# 18650 battery design and charging questions



## Yabert (Feb 7, 2010)

Welcome aboard! Nice bike!

1- No. 85v / 23 cells = 3,7v per cell and that is roughly 50% charge. You will need to charge between 4,1v and 4,2v. So, to stay under the 90v rating of your controller, I suggess you 20S or 21S (cell in series).

2- maybe start to look at AC/DC 48v Power Supply or RC charger http://www.progressiverc.com/chargers/battery-chargers/lipo-chargers/

3- What you want is 20S 24P, so build 20 time 24P and connect it in serie after.

4- Look at awg ampacity. In your case, there will 300-400A max who will pass by your battery, but theoretically this current will be share between the 24 parallel cells (so 12-17A will pass between cells). That mean big conductor at the end of your battery, but little conductor between cells.

5- Learn a lot about 18650 cells before start to charge/discharge, if no you will en to burn your house...

Good luck


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## Romack (Nov 12, 2016)

Thanks for your reply!
1. Ok. I'll stick to 21s

2. I didn't know you could charge a 72v pack with a 48v charger due to the upper limit safety cutoffs. The 72v chargers I've been looking at usually stop the charge at about 84v. Did you recommend those hobby chargers because they provide balance charging? I'm not using balance leads, and will be bottom balancing.

3. Are you recommending 20 24p wired in series due to balance charging concerns?

5. I've been sorting 18650s salvaged from laptop batteries for about a month now using two sky rc hobby chargers to measure discharge capacity, and a nitecore charger to top them up to 4.2v before I test capacity. I've got about 300 sorted so far, and about 50 of those are bad cells. 

My plan with the completed packs is to solder wire fuses to every cell, and to carefully bottom balance my completed packs. Hopefully all this will help me not burn my house down!


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## Yabert (Feb 7, 2010)

Romack said:


> I didn't know you could charge a 72v pack with a 48v charger due to the upper limit safety cutoffs.


You can't. I mean using two power supply.


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## e^2 (Jul 22, 2017)

Did you get this built? I'm going to do something similar.


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## Karter2 (Nov 17, 2011)

Romack said:


> My plan with the completed packs is to solder wire fuses to every cell, and to carefully bottom balance my completed packs. Hopefully all this will help me not burn my house down!


... As this thread has been revived, i would comment...
Unless you plan on regularly draining the pack to very low levels (bad idea for pack life)..then bottom ballance is not much of a comfort,...especially with mixed, recycled, cells.
Most find it better to cycle the packs closer to the upper end of the capacity curve, hence why the majority users of Lico/NMC/NCR/INR, etc , cells, prefer top balancing...the way balance chargers work.
Are you going with a good BMS ?
Fuse wires will protect against some failure modes, but not all, ..
....so better keep that fire extinguisher handy still !


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## e^2 (Jul 22, 2017)

If he is running 24P, why wouldn't that tier of batteries average out their capacities? As long as each "S" tier doesn't go below the safety margin, no harm no foul.

You could even go as far as rating each tier and grouping the individual cells appropriately so the average is correct for each tier. Example, each battery is 2000mah plus or minus 400mah. As long as the average cell capacity is 2000mah (albeit 24P), then the pack is 2000mah. Just don't have one pack be 1800mah average with another at 2200mah average.


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## Solarsail (Jul 22, 2017)

Romack said:


> 1. If I stop the charge @ 85ish Volts, will my plan work?
> 
> 2. Would this charger work?


It appears you are using LiCo cells as opposed to LiFePO.

1. Stopping at 85.1 volts is a bad idea. 85.1/23 = 3.7 volts. At this voltage, the cell state of charge is only 51%! Furthermore, if you are charging at 1 A and cut at 3.7 V, the cell will relax to about 3.6 V, which means your real SoC is only 38%! You are wasting 62% of the storage. Might as well build a pack with 62% less cells, but charge it fully to 4.2 V. You need to charge it to 4.2 x 23 = 96.6 volts. If you want your cells to last you a long time, then charge it only to 4.15 x 23 = 95.5 V. You will leave unused about 3% of the capacity, but you will have cells that live a lot longer.

