building Li-Ion battery pack

[miky94]

Hi,

I'd like to build a 6S2P Li-Ion battery pack using 18650 cells but since I don't have a spot welder and solder nickel strips using a soldering iron would damage the cells I was thinking about 2 other solutions:

1 ) buy cells with presoldered nichel strips and solder them toghether using a normal iron soldering
PROS: lighter, customizable shape
CONS: BMS needed, trickier

2 ) combine case holders
PROS: no BMS needed
CONS: heavyer, constrained shape, not sure they can carry 15/20A draw per cell

I also know the vruzend solution but it is heavyer and more expensive

what do you advise me?

Thanks

 

[wafflejock]

Along with Vruzend there's NESE https://18650.lt/index.php/product/n-e-s-e-module/ people on the esk8 forum entertain these every once in a while, but ultimately having someone professionally build the packs for esk8 always ends up being the solution (finding someone with the spot welder to borrow locally or making your own spot welder are options but a bit scary either way). Personally I use two 5S 5Ah LiPo connected in series for my esk8 with a custom xt-90 based y harness with an antispark on one side to stop big sparks from inrush current into capacitors (just an issue with the esk8 escs needing big caps in case of long wires from battery to ESC). Literally just got done un-bricking my esk8 ESC, had messed it up flashing firmware over Bluetooth but the USB connection on it seems to be dead so it's the only way now, just upgraded my custom controller so had to update some settings (yay! if it weren't so cold would go for more than a few block test cruise).

 

[miky94]

Thank you for the information, it seems a better solution than Vruzend one, but still expensive.
What about the 2 solution I proposed? Would they work? Which is the better in your opinion?
On one side solution 2 is easier, I only have to solder 2 3x case holders in series and the 2 6x resultants in parallel right? they weight only 8g x 4 = 32g, and I could charge the cells separately using any 18650 charger.
On the other hand using cells with presoldered nichel strips I could integrate better the battery pack with the frame and with a BMS (I don't know how does it cost) I could charge it using my iMAX B6 mini, but there's more welding work.

 

[Al Parmer]

I use li-ions off of bad battery packs and just use good ole solder and copper wire. Put a dab of flux on the batt and solder a spot on it. Tin your wire and then solder onto battery. I use them mainly for Tx or Rx packs and work great. I wouldn’t use 18650s for flight packs for a couple of reasons. One, they are heavy; and two, they are not designed for fast heavy discharge. On the other hand they are more resilient than lipos.

 

[miky94]

I think they are lighter, a 6S1P LiPo weights about the same of a 6S2P LiIon so you can have higher capacity and also they have a wider voltage range. My quad draws only 30A so they are enough.
having only a soldering iron which of the solutions I posted do you think is better?

 

[Al Parmer]

I think they are lighter, a 6S1P LiPo weights about the same of a 6S2P LiIon so you can have higher capacity and also they have a wider voltage range. My quad draws only 30A so they are enough.
having only a soldering iron which of the solutions I posted do you think is better?

I’m confused because this is the first time I hear of someone using lions vs lipos on a quad or anything other than Rx or Tx packs. Not sure what you mean by wider voltage range. They are both 3.7 per cell unless you use LiHv. Maybe I haven’t caught up with new lion tech.

 

[miky94]

I mean that you can discharge them to a lower voltage
watch this

 

[wafflejock]

If you could find the cells you want to use with the nickel strips already attached that sounds like the best bet really otherwise would expect need to spend some time tuning a spot welder for whatever strips you get and experimenting with different strip thickness etc to get a good connection without overheating the cells (seems like a lot of variables to figure out to me).

 

[Al Parmer]

I'm pretty sure I am behind the curve when it comes to newer lions. This diagram is what I am used to.

 

[miky94]

and do you think the second solution could work?

 

[wafflejock]

Basically same feeling as you not sure they could sustain the discharge amperage through the connectors with just spring loaded contact it might be worth trying since it's probably much cheaper/easier solution but would maybe bend the contacts into the cells a bit more than how they ship just to be sure that connection is solid otherwise imagine that's where things will heat up or spark if the connection is weak. If you go that route would just be sure to closely monitor all the contact points while testing things out to be sure the tabs aren't getting roasted from the discharge current. Plus side with those is it would make checking individual cells while testing easier compared with more permanent connections.

