Help me design/build a 18650 battery pack

[Synon]

Help me design/build a 18650 battery pack

So I finally got my 48v 1kw motor working, it's one of the yescom kits. I've been using about 40lbs of SLA's just to try things out, it gets me up to 25mph and it's fun but the range is dismal and the weight is far too much.

I've been collecting laptop batteries for a while now I have a pretty impressive collection of matching 18650 cells (going by the label, the cells are 2850mAh per cell). Unfortunately I started dissembling the batteries last year before I had done as much research as I should have, and I took the tabs off the cells I want to use, going to make more work for me but oh well. I’ve got about 150 of these cells ready to go. I’ve been reading lots of threads (especially the 18650 thread that DrkAngel put up) but I still have questions.

The proposed battery pack –
12s12p – 50.4v fully charged to 4.2v per cell, 33.6aH
OR
13s11p – 54.6v fully charged to 4.2v per cell, 31.7aH

Is 54.6v too much for a 48v controller? Am I better off with the 12p to help with the low C rating of these batteries?

Testing –
Do I need to test each individual cell? Or is this something I could do several at a time? What equipment is recommended for testing these cells? I haven’t seen a simplified version of what needs to be done.

Charging/balancing –
Recommendations on a charger for this system? I see that a PCB is usually a good idea, any recommendations on PCB’s that would work for this pack?

Power supply –
I plan to use a 750w computer power supply to go with the charger

I see that welding on new tabs would be best, soldering is also very good, but I still would love to see an option that requires neither. Does a case exist that will press contacts onto the batteries? This would eliminate the need for me to do any soldering and would make swapping batteries out or possibly adding them MUCH easier. Any thoughts on that?

 

[teslanv]

Everything you want to know is in Drkangel's thread here.

http://endless-sphere.com/forums/viewtopic.php?f=3&t=26383

I just finished my 12S9P pack a couple weeks ago. It is a large undertaking. I bought a big soldering iron and "built" the pack in one day, but before that spent many hours testing and sorting cells so the pack would be well-balanced. So far it seems to work satisfactorily.

 

[Synon]

Thanks, like I mentioned, I've been reading DrkAngels thread but I still have the questions above.

Maybe this is too many questions at once, so maybe I'll start with one.
For a 12s12p 18650 battery pack, are there any chargers that come highly recommended for this setup? A friend of mine recommended the Thunder 1220, but I don't know the strengths and weaknesses of chargers.

 

[teslanv]

I think you would be fine to go 13S as many "off-the-shelf" 18650-based batteries use this configuration.

However, if you plan on charging it with an RC charger like I do, you may want to break up your Larger pack into 3- 4S or 2 - 6S packs so you can buy a cheaper charger and parallel charge in 4S or 6S.

I also used the rc charger to test each cell individually for mah capacity, then balaced the parallel cell packs based on these tests.

 

[Synon]

I do plan to use an RC charger, so maybe breaking up the pack would be a good idea, I like the sound of the 2 - 6s packs.

The Thunder 1220 supports up to 12s, however with the low C rating of these batteries am I wasting money with a charger that has more capability then I can use? Am I better off getting a couple IMAX B6 chargers or something else if I plan to split the pack into 6s?

 

[teslanv]

I have the imax b6.
It only charges at 50 watts, so it's pretty slow but gets the job done.

If you are building a 12S10p or larger pack, and if your individual cells are at least 2000 mAh, that would be over 20 Ah in 12S and 40 Ah in 6s and you should be able to charge it at 800 watts and still be under 1C
If you have the budget, check out a 200w or 400w charger.

 

[teslanv]

Am I better off getting a couple IMAX B6 chargers?

You just need one charger. Charge and balance multiple packs (up to 6 packs)
at one time in parallel. Works great. They just need to be all the same cell-count.

 

[liveforphysics]

If by chance you're looking for a good pack design, I would not read the above linked thread and try to forget what you saw in it, and instead view this one.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=57810


Making a pack from unknown condition used laptop batteries seems like a bad idea at it's best though IMHO.

 

[Jonathan in Hiram]

Making a pack from unknown condition used laptop batteries seems like a bad idea at it's best though IMHO.

It all depends on your financial situation, your eventual aim and your ability to hack. If you have more time than money, just want low cost transportation that doesn't have to break land speed records and you are willing and able to deal with the necessary hacking it's not a bad idea. I'm retired and on a limited fixed budget that really doesn't allow for sending hundreds of dollars overseas with the possibility of not getting a decent product back. I've been burned a few times buying stuff online particularly from China and risking a distinctly non-trivial percentage of my monthly income is a powerful disincentive to dealing with Chinese suppliers. All my ebike gear has been hacked from ebikes with dead or missing batteries I've found locally on Craigslist and bought for pennies on the original dollar.

