Modular 18650 packs as RC lipo alternative

[mlt34]

I had an idea for an alternative to RC lipo for use in ebike batteries. Why not build small 18650 packs with approximately equal voltage and capacity to commonly used RC lipo packs (~5AH, 4s-6s) that have no BMS but do have balance wires and discharge wires just like lipo packs. You could use them to build larger ebike packs and charge/balance them just like lipo packs either by bulk charging or balance charging.

View attachment 1

The main reasons/advantages are as follows:

• this gives modularity for pack building and breakdown, especially for stuffing into odd shaped bags
• makes replacing a bad cell group easier as you only throw out (or repair) a small ~12 cell pack instead of a 65+ cell pack
• gives lots of cell choice as there are more variants of 18650s than most other cell types
• would be physically smaller and lighter than most RC lipo packs of comparable capacity
• allows you to keep using your same charger and wiring if you already use lipo bricks
• safer than lipo
• longer life cycle than lipo
• could add plug-and-play external BMS via balance and discharge wires

Some potential disadvantages are:

• hard to match the power output of lipo without using very expensive 18650 cells
• requires spot welder to join cells without damaging them (soldering is possible but damages the cells)
• requires care to not overcharge or over discharge just like lipo (however without the same associated safety issues)
• not for novice ebike riders (just like lipo) as making parallel/series connections between bricks requires care and understanding.

I am going to work on creating a few prototypes, maybe 4 packs of 6s2p. This initial mock up would be 6s and 5200mah:


I've got some jst-xh connectors on the way and then I'll be able to build some of these packs for testing. I think it could be a good missing link for ebike pack building. Testing will show me if I've missed any important considerations here.

 

[dogman dan]

I'd love to see 12v 10 ah sections of good 18650's with balance plugs for sale at decent prices. 24v-36v-48v-60v-72v, easy.

What I mean is, you don't have to build them yourself. They'd come with bullet plugs just like a hobby king pack. They'd be about the size of a lawnmower pack, but 12v instead of 48v.

Add a bms, just as easy. Any problem with a 12v section, swap in your spare. easy.

This would really help ebike techs in shops if you could fix batteries easier. Today, they are not made for easy servicing.

 

[Ykick]

I’ve been looking into the idea of sourcing 2, 3, 4, etc., P groups welded up and just do my own serial wire connections (10-20S) while adding a BMS.

 

[spinningmagnets]

If you bought the raw connectors (with loose pins) for jst-xh, I found them to be extremely difficult to work with. I was much happier using "pigtails" that were pre-crimped from the factory.

 

[mlt34]

If you bought the raw connectors (with loose pins) for jst-xh, I found them to be extremely difficult to work with. I was much happier using "pigtails" that were pre-crimped from the factory.

you're exactly right and I've only made that mistake once. I got extension cables. The annoying thing is you're left with the other end which inevitable never gets used, but it's a small price to pay for the convenience of not crimping a million of those little suckers.

 

[Ykick]

If you bought the raw connectors (with loose pins) for jst-xh, I found them to be extremely difficult to work with. I was much happier using "pigtails" that were pre-crimped from the factory.

Yep, they're tricky but practice makes perfect. One tip - slip the mating connector onto the contact you're crimping, much easier to hold and align properly in the crimping die.

 

[mlt34]

I'd love to see 12v 10 ah sections of good 18650's with balance plugs for sale at decent prices. 24v-36v-48v-60v-72v, easy.

What I mean is, you don't have to build them yourself. They'd come with bullet plugs just like a hobby king pack. They'd be about the size of a lawnmower pack, but 12v instead of 48v.

Add a bms, just as easy. Any problem with a 12v section, swap in your spare. easy.

This would really help ebike techs in shops if you could fix batteries easier. Today, they are not made for easy servicing.

This one I've considered too, but it's hard to do 12V packs with li-ion 18650's because of the 3.7V thing. Thats why LiFePO4 is used for those "SLA replacement" 12V packs because 4s increments of LiFePO4 is reliably equivalent to 12V increments.

Li-ion on the other hand doesn't do as well. 3s is 11.1 which is what my "12V" power tools use, but building big packs wouldn't work that way because 36V would be 9s instead of the industry standard 10s, and 48V would be 12s instead of the industry standard 13s which is already a little low for 48v which is why EM3EV builds their '48v' batteries with 14s.

 

[leelorr]

I saw a topic a few days ago where a supplier was offering to use his spot welder to build 1s4p or 1s5p "blocks" of these cells and shrink wrap each block. You could buy as many blocks as you wished and the wires coming from each block could then be connected in serial to obtain the voltage you wished. I thought it was a pretty useful product, and would mean that anyone could build their own pack just by connecting the blocks together and adding a BMS...

 

[chas58]

That is a very intriguing idea. It would be nice to either add packs to add capacity (parallel), or even add packs (series) to change voltage/speed (would require different BMS though).. The modularity is one big advantage of Lipo

 

[mlt34]

That is a very intriguing idea. It would be nice to either add packs to add capacity (parallel), or even add packs (series) to change voltage/speed (would require different BMS though).. The modularity is one big advantage of Lipo

I'm imagining these without a BMS just like lipo. And just like lipo, you have to be responsible about monitoring your cells to make sure you don't overdischarge. Unlike lipo though, if you did over discharge the consequences wouldn't be as dangerous.

