Building a 48 Cell 18650 Li-Ion Battery Pack?

[HacknMod]

Hi guys,

I need to build a 12s 4p battery pack to power a hub motor for an electric unicycle project. I found that the cheapest batteries with the greatest density are 18650's (the same ones Telsa Motors uses). So, I need to combine them as described above, however, I fear I won't build the pack properly and they may overheat and/or explode.

Hub motor specs:
Max current: ~20A
Most efficient at 7.5A
48v/1000W (up to 60v max)

I was planning on buying these Li-ion cells since they are the best deal around. Of course, it's eBay, so I ordered a few to test before ordering 48 of them.

I've seen a few packs on this site with 200+ cells, and no one seems to worry about protection PCBs?

Thanks in advance!

 

[icecube57]

Those cells are going to underperform. Ypedal has already tested those cells. Search the forum and you will find his test results on the ultrafire. They are supposedly qc regects... or cells from pack salvage operation. They have false bottom... endcaps to cover up the spot welds and they barely give 1/3 to 1/2 of the rated capacity.

 

[icecube57]

http://www.endless-sphere.com/forums/viewtopic.php?f=3&t=25904&hilit=ultrafire+ypedal&start=45#p383845

 

[HacknMod]

Thanks for the info, I read the entire thread. VERY interesting information.

Is there a source for some quality 18650's at a decent price? Or perhaps instead of the 18650's, is there something more reliable at an "affordable" price (LiPo, LiFePo, etc seem awfully expensive).

Thanks!

 

[AussieJester]

I found that the cheapest batteries with the greatest density are 18650's

I would have thought nano tech lipo would be a better choice
maybe half a dozen packs from Hobby King...?

Best of luck with your project, and welcome to Endless Sphere

KiM

 

[SamTexas]

18650 LiCo (lithium cobalt) cells indeed have a very high energy density. However their power density is very low. They can sustain 1C discharge. 2C is possible for a short time, 60 seconds max, before they get hot. Keep in mind that these cells are designed for use in laptops where the typical discharge rate is 0.5C. Tesla, too, uses these cells at 0.5C or less (the range on their cars is 2 hours driving or more.)

So your proposed 4p pack is small for your intended application (20A max). 4p is about 8Ah meaning 8A or less draw for safe and reliable operation. If you have a way to limit the maximum draw to 8A, then 4p is adequate, otherwise go for 8p.

Panasonic CGR18650D (light green) and CGR18650E (pink) are the best cells I have seen so far. I found them mainly in Dell laptop battery packs. You can buy used/defective packs from computer repair shops or you can get them free from recycle centers (if you're lucky). Properly built, properly used, 18650 LiCo packs are small, light and economical.

Just like other type/format of Li-Ion cells, protection (charging/discharging) is at your discretion assuming you know what you're doing.

 

[HacknMod]

I would have thought nano tech lipo would be a better choice
maybe half a dozen packs from Hobby King...?

Best of luck with your project, and welcome to Endless Sphere

KiM

Nice! I'll check out the nano-techs.

1. Is this THE best deal for LiPo's online? Or are there others you can recommend? Thanks!
2. Why did you say "half a dozen?" Is using multiple cells better than something like this beast?

18650 LiCo (lithium cobalt) cells indeed have a very high energy density. However their power density is very low. They can sustain 1C discharge. 2C is possible for a short time, 60 seconds max, before they get hot. Keep in mind that these cells are designed for use in laptops where the typical discharge rate is 0.5C. Tesla, too, uses these cells at 0.5C or less (the range on their cars is 2 hours driving or more.)

So your proposed 4p pack is small for your intended application (20A max). 4p is about 8Ah meaning 8A or less draw for safe and reliable operation. If you have a way to limit the maximum draw to 8A, then 4p is adequate, otherwise go for 8p.

Panasonic CGR18650D (light green) and CGR18650E (pink) are the best cells I have seen so far. I found them mainly in Dell laptop battery packs. You can buy used/defective packs from computer repair shops or you can get them free from recycle centers (if you're lucky). Properly built, properly used, 18650 LiCo packs are small, light and economical.

Just like other type/format of Li-Ion cells, protection (charging/discharging) is at your discretion assuming you know what you're doing.

Ah, good point. I didn't realize the 18650's had such a low discharge rate. Perhaps I should consider Li-Poly instead? Is that a good idea for this application?