My approach works for me as I have a friend who is employed at an electronic recycling company, I'm just looking for the cheapest possible transportation with a little style and I spent a career fixing electromechanical and electronic widgets for which I rarely had any technical data at all so I'm a better than average hacker.

I also like the idea of taking other people's junk and turning it into something useful, it seems like a fairly "green" way of doing things and it's kind of my way of "doing art". My home when I was growing up had a shop in the basement where my dad restored and rebuilt antique grandfather clocks so the attitude is something I've had all my life.

 

[liveforphysics]

I'm all for saving money, recycling, DIY'ing and building things for yourself.

I will tell you the part that doesn't make sense to me.

You don't know what the cells have been through. If they have been compromised in the wrong ways, you may burn your home down even if you did a great job on the pack build, because you've got a cell that sat over-discharged or whatever that you didn't know about, and it's loaded with copper ions waiting to precip into a dendrite and internally short on the 20th time you charged it or whatever.

Cells that aren't aged similarly that are in series just doesn't seem like that good of an idea to me, even if you were being paid to use them rather than just being free.

But... I used to do things like it many years ago, so who am I to judge. Just charge and store someplace you're ok with burning down.

 

[Hwy89]

I totally agree with Jonathan and also understand Luke's point of view. Building homemade packs from 18650 cells may not be for everyone but it has worked well for me.
After reading and studying all of DrkAngel's thread I set out to build my first pack. I soon discovered that trying to get enough matched cells by buying used laptop packs on Ebay was going to be a nightmare so I broadened the search and found several lots of unused surplus packs for discontinued equipment. The first lot was packs for a medical device and the second lot was a whole pallet of two cell packs designed for hand held scanners.
I have put together four bike packs, 2ea. 12p 12s, 1ea.16p 12s and one 14p 12s.
The 12s packs are separated into 6s sections and are charged on a four channel balance charger and the 14s has a BestTech D167 bms and seperate charger.
I am very happy with these batteries. My riding style is mostly in the 15 to 22 mph range because I like to pedal. I occaisionally run up to 31 mph for short bursts and have lots of hills to climb.
The range is fantastic. I usually put 50 miles on the bike over several days before recharging but have seldom seen the cell voltages drop below 3.7.
On the subject of safety. The cells were carefully matched for capacity before assembly. Cell balance is constantly monitored. The packs are large enough that they seldom see 1c discharge. Charging is done in a steel trailer with solar panels away from the house.





.

 

[Synon]

“Just charge and store someplace you're ok with burning down.”

I appreciate your concern and understand that this does involve a level of danger. I won’t be charging this in my house.

That pack design looks really good! I’ve seen those 18650 holder brackets around, I think I’ll have to invest in some. My only fear with that design is slack in the middle of the case could cause it to lose contact, maybe even with that compression blanket? Anyway, I’m sure there are easy ways to fix that, but I’m not quite to that stage yet.

So back to the charger as this is the missing piece before I can start testing my cells, I’m definitely willing to invest in a more powerful charger if the C rating isn’t going to be a factor. Teslanv you mentioned one charger will charge “6 packs”, I guess I don’t understand exactly what is meant by packs. I was under the assumption it meant 6 sets of batteries in series. For example it could charge a 6s12p battery, but not a 12s12p battery (at least not all at the same time). Is my thinking incorrect on this?

 

[teslanv]

If you build your Big 12S-10P Pack in say "halves" then you would basically build two smaller 6S-10P packs. - When you use them on your bike, you would connect the two 6S packs in Series so that they become the big 12S Pack. If you connect those same 6S packs in parallel, then you essentially have a 6S-20P pack, which has twice the Amp-hours as a single 6S-10P pack. - You would also Charge the packs in Parallel, since you can get a 6S RC charger mush cheaper than you can a 12S charger. Then to determine charge time, look at how many amps the 6S charger will output in 6S. My Imax B6 will only charge at 2A or so in 6S, so that 6S-20P pack, if it is 40 Amp-hours would take 20 hours to fully charge from dead flat to full charge. Better chargers will have output more watts and Amps and thus charge your 40 AH pack faster.

Here is a video on parallel charging several LiPo packs with an RC charger.

https://www.youtube.com/watch?v=WY194Md6KNg

Also check out the build thread in my Signature for some additional photos. - I use LiPo batteries in 4S bricks, since they are cheaper and then wire up 3 in series for my 12S Pack.

 

[Jonathan in Hiram]

Everything you do involves a level of danger nothing in life is completely safe, drinking too much water can kill you. My personal safety calculation is that I'm considerably more likely to be taken out on the road by some vapid clueless mouth-breather texting while driving their Kenworth Pilgrimage to the 7-11 for lottery scratch offs than to burn up in my home from charging laptop cells on top of fire bricks inside a steel filing cabinet.