2. No, this is the wrong charger. It is for LiFePO cells which max out at 3.7 volts. You need to charge it to 96 V.


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## Solarsail (Jul 22, 2017)

Romack said:


> 3. What disadvantages/advantages are there in different cell configurations? To clarify...would there be advantages/disadvantages to building 3 packs 23s8p wired in parallel when compared to building 23 packs of 1s24p? wired in series? A lot more wiring and crimps is one obvious disadvantage! But is there any advantage that would justify the added work?
> 
> 4. Why do people use such huge bus bars?? 1/16 x 1/4" copper is rated for about 75amps of power, yet I see people building packs that have what appear to be 1/4" by 1/2" bars. If I make 3 23s8p packs, I estimate the total max draw on each pack to be under 66amps, which would only be carried by the mains bus bars, and the wires between packs, the individual lines of parallel rows of 8 will only be carrying about 5.7amps max if I'm fusing rows of two cells together, yet I constantly see people using 1/32" x 1/4" bars when simple, cheap household romex is rated for 20 amps, and is way easier to solder. Are people just way over-building their packs?


3. That is a good question. First off, you would need a 23S balancer. This may be hard to find. And you are dealing with dangerous 95 V voltage. So I would break that down to at least 2 modules, each 11S in series - with 46 V safe voltage per module. You can now use a 13S balancer to balance 11S. So the configuration suggested is 2S11S27P which is 594 cells, 92.4 V. S stands for modules in series. You may want to further break that down in order to make the charging easier. Instead of having one gigantic charger, it is better to have four small chargers. Thus I would recommend the configuration 2S2P11S14P. At 3.4 Ah per cell, that is about 7.5 kWh, which probably will give you a range of about 200 km. Each module is about 1.8 kWh.

4. I believe the reason people over-build their bus bars is because they are planning to race, and thus they need to draw huge amounts of current. If you do not plan to race, then let's say a 10 kW motor will draw at most 110 A, and you can design for this.


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## Solarsail (Jul 22, 2017)

Romack said:


> Thanks for your reply!
> 1. Ok. I'll stick to 21s


Not sure how you plan to bottom balance, but if you top balance, it will be automatic, and also will give you protection against undervoltage, overvoltage, overcurrent, and short. This secondary protection is a must. Primary protection is the charger.

So again, I suggest 2S11S27P or 2S2P11S14P. 11S is a safe voltage and balancers can be found for that (use 13S balancer). With 11S, you need a charger for 46 V. At 1 A charge, you need a 600W charger. 

For 2S2P11S14P you need the following:

1- Power supply 24 V or 36 V, 2 x 1200 W or 4 x 600 W.
2- CCCV charger step up to 48 V, to be adjusted to 11 x 4.15 = 45.7 V, 4 x 600 W.
3- combination balancer and protection board, 4 x 13S, 60 A cutoff rating
4- micro fuses (as you have suggested)
5- 616 cells, 91.4 V, 5 kWh at 2.2 Ah cells.


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## e^2 (Jul 22, 2017)

So again, I suggest 2S11S27P or 2S2P11S14P.

What do you mean here? And if you bottom balance, do you stop at what would be 4.0v per cell?


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## Solarsail (Jul 22, 2017)

e^2 said:


> So again, I suggest 2S11S27P or 2S2P11S14P.
> 
> What do you mean here? And if you bottom balance, do you stop at what would be 4.0v per cell?


2s2p11s14p would mean groups of 14 cells in parallel, are tied 11 in serias, to make a module. Then two modules in parallel, and then two of these module pairs in serial. Total number of cells would be 2 x 2 x 11 x 14.

How do you propose to bottom balance? Is it automatic?

You cannot stop at 4.0V per cell. That will leave about 15 to 20% of the storage space unused. Might as well charge to 4.15 or 4.2V and use 15% less cells.


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## e^2 (Jul 22, 2017)

Alright, thanks. I see what you mean, I just wasn't familiar with it in that format.

