RageNR
100 W
- Joined
- May 23, 2016
- Messages
- 216
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.
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.