Charging system & Build of 20s7p 18650 pack. Need assist pls

[RageNR]

I am looking to build a 20s7p 18650 pack. Would greatly appreciate if some of you with experience with a pack of this size could give some guidance.
This pack will be build from Tesla 18650 cells provided from a member her on E-S that is selling them from a removed Tesla power pack.

My criteria:
- 20s7p - 84v @ 4.2v charged (72v Nominal @ 3.6v)
- Will be charging to 4.1v quite often to extend life. 82v
- Tesla cells are capable of supercharging. 50% in 20mins, 80% in 1hr. I would like to be able to take advantage of this when needed.
- 2.5hr charge time to 100% is ok for me when supercharging is not needed.
- I need a home charging station, as well as something portable to take along. Prefer a faster charging solution when away from home base.
- Portable solution should be as compact and light weight as possible. Will only be used on occasion. Home base charger will be used 90+% of the time.
- I want something that will balance these well and provide a long service life.
- Prefer one controller that can balance them all at the same time. 2-3 controllers are also ok if that is what is needed.
- Cheaper the better, but I do not want the cheapest quality gear. I will spend a few extra $$ for good equipment.
- 115v equipment. I do not what to have to hunt down 230v.
- One beefy powersupply for home base would be great
- I am not sure if I want an internal BMS. I am pretty attentive and plan on watching the pack closely while in use. But I am not entirely against a BMS.
- Robust connectors. May be carrying batt pack in a backpack and having the connection (breakaway?) hiding just under the back of the saddle. Read notes below...

Before deciding on the 18650 pack, I was considering 3x Multistar 20,000mAh 22.2v 10C packs from HobbyKing. I may buy some of those eventually anyways, so I would like the charging setup to accommodate those as well (unless it will cause a large deviation from a simpler setup just for the 18650 pack)
With my original setup, I had planned on using the 3x Multistars in a backpack. That would have been roughly 16lbs.
I expect the 18650 pack to be in that region. Not quite sure if I will be using them in a backpack or not. I may try to configure the pack in such a way that it will work mounted on the bike and work ok in a backpack. Once I get my hands on all the cells, I'll experiment with placement.

If anyone would be so kind, could you please advise on pack design? A 20s7p will be 140cells in total. That should make the wiring setup have stings of 7cells x 20strings. Is this a good idea for cell balancing?
I would think smaller clusters would allow better balance control. Are there recommended string configurations that yield better output/charging?
These cells I will be buying will be bare. If I am going to build a pack, allow for 1-2mm spacing between each cell, and make a rigid structure, do I still need to wrap them? Liquid or air cooling is also something I am considering. Comments?



So on the pack itself. These are the Panasonic Tesla cells. The ones the E-S member are selling are from a 2015 Model S (was a 2014 model before, he ran out of stock).
According to the information I could dig up online, these 2015 cells should have a capacity of 3,600mAh. But for the sake of error margin, we will claim 3,000mAh while discussing the pack build.
These Panny cells should be able to sustain 10a discharge for at least 10mins. Test show they can handle 15a bursts without damage.
So at 7p, that should be a solid 70a discharge rate. Say the Cyclone is pulling 50a on avg at high speeds. That would pull ~7.15a per cell correct?
And as mentioned before, we are going to claim these have 3,000mAh. That would be 21Ah total.

Here is where I am confused. How do you calculate the Wh of the pack? Do you use the Nominal voltage or Fully Charged voltage?
20s @ 3.6v = 72v, so (72v x 21Ah) = 1440Wh
20s @ 4.2v = 84v, so (84v x 21Ah) = 1764Wh ----------- Which one is correct?

Then you consider that these cells can be discharged to 2.5v regularly without much sweat. I don't quite understand what the actual Wh rating should be. Seems it can vary greatly depending on how you use the pack.
And then there is the voltage sag when under load. I expect a sag to 3.65-3.70v @ 7amps._______ 3.65v x 20 = 72v. That is right in line with what the stock Cyclone controller supports. Obviously the sag will not be this low at all times, but I figure that the most stress on the internal controller components will be at high states of power use. And that voltage sag is assuming fully charged to 4.2v/cell. After the initial drop, it will continue to fall with use.

Is there anything I am missing here. This has been a lot of knowledge to absorb in the past week. Doing my best to know what I am working with here.

 

[t_tberg]

Do you have a spot welder? Get a BMS and a bulk charger. You'll have 1440Wh. Cells below 3.6v have very little capacity left, maybe a 8%, if 2.5v is the cut off this is under load, I don't think I've ever seen my cells sag that low but then again I have a 20Ah pack and a 26Amp controller however it is a cheap aliexpress battery; I never run my pack down all the way so i guess I don't ever really see a true low voltage. You wont have to cool your batteries, if you do your over-discharging them. Hot glue is a better conductor of heat than air so you should probably fill up the empty space, definitely insulate the pack!! You don't want exposed terminals, if you leave it bare you're asking for a short. Also Dogman will probably poke his head in to tell you that you need chafing protection if you mount it on the bike. This is definitely a good idea, heed to his advice. I'm no expert on battery building but I would imagine that you would want to test all the cells first (discharge test, self-discharge test, internal resistance) and group all of the questionable cells together in a parallel group so if one cell does have a problem down the line it will only ruin the cells in its parallel group (I think).