Not sure why either of these setups would require a BMS though? typically with esk8 people will use a BMS for charge only to balance the cells while charging but then have discharge come straight off the battery main leads (BMS with high discharge capability are typically much more expensive). Personally that's part of the reason I just stick with lipos and use a balance charger for my esk8 stuff too, didn't want to be carrying all that charge circuitry everywhere with the board.

 

[miky94]

Basically same feeling as you not sure they could sustain the discharge amperage through the connectors with just spring loaded contact it might be worth trying since it's probably much cheaper/easier solution but would maybe bend the contacts into the cells a bit more than how they ship just to be sure that connection is solid otherwise imagine that's where things will heat up or spark if the connection is weak. If you go that route would just be sure to closely monitor all the contact points while testing things out to be sure the tabs aren't getting roasted from the discharge current. Plus side with those is it would make checking individual cells while testing easier compared with more permanent connections.

Not sure why either of these setups would require a BMS though? typically with esk8 people will use a BMS for charge only to balance the cells while charging but then have discharge come straight off the battery main leads (BMS with high discharge capability are typically much more expensive). Personally that's part of the reason I just stick with lipos and use a balance charger for my esk8 stuff too, didn't want to be carrying all that charge circuitry everywhere with the board.

the BMS is intended only for charging, where can I find it? does it exist specifically for 6S2P?
I'd probably use an elastic to keep the cells in position.
looking at the Vruzend kits the V1.5 components are in stainless steel and support 3.5 A continuous per cell while the V2.0 components are in copper coated in nickel and support 20 A continuous per cell so with pure nickel case holders I should be safe.
the operating temperature of the cells is 75℃ so shouldn't have problem with plastic.

 

[miky94]

but without a discharge BMS if in a 2P pack I draw 30A and cells are 20A rated, even if I should have 40A constant discharge current, if the 30A are not balanced between the 2 parallel packs they could be divided 25A + 5A or if they are wired the same the amperage is closer (16A+14A / 17A+13A / 16A+14A / 18A+12A) ?
where can I find BMS for 6S2P with JST balance plug ?

 

[wafflejock]

I'm pretty sure I am behind the curve when it comes to newer lions. This diagram is what I am used to.

Really there's a lot of minor variations in the anode/cathode material and exact chemistry in the electrolyte (https://batteryuniversity.com/learn/article/types_of_lithium_ion) Then there's different packaging standards 18650 (18mm tall 65.0mm long) being most popular and 21700 being new standard Tesla is using for their packs (used to use 18650 though too) or what people refer to as LiPo typically which are prismatic cell https://batteryuniversity.com/learn/article/types_of_battery_cells also https://scottiestech.info/2015/06/21/lithium-polymer-vs-lithium-ion-batteries-whats-the-deal/ )
Basically there is a difference between Lithium Polymer and Lithium-Ion but the packaging type isn't actually it and many times other parts of the cell effect the discharge amperage more than the packaging type. People use the 18650 in esk8 usually because you can create a low profile battery with a decent capacity and discharge by putting 4 or 5 cells in parallel and stack them 10 tall (40-50 cells total) and have about 460-500Wh battery (whereas my couple of 5S 5Ah bricks only adds up to 185Wh). Advantage with my setup is it's much easier/cheaper to get the cells packaged this way and I can easily swap out the bricks (also can charge with standard balance charger in bat-safe box and it's flight safe when batteries are separate, under 99Wh I believe is the limit).

 

[wafflejock]

the BMS is intended only for charging, where can I find it? does it exist specifically for 6S2P?

http://www.bestechpower.com/ <-- this is the go to for people on the esk8 forum they are the best quality BMS but not cheap from what I understand (I've not used these myself since like I said above I prefer using a balance charger at home and not having the extra charge circuitry getting rattled to death on my board ).