 

[AussieJester]

I would have thought nano tech lipo would be a better choice
maybe half a dozen packs from Hobby King...?

Best of luck with your project, and welcome to Endless Sphere

KiM

Nice! I'll check out the nano-techs.

1. Is this THE best deal for LiPo's online? Or are there others you can recommend? Thanks!
2. Why did you say "half a dozen?" Is using multiple cells better than something like this beast?

Yes HK is the cheapest for LiPO, nothing wrong with the one you linked to, only
reason i suggested a few smaller packs was ease of containing them, rather than a few large packs
in one spot off balancing the uni cycle somewhat i thought perhaps smaller packs might
be easier to spread over the bike, possibly making it a little better balanced?

Best of luck anywayz

KiM

 

[SamTexas]

You need to come up with an educated estimate of power consumption for your unicycle first. How many watts does it need 95% of the time? 100w, 150w, 200w... ??? Then you can choose your battery chemistry/format. Don't discount 18650 LiCo just yet. For the same energy content, the 18650 LiCo pack will be much smaller and lighter than the HK RC Lipo pack. Plus, since you are going to build the pack yourself, you can shape it any way you want.

Again, first you have to determine your power requirement.

 

[HacknMod]

Yes HK is the cheapest for LiPO, nothing wrong with the one you linked to, only
reason i suggested a few smaller packs was ease of containing them, rather than a few large packs
in one spot off balancing the uni cycle somewhat i thought perhaps smaller packs might
be easier to spread over the bike, possibly making it a little better balanced?

Best of luck anywayz

KiM

Solid points, thanks.

You need to come up with an educated estimate of power consumption for your unicycle first. How many watts does it need 95% of the time? 100w, 150w, 200w... ???

I'm not 100% sure to be honest since I don't have the motor yet (comes next week) and I haven't done tests. However, I know that a commercial product (the SoloWheel) which uses the same motor utilizes a "LiFePO4 38.4 volt battery." A poor quality torque curve on the motor is here.

Then you can choose your battery chemistry/format. Don't discount 18650 LiCo just yet. For the same energy content, the 18650 LiCo pack will be much smaller and lighter than the HK RC Lipo pack. Plus, since you are going to build the pack yourself, you can shape it any way you want.

Again, first you have to determine your power requirement.

The reason I "discounted" the Li-ion batteries is because of the cost of "quality" cells. I can get ones with high ratings on eBay for $1.75 each, but as I've learned, they're usually crap. I have a few in my possession which I'll test though. It seems that high quality 18650's are $4-5 each, so they're quite a ways out of what I wanted to spend on batteries. Alternatively, it appears the LiPo packs might be a little more affordable.

Do you know of a source of quality (as rated) 18650's at a decent price?

 

[SamTexas]

I don't even know what a SoloWheel is. If I have to guesstimate, I would say that a unicycle consumes at most the same amount of power as a regular bicycle. I would also assume that a unicycle runs at a much lower speed, say 15mph max. BikeCalculator.com says I need 100W.

Re: Where to get laptop 18650 LiCo cells?
See my previous post. I bought only Dell laptop batteries at $6.00/pack (I think). I have realized that I paid too much. Most people paid a lot less. Anyway I only bought authentic Dell batteries, 56wh or 53wh per pack, all Panasonic cells as described in my previous post. Each pack contains 6 cells (3s2p). At most 2 of them are bad. The remaining 4 are good and have capacity ranging from 1.5Ah to 2.2Ah. So assuming 4 good cells per pack, with an average capacity of 1.8Ah, we're looking at 27wh for $6.00 (again I paid too much).

Your proposed 12s4p pack would have a capacity of 324wh. Assuming you're consuming 100W 95% of the time, the draw rate is only 0.31C which is well below the pack's ability to deliver.

Of course, all above calculation are based on my "scientific" guess. You'll have to confirm all numbers by yourself. Have fun and good luck.

 

[HacknMod]

I don't even know what a SoloWheel is. If I have to guesstimate, I would say that a unicycle consumes at most the same amount of power as a regular bicycle. I would also assume that a unicycle runs at a much lower speed, say 15mph max.

Details on the SoloWheel. Yes, mine will go about 15 mph max.

BikeCalculator.com says I need 100W.

Wow, that seems oddly low. But if that's the case, that'll save me some cash. I need to do more research on the power consumption I suppose.

 

[HacknMod]

Ended up going with LiPos from HobbyKing. A bit more expensive, but worth the investment for quality purposes.