 

[RageNR]

Thanks. I didn't know you replied. Still figuring out how this forum works.

No I do not have a spot welder, but I am going to make one for this project. Seems fast and easy enough to make.

Definitely will be making a hard shell for the pack as a whole. It has to be versatile enough to hang on the bike or carry in a backpack. No exposed connections.
I just don't like the idea of a BMS. As a trade worker, I've seen so many good batteries thrown away due to failed BMS (looking at you Makita...). So if I can stay away from that, I will.

As for testing the cells, these are right out of a Tesla pack. Tesla runs a death grip on those poor 18650s. Indeed I will test them before, but they should all be within less than 1% of another.

 

[t_tberg]

A BMS isn't necessary. I use raw lipo for one of my bikes. If you carefully charge and discharge and keep an eye on the voltages you should be good. For some reason when I think 18650 I think BMS, probably because its what all the plug and play packs use. I guess it's not very different from using lipo without a BMS, it is important that your low voltage cutoff (LVC) is properly set but if you're getting a 74v controller that should be no problem. Post some pics of the battery build. We love pictures around here, it's not hard--just search jpeg (or whatever file type) compressor in google and compress your pictures and then upload them as attachments.

 

[parabellum]

So, Let me readjust, I understood you wrong in the other tread apparently. You need a 72V nominal charger for 7cell (21Ah at 3Ah cells) parallel pack, 20 serial blocks of 7 paralleled cells?
If yes, look here https://bmsbattery.com/13-ebike-charger-ev-charger 128-139USD and you pretty max out 110V socket.

 

[RageNR]

So, Let me readjust, I understood you wrong in the other tread apparently. You need a 72V nominal charger for 7cell (21Ah at 3Ah cells) parallel pack, 20 serial blocks of 7 paralleled cells?
If yes, look here https://bmsbattery.com/13-ebike-charger-ev-charger 128-139USD and you pretty max out 110V socket.

Yes, correct. BUT, I am going to configure the cells in such a way that I can mount them on the bike, OR carry them in a backpack. For the majority of the time, I plan on using them in the backpack.
So with that in mind, I might split the entire pack up in 2 or 3 sections to make it work. That is not for sure though.
Click on the build link in my signature to see the bike and spacing I am working with.

That link for BMSbattery, those are just power supplies right? I still need a way to balance and monitor them. Looks like the best, most compact solution at this point is going to be an onboard BMS.
And if I am to use a BMS, it has to be one that can play on the strengths of the Tesla cells. That means a BMS that is capable of keeping up with up to 105A and voltages down to 2.5v (voltage sag under load).

 

[macribs]

I think what you want is an Adaptto controller with their BMS. Then you can have it all with only one charger.

If you are in the market for a portable charger then it seems counter productive to use chargers like the iCharger301B etc. Because you can not charge your complete battery pack with those chargers. If I encoded your post correctly you would like to have "one the fly quick charge" for when you are out riding? Sort of a safety net in case you run out of juice. To top off your pack you need a 20s charger.


If you use adaptto controller and bms you can use a PSU to charge. The more power the PSU can put out the faster you will charge.
Look at PSU like ie Eaton APR48-3G. Reason I mention the one specific is that it is 1800 watt of charging power, yet the size is relatively small so you could maybe even build the PSU into you bike, like some has done. Then you just need a power chord in like those on PC.

If you are doing a long trip so you need to charge messing about with those hobby chargers will be PITA and so slow to charge.
With the adaptto and Eaton APR48-3G combination you are golden, and if you can manage to fit the PSU onboard the bike you don't have to worry about charging as it is always with you. And you won't need a second charger.

 

[parabellum]

That link for BMSbattery, those are just power supplies right?

No, this is a fully functional CC/CV charger, you only need to add BMS to the battery, if you want some protection and automatic balancing.
1) In this case your system will be modular and you can use any controller, any BMS, any CC/CV charger that meets your voltage.

2) Other, but kind of extravagant VIP option is what macribs mentions, complete MaxE system (controller, BMS and charge inductor) + matching PSU, where Eaton PSU is the most compact but also most $$ solution. Server PSU can result little bigger but most economical solution.

 

[macribs]

That link for BMSbattery, those are just power supplies right?

No, this is a fully functional CC/CV charger, you only need to add BMS to the battery, if you want some protection and automatic balancing.
1) In this case your system will be modular and you can use any controller, any BMS, any CC/CV charger that meets your voltage.

2) Other, but kind of extravagant VIP option is what macribs mentions, complete MaxE system (controller, BMS and charge inductor) + matching PSU, where Eaton PSU is the most compact but also most $$ solution. Server PSU can result little bigger but most economical solution.