I am going to use 30S3P in my EV (60Ahr cells). Should I do 3P packs and then 30S, or run 30S and parallel them only at the top?

Good to know on the max charge, but I thought the point of bottom balancing was to allow some wiggle room for the batteries to have different voltages when fully charged?


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## Solarsail (Jul 22, 2017)

e^2 said:


> Alright, thanks. I see what you mean, I just wasn't familiar with it in that format.
> 
> I am going to use 30S3P in my EV (60Ahr cells). Should I do 3P packs and then 30S, or run 30S and parallel them only at the top?
> 
> Good to know on the max charge, but I thought the point of bottom balancing was to allow some wiggle room for the batteries to have different voltages when fully charged?


Are these LiFePO4 cells? Or Lico or Lipo prismatics? I am assuming the latter.

Your question is, shall it be 30s3p or 3p30s. Problem with 3p30s is that you will need 3 balancers and protectors. Which is what you may want to have anyways because at 60 Ah per cell, you may not find balancer/protector with a rating of 180 Ah for the 30s3p design (depending on your discharge current). However I believe 30s3p is preferred because it only needs one balancer/protector (but again, at 60Ah per cell, you may not be able to find one - note if your continuous discharge current is low like 0.2C, then you should be able to find balancer/protector). Another problem you will discover is that 30S balancer/protector are not easy to find. This is outside of the domain of eBay. Then either architecture has another problem - you would need one huge charger 3x60x0.2(C)x4.2x30 = 4.5 kW and 135V. This is certainly outside of the cheap chargers and power supplies you can find on eBay, and will cost an arms and a leg if you find one.

May I suggest the following architecture instead: 3s1p10s3p. So you make 10s3p modules, and then put three of these in series. You would need three balancers, one per module, however 10S balancers are easy to find. Also each module is max 42V which is a safe voltage. You can pack each module in its own enclosure, and have three smaller lightweight packs, instead of one huge heavy and dangerous high voltage pack. Another advantage of this architecture is that you can also supply 36V or 72V in addition to 108V, if you ever need that, because each module is nominal 36V.

You would then be charging each module separately. So you need a 3x60x0.2x4.2x10 = 1.5 kW 42V charger and power supply which is pretty standard and cheap. For fast charging put one charger on each module, for a total of three chargers, and you can charge at 4.5 kW with three power supplies. If you have only one 1.5 kW power supply, rotate it between the three chargers. This is how I am building my 12 kWh pack (8s1p13s10p using 3.4 Ah 18650 cells) to arrive at 345V to 384V (note, for 345V, the architecture is 7.4s1p13s10p). The advantage of 13S (48V) over 10S (36V) is that traction motor suppliers are going from 24V to 48V and bypassing 36V. Then they will go to 96V. So I can supply 48, 96, 192V, etc. which seem to have become the standards.

If you are using 3.2V LiFePO4 cells, then I suggest standardizing on 15S to give you the 48V. So go for 2s1p15s3p. You can easily find 16S balancers that will balance 15S.

I hope this makes sense.

How do you plan to bottom balance? How are you going to drain the cells (or groups) that are let's say 3.2V to make them drop to the balance at 3.0V (again, assuming Lico batteries)?


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## e^2 (Jul 22, 2017)

Hey solarsail. Looks like posted up another thread as I didn't see this reply!

My batteries are in 5S groups (LiNiCoMg) and I cannot split them. I'm also running an 8" ADC motor that is rated at 96V so I'll be using a 120V rail. I have a charger already, a 3.5kW TSM2500 that should charge at 25A up to 123VDC, so I'm good there.

I don't plan on running any protectors or balancers. Currently, I have a string of six battery packs on my garage floor all paralleled up and at the same voltage. All I need to do is drain them down and they will be bottom balanced and ready to install.

I think I have room in the car to do chunks of 6 batteries. I'll take a look today and see. If a 10S3P chunk gives me flexibility for future options of management and monitoring, I'll go that route.


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