Building a 21 cell 18650 battery. need advice.

[johanv]

Hi All,

I want to build a 21 cell 18650 battery. 7 in series, 3 parallel(24V, about 5Ah). I need this config because I want to put them in a aluminium tube. Could you advice me which cells to use (cheap, and OK, not necessarily the best). And more important: where can I get(if anywhere) a suitable BMS for this home made battery(preferably in Europe). Thank you for your answers.

 

[mistercrash]

DoctorBass is a good source to get used but very good Sony 18650V cells. That aluminum tube would have to have something like a 40mm ID and be something like 500mm long to house a 7S3P pack. If your aluminum tube is at least 45mm ID then you could build a 7S4P pack. There's a few people here, including me, that use these cells with no BMS and just balance charge with a Hyperion 1420i charger.

 

[johanv]

Hi Thanks for your answer.

It's the top tube of a vanMoof bicycle. which has a 47mm id and a length of more than 500mm. It's accessible via the taillight, so ideal for housing a battery.
I guess, the easiest way to build this, is just buying a typical Chinese battery pack (S7P5), disassemble the pack and remove one set of 7 batteries.

Has anybody disassabled one of these 24V/10Ah packs from BMSbattery?: What kind of cells are in there? 18650s?

http://www.bmsbattery.com/24v/191-36v-10ah-lithium-ion-electric-bicycle-battery-pack.html

http://www.bmsbattery.com/24v/443-36v-15ah-lithium-ion-electric-bicycle-battery-pack.html

http://www.bmsbattery.com/24v/245-24v-lithium-ion-little-frog-abs-shell-ebike-battery-pack.html

http://www.bmsbattery.com/24v/451-24v10ah-lifepo4-ebike-battery.html
http://www.bmsbattery.com/24v/315-24v-lithium-ion-little-frog-abs-shell-ebike-battery-pack.html

 

[HypnoToad]

Hi, I'm using a 12s 6p 18650 based battery and it works well.

My advice is use cells with tabs already on.

Don't mix brands and capacities.

I do not use a BMS myself, I simply have a volt meter on the handlebars.

Just made sure they don't sag too much and you'll be good to go.

 

[dogman dan]

Get good cells from the Doc. Seriously. Or buy similar new tool packs for the cells. The cheap chinese 1c cells won't cut it with a battery that small.

Cheap china lifepo4 for example would only get you 3 amps of 24v power. Hot damn, a 75 watt ebike. With better cells, like real A123's or konions, you could at least discharge 3-4c and have closer to 250w. Still pretty puny, but ok if your needs are just a little assist on one hill.

3 ah or so, is still going to run out very quickly.

 

[dnmun]

the BMS battery in the picture is a headway based pack. 38mm diameter cylinders. without a BMS to protect them, you should plan on rebuilding it regularly so make it easy to remove and work on.

your cell to cell connections have to be confined within the radius of the cells also or they will short out on the inside of the tube. with time the plastic insulating sleeve on the cylinders will wear through and eventually it will short out inside the tubing. you will know when the tubing gets hot.

 

[johanv]

Thanks for all the replies.

I found this chart on some forum:

And it seems that Ultrafire 2600 and Ultrafire 3600 protected are not that bad. I read somewhere that 'protected' means some small PCB which protects the cell from under- and overcharging. So it seems to me, a protected cell can be used without a BMS, and it will still be safe to use for a n00b. Is this correct?
Is there an overview of different types of batteries on endless-sphere?

 

[999zip999]

An ebike battery should have 10ah. But If you want to gamble on a small pack, Konions found in makita drill packs. LiMn as they do well without a bms. Cellman has an A123 pack that fits in the triangle perfect. Batteries arn't so powerful to haul you and your bike around 200lbs. that will fit in that tube. I understand stealth, but it not uraimun or a peice of the sun. Nice bike. What are your travel needs ?

 

[johanv]

Thanks. This is kind of a pilot project, to find out how it works. I'm riding about 12 miles a day, so I guess I my capacity can be a little less than 10 Ah. As pointed out by mistercrash, I might be able to squish in an extra set of batteries to increase my capacity.

 

[dogman dan]

At 36v, at 25 mph, 1 ah = 1 mile. So unless you go slow, or pedal your guts out, you will need a lot more than will fit in the tube. For a test, you could run a bike on a 36v drill pack, but to do it all the time you need to paralell in more capacity.

Soon as you said 12 miles, I see the battery in the frame tube as a waste of time. Cool if you have only about 2 miles to get there and 2 miles back, but it's just going to take more to go 12. So why bother with trying to make a pack fit that tube.

If it was a steel bike, it would definitely be worth looking into rebuilding it with a 2 1/2" square tube though. RC Lipo would fit great in there then. With 30 c or more discharge rates, 5 ah is plenty.