I'd still lean towards you don't need a BMS just add your own balance leads to the pack once you're done making it and charge with a regular balance charger on the ground (reduce weight in the air). Regarding discharge if the cells are hooked in parallel then from "the outside" they act as one and between the two cells they will self balance to the same voltage (it's important your cells are already very close in voltage when you put them together), the discharge amperage from each will be close to equal if the internal resistance of both cells is close to equal (you can check the internal resistance of each cell either by discharging measuring and calculating or by getting a balance charger that also reports internal resistance on cells (my AR1 from keenstone does it but not sure they sell them still). Basically if cells are matched in a P group with regard to internal resistance then the discharge amperage out of them will be roughly equal.

 

[miky94]

http://www.bestechpower.com/ <-- this is the go to for people on the esk8 forum they are the best quality BMS but not cheap from what I understand (I've not used these myself since like I said above I prefer using a balance charger at home and not having the extra charge circuitry getting rattled to death on my board ).

I'd still lean towards you don't need a BMS just add your own balance leads to the pack once you're done making it and charge with a regular balance charger on the ground (reduce weight in the air). Regarding discharge if the cells are hooked in parallel then from "the outside" they act as one and between the two cells they will self balance to the same voltage (it's important your cells are already very close in voltage when you put them together), the discharge amperage from each will be close to equal if the internal resistance of both cells is close to equal (you can check the internal resistance of each cell either by discharging measuring and calculating or by getting a balance charger that also reports internal resistance on cells (my AR1 from keenstone does it but not sure they sell them still). Basically if cells are matched in a P group with regard to internal resistance then the discharge amperage out of them will be roughly equal.

if I understood I can balance charge using my iMAX B6 Mini just by soldering a JST-XH plug to the pack? can you explain me how the wiring should be in a 6S2P?
and are there other Li-Ion standards with better quality/price to get 22.2V, at least 5000mAh capacity, at least 30A constant discharge current?

 

[wafflejock]

Yup usually when using a BMS people end up soldering on their own balance leads as well, basically they just go to each parallel group of cells so your balance charger can read the voltage of the group as a whole (it basically treats each P group as a cell) https://www.electric-skateboard.builders/t/battery-wiring-diagram-help/39011/4 <-- if you use the magnifying lense/search in the top of that page can find a lot of examples of builds and diagrams
https://www.electric-skateboard.builders/t/please-review-my-battery-pack-so-i-dont-make-a/70102/13

Wouldn't hurt to actually join that forum too and just post your battery build as a thread with context that it's for a quadcopter but looking for people who know their way around building a good battery. Also be sure to search and read there first or people will get on your case there's a lot of good info already out there but if you post your actual components and pics of build then you'll get good feedback.

Regarding which cells to use also good to search the forum there for places to acquire known good cells and occasionally threads pop up with deals on cells, believe most people go with Samsung 30Q on esk8 but requirements there are usually high capacity somewhat high discharge, but weight doesn't matter so much (usually the motor is doing the pushing, for myself weight matters more since I take it on the train sometimes and don't like lugging 20lbs up the stairs).

 

[miky94]

Thank you I'll ask in that forum
I found cheap HG2 cells on AliExpress which suppliers told me they are not genuine but with same specs, can I trust them or where do you advice me to buy them?

 

[wafflejock]

Thank you I'll ask in that forum
I found cheap HG2 cells on AliExpress which suppliers told me they are not genuine but with same specs, can I trust them or where do you advice me to buy them?

Kind of depends on the volume if you can buy a couple and have a multi meter to test with and/or balance charger that can measure IR ( or separate IR tester ) or can just buy a handful for cheap enough then probably worth the gamble but with esk8 people buying 60 cells at a time don't usually take the chance.

 

[miky94]

Kind of depends on the volume if you can buy a couple and have a multi meter to test with and/or balance charger that can measure IR ( or separate IR tester ) or can just buy a handful for cheap enough then probably worth the gamble but with esk8 people buying 60 cells at a time don't usually take the chance.

how can I check specs using IR value? the iMAX B6 Mini measures it but which value should I get?
If seller guarantees me the specs I'll try and if it is not I'll ask for refud