hour of work...

 

[Syonyk]

How are you planning to test them?

 

[Storx]

Ok, after a lot of calling around to get used laptop batteries to salvage, i managed to get a few laptop batteries and 2 cordless drill batteries from home depot out of their bin...

Now that i have the cells, all but 3 test above 3.0v on my multi-meter with the red colored ones being the lowest batteries overall.. most just barely over 3.0v, the 3 that didnt have 3.0v were down in the 1-2v range...
I dont own a tester yet to determine their capacity, or a means to charge all of them up, so i figured i would ask you guys what test/charger/balancer combo should i purchase, seems that most use the imax b6 off ebay for $20...if there is something similar that would allow the balancing of 13 strings instead of the 6 the imax can only balance that would highly benefit me since that is the amount of strings i plan to build the pack out of...

Also, if the Imax is my best bet for charger, i was thinking of picking up 2 of the 3 cell 18650 battery holders and solder them up to the balance leads on the imax to charge the cells 6 at a time..

I am aiming for an 48v 20ah pack, so i know im going to need to gather a few more packs to salvage from to meet my goal... i was thinking if the capacity is over 2000mah i should go ahead and keep the cell to be used in the pack?

Also when i go to assemble the pack, i was thinking of using pico fuses like a few others used.. is that the best option or are fuses really needed for an ebike for each cell?
http://www.ebay.com/itm/QTY-100-1A-125V-PICO-FUSE-VERY-FAST-ACTING-LITTELFUSE-R251001-AXIAL-FUSES-/111703999329?hash=item1a0213c361:g:-X8AAOSw3ydVjL8U

If i can't find an charger deal similar to the imax that can support 13 strings, i was thinking of using this bms i seen someone else use along with an 48v charger...
http://www.ebay.com/itm/13S-48V-Li-ion-Lithium-Cell-20A-18650-Battery-Protection-BMS-PCB-Board-w-Balance-/331735442078?hash=item4d3cf9229e:g:YqoAAOSwHPlWczLz

Here are some photo's of the cells i managed to get today, i have a feeling the red ones are rewrapped, because there is a different inner color on them and the markings are weird looking, i am a little unsure on the blue ones, but the rest seem like legit brand name batteries, the panasonics i got out the one dewalt drill battery i took apart.

 

[Storx]

How are you planning to test them?

that is what im looking into, i know i can use an Imax B6 since that seems to be a very common one, i have just been looking to see if there was one available with higher series balancing to possibly duelly used as a charger later as well.. The plan is to buy 18650 parrellel cell holders for a few bucks to max out the amount of batteries i can charge at once during that phase, then i will use the tester to test the capacity and then mark it on the cell and put them away tell im ready to organize each string so they have similar capacity among all strings....but the more i think about it the more it seems just a single cheap tester is fine for me.. yes it will take longer to test all of them, but using an actual bms circuit card along with a generic lithium ion power pack seems better....

But then i found an 8 cell tester for $40 bucks.... that i may invest into 2 of them and then i can use both at the same time during the testing phase as well use them duelly as the charger later on by just changing my battery from 13s to 14s from 48 to 52 volts..., but i have not made up my mind..

An 13s 48v BMS is $13, then a 48v lithium charger is $30 for a total of $43 bucks for BMS and Charger to do all strings at once at a much faster pace... so its $80 vs $43 right now on if i want 2 testers to speed things up...

Right now the chargers that seem to be super popular are these, both can only balance up to 6 cells at once, i really wish they did 8, because that would make life easier...

Imax B6 (6 series) $24.99
http://www.hobbyking.com/hobbyking/store/__5548__IMAX_B6_50W_5A_Charger_Discharger_1_6_Cells_GENUINE_.html

HobbyKing Eco 6 (6 series) $18.99
http://www.hobbyking.com/hobbyking/store/__11060__HobbyKing_ECO6_50W_5A_Balancer_Charger_w_accessories.html

Turnigy Accucell-6 (6 series) $26.82
http://www.hobbyking.com/hobbyking/store/__58457__Turnigy_Accucell_6_50W_6A_Balancer_Charger_LiHV_Capable.html

 

[Storx]

Well i made my decision and placed an order for an Imax B6 Mini Pro V2 w/ 108watt, 17v, 7a power supply for $45 off amazon... they will be here on wednesday

 

[DrkAngel]

Should have left cells attached in pairs!

Half the work and nice safe solder points ...

Testing a pair is just as easy and effective as testing a single.
If pair quick tests as poor ... then cells can be separated and tested for 1 bad?

Soldering to the spot welded tabs is easier, simpler and safer than soldering to cells!
You could spot weld tabs to cells ... but you just ripped of the tabs that were already spot welded to the cells?

 

[Storx]

Should have left cells attached in pairs!

Half the work and nice safe solder points ...

Testing a pair is just as easy and effective as testing a single.
If pair quick tests as poor ... then cells can be separated and tested for 1 bad?