That was a great tip PB. Modular system is a huge plus if testing various controllers, or even if charging several bikes.
And we all know how it goes, first bike is not even optimized to the max before the urge to do another one. So expect to stack up and accumulate e-bikes. :lol:

I wonder how safe are those CC/CV chargers to use without a BMS? From the link you posted the pics and specs was kind of limiting, so I don't know/didn't understand if it is possible to set a cut off voltage? But I guess that kind of goes with the territory of CC/CV?

 

[parabellum]

I wonder how safe are those CC/CV chargers to use without a BMS? From the link you posted the pics and specs was kind of limiting, so I don't know/didn't understand if it is possible to set a cut off voltage? But I guess that kind of goes with the territory of CC/CV?

Having about 8 working BMS's in my junk piles I personally never used BMS on by bikes and mopeds by now only pack level LVC, CC/CV charging to 4.1V/cell and balancing with battery medics once every 4-6 months. BMS battery adjusts the charger to your requirement before sending and most of them can be opened and readjusted by trim pods inside. I would lower the current a little below their standard setting, I have few that cooked coils after a year or two of use (may be falling cooling efficiency related, dust accumulation and fan speed decay).
BTW I would never give BMS less vehicle to someone else for more then one ride.

 

[macribs]

Having about 8 working BMS's in my junk piles I personally never used BMS on by bikes and mopeds by now only pack level LVC, CC/CV charging to 4.1V/cell and balancing with battery medics once every 4-6 months. BMS battery adjusts the charger to your requirement before sending and most of them can be opened and readjusted by trim pods inside. I would lower the current a little below their standard setting, I have few that cooked coils after a year or two of use (may be falling cooling efficiency related, dust accumulation and fan speed decay).
BTW I would never give BMS less vehicle to someone else for more then one ride.

Battery medics must be plugged/unplugged each time one uses them, right? Those balancing leads seems so flimsy, I am not sure I would trust to connect/disconnect over and over again in fear the plug would break. Is there maybe another way to balance pack without the need to disconnect each time? Like something that I could just leave plugged in all the time?

How well does your battery packs hold the balance?

 

[parabellum]

Battery medics must be plugged/unplugged each time one uses them, right? Those balancing leads seems so flimsy, I am not sure I would trust to connect/disconnect over and over again in fear the plug would break. Is there maybe another way to balance pack without the need to disconnect each time? Like something that I could just leave plugged in all the time?

How well does your battery packs hold the balance?

No worries, they will hold for many years considering ~ 12 connect-disconnects per year. I check balance once a month at most. I balance when highest to lowest cell difference is 0.03V, it never happened in under 4 months everyday use. My daily duty cycle is ~50% capacity, sometimes up to 90% on weekends. Excess capacity and conservative LVC HVC make wonders.

 

[RageNR]

Thanks for the suggestions guys. Right now, I simply can not afford to go with the addapto and charger. That may have to wait for a future upgrade..

Here is a nutty idea...
Since I am trying to design the pack to be modular enough to fit on the frame and in my backpack, why not split the the pack in half-ish?
I could make 20s4p and 20s3p sections with a connector between. Each string of 7 would be charged separate anyhow.
Several of the 6s chargers have 4p and 3p connectors. Can any of those actually handle a total of 7 at a time? A 4p plugged in one port and a 3p plugged in the other?

In order to do this, I imagine I will need to have connectors to connect each 4p to each 3p along all 20s. Would 20 XT60s be enough?

Thinking outside the box here

 

[parabellum]

Thanks for the suggestions guys. Right now, I simply can not afford to go with the addapto and charger. That may have to wait for a future upgrade..

Here is a nutty idea...
Since I am trying to design the pack to be modular enough to fit on the frame and in my backpack, why not split the the pack in half-ish?
I could make 20s4p and 20s3p sections with a connector between. Each string of 7 would be charged separate anyhow.
Several of the 6s chargers have 4p and 3p connectors. Can any of those actually handle a total of 7 at a time? A 4p plugged in one port and a 3p plugged in the other?

In order to do this, I imagine I will need to have connectors to connect each 4p to each 3p along all 20s. Would 20 XT60s be enough?

Thinking outside the box here

I think you are still confusing S and P, read this tread https://endless-sphere.com/forums/viewtopic.php?f=14&t=26621#p384360
If you have 20s or 72V nominal system, forget about existence of RC chargers. Then most of RC chargers still need PSU to work. Your described battery will be about 14lbs, you do not want another 10lbs on charger equipment and complexity on pack reconfiguration. See and read some pack build treads to make clear how series and parallel connections look like.
You just need bms on your battery and CC/CV charger, that is all.

 

[RageNR]

I understand S and P. Maybe there are things I am misunderstanding about how they work in a pack design like this? Also possible that I am just not getting my point across straight.

Tell you what i'll do... I am going to make a diagram or drawing of what I am thinking of and see what you guys interpret out of that. Sometimes our words just don't do a very good job telling the story.