Soldering to the spot welded tabs is easier, simpler and safer than soldering to cells!
You could spot weld tabs to cells ... but you just ripped of the tabs that were already spot welded to the cells?

Its ok, im learning here.. this pack may be my initial budget pack tell im able to source better cells for a better higher quality pack in the future.. either way i have only about $55 into building this pack and that includes gaining a tester (Imax B6 Mini pro v2), so if i leave that out of the pack cost... the 200 cell spacers i ordered was $18, and the pico fuses was another $8 for 100 of them, so i have around 26 dollars in parts currently in this pack and all i really need to finish it is to find more cells and test them out.. so $26 for my initial 20ah pack is an ok learning experience.. i have a good month before my ebike kit will arrive due to chinese holidays.. so i have time to test some of them and go from there..

 

[999zip999]

Pico fuses ?

 

[Storx]

Pico fuses ?

Ya, they are cheap 4amp fuses that are found on the tesla battery packs, so a lot of diy battery builds i read about use them for using each cell, i got 100 for $8, so they are not all that expensive and you can use the fused connection as the solder link between strings...

 

[Syonyk]

Don't solder directly to the cells.

 

[DrkAngel]

4A fuse will be blown with <2C discharge!
Which will cause a cascade failure disabling entire pack.

 

[Storx]

4A fuse will be blown with <2C discharge!
Which will cause a cascade failure disabling entire pack.

Could you explain this more plz, i only ordered them because tesla uses them on their 18650 cells and many others.. why would it be different for my setup?

 

[Storx]

Don't solder directly to the cells.

Why do people keep bringing this up, yet many solder the connections to the batteries without issues? am i missing something?? i just have to make sure i dont leave the soldering iron on the cell very long, to try and flash solder it... am i wrong?

 

[Syonyk]

Why do people keep bringing this up, yet many solder the connections to the batteries without issues? am i missing something?? i just have to make sure i dont leave the soldering iron on the cell very long, to try and flash solder it... am i wrong?

Well, it "works" in that it makes an electrical connection to the cell and the cell survives afterwards.

But many, many data sheets specifically say, "Do not solder these cells, spot weld only."

Soldering dumps a lot of heat into the ends of cells that are not designed to be heated that much. The damage you do is pretty much unknown at the time, and will depend on the cell internal design, electrolyte, chemistry, etc.

And then there's the question of consistency. Hand soldering is less likely to involve a consistent amount of heat into each cell.

*shrug* Do whatever you want. If you're going to mix and match a bunch of scrap cells, you probably won't make them much worse with soldering.

 

[bikegeek]

But if you cool the cells in the refrigerator to about 4-5 degrees Celsius and you solder them I think will not be damaged that much or at all. But it is just a theory

 

[Skrzypas]

I've never met the darker blue (with barcode), I suspect these could be rewrapped or unknown brand.

Red ones are Sanyo. Very good ones.

 

[Storx]

But if you cool the cells in the refrigerator to about 4-5 degrees Celsius and you solder them I think will not be damaged that much or at all. But it is just a theory

I may actually try this if it's actually safe to do... ive talked to a few people who did soldering on their packs and none of them reported any issues with soldering, Rinoa Super-Genius from youtube recently built another pack for his ebike 48v16ah pack and after talking to him, he told me when he tested the imax b6 on all the cells that went into the pack, he had a xx,xxx number, after creating the pack and actually riding with it for 2 weeks before doing a capacity test on the entire pack assembled, he said there was less than 90mAH of difference between the before total cell capacity added together compared to the test result on the overall pack. I also talked to 2 others who soldered their packs together, one stating he has soldered 1000+ 18650 cells without an issue outside of the solder mistakenly running to the side of the positive end and shorting it to the negative side.., but he has since resolved this issue by installing cheap hole punch repair stickers on the positive ends of the cells...

 

[TheBeastie]

But if you cool the cells in the refrigerator to about 4-5 degrees Celsius and you solder them I think will not be damaged that much or at all. But it is just a theory

Rinoa Super-Genius from youtube recently built another pack for his ebike .

I watched some of these guys videos on the topic and I am a bit dubious that a lot of his repeated videos on this subject are about getting video hit clicks then actually being a practical way to build an 18650 pack imo. Considering how complete DrkAngel threads are full of information and complete with easy to find indexes maybe this path isn't for u?
Rinoa clearly joined the hoverboard exploding video for hits game as this video here you can see the large wires protruding from the hoverboard as he deliberately overcharged it to make it burst into flames/explode https://www.youtube.com/watch?v=1PlEPCjMdbM
I have built two 18650 battery packs with no solder or welding methods but I always built them with new or practically new cells.

 

[Lurkin]

Why not learn to do it properly? A JP Welder off here is pretty inexpensive for what it is and you have a decent number of cells to practise spot welding on.

Perfecting soldering to cells is like perfecting polishing a turd.... an unnecessary skill and a potentially damaged product before you start using it.