 

[Hillhater]

Good idea this, but first you have to find a reliable supplier of genuine cells at a realistic price.
There are too many fake cells about and too many overpriced sellers, together with the insecurity and cost of shipping from source countries.
Do you have a supplier and cell model in mind ?

 

[mlt34]

So far I've got two suppliers of genuine Samsung ICR18650-26F cells. They are good quality cells that are all delivering over their rated capacities. The only problem is they aren't very high power cells and should be limited to about 5.2Amax continuous discharge for good health. I'm sourcing a good place for genuine 25R cells currently.

 

[teslanv]

I would welcome this type of product.

The nice thing is that you can usually get comparable WH capcity with even the Multistar batteries, if you choose the right 18650 cells.

I would love to see 5S-10AH packs with HIgh-quality cells. Perhaps a 5S4P configuration. A pair of those would be perfect for small BMX and light-weight cruiser bikes, and you could run and bulk charge in series (10S) and balance charge as needed in parallel (5S x 2P) with an inexpensive 6S Balance charger. This is essentially what I do with my current 4S LiPo packs.

 

[dogman dan]

Ok, 4s or 5s packs then. True enough, hard to do 48v with just 4s or just 5s. If you could get both, you could build pretty much any voltage. 8s for 24v, 10s for 36, 14s for 48, and so on. The 24v pack would be a bit high volts.

7s sections would work good too. "24v" blocks you can stack up like lipo. But that leaves out "36v"

 

[mlt34]

Ok, 4s or 5s packs then. True enough, hard to do 48v with just 4s or just 5s. If you could get both, you could build pretty much any voltage. 8s for 24v, 10s for 36, 14s for 48, and so on. The 24v pack would be a bit high volts.

7s sections would work good too. "24v" blocks you can stack up like lipo. But that leaves out "36v"

I like the 4s and 5s block idea. Having just two standard sizes allows the user to buy the ones they need for their specific project. Of course many people have gotten used to 6s and run 12s system voltage. Then there's those 18s people. Ugh, I just can't decide on a good 'best' voltage. It seems like everyone will want something else. That's why HobbyKing is so nice, they have everything (even if half of it is backordered).

Another thought is that if you used a two wire BMS that charges and discharges via the main leads, then I believe these could also be protected bricks, so you wouldn't even need to worry about monitoring or balancing cells, you just build them up to whatever final voltage you need and buy a simple charger for that voltage, leaving them assembled for charging. I think. Or maybe if one of the packs 'upstream' finishes charging first and the BMS cuts off, that would cut off current to the downstream packs. Gotta think about that one. Where's dnmun when you need him?

In other news, I'm now only a few colors away from being able to make a "pride pack".

 

[teslanv]

I would request Anderson PP45 Power-Pole Connectors for the Main + and - Leads of each pack. This makes the Icecube57 Harness method a breeze.

http://endless-sphere.com/forums/viewtopic.php?f=4&t=32096

 

[mlt34]

Hmm, I had been planning on 5.5mm bullets. IceCube57's harness has bullet connectors at the pack end and PP45's at all the other connections, right? So I thought to be compatible with this stuff that I'd need bullets on the packs, just like most lipos?

 

[wesnewell]

I had an idea for an alternative to RC lipo for use in ebike batteries. Why not build small 18650 packs with approximately equal voltage and capacity to commonly used RC lipo packs (~5AH, 4s-6s) that have no BMS but do have balance wires and discharge wires just like lipo packs.

Because 18650's suck for high power apps. Now if you can find some rated for 10C or higher discharge they would work fine, but most don't work well over 1C, and some don't work well even at 1C. And they sag like crap, even at low C discharge rates. Just look at the discharge charts that can be found here.
http://lygte-info.dk/review/batteries2012/Common18650comparator.php

 

[spinningmagnets]

Here's another thought to keep in mind. You are already assuming rectangular "sub packs" that can be assembled by the end user into a variety of shapes to suit a particular builder. By adding only one other sub-pack shape, you can greatly ease the building of a triangle pack (for those who are so inclined to try that). Shown below is a green-cell 4P string, and a couple of triangle pack shapes using 5P strings (most are 5-inline, some are 2+3 stacked).

Doing so "might" make arranging packs like Legos, and very easy to do?...

http://endless-sphere.com/forums/viewtopic.php?f=14&t=48342

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[mlt34]

I had an idea for an alternative to RC lipo for use in ebike batteries. Why not build small 18650 packs with approximately equal voltage and capacity to commonly used RC lipo packs (~5AH, 4s-6s) that have no BMS but do have balance wires and discharge wires just like lipo packs.

Because 18650's suck for high power apps.

Saying "18650's suck" is like saying "food sucks". Some does, some doesn't. Generalizations are rarely accurate and almost never helpful.

You can get 18650's with over 30A capability. Check out the Sony VTC4's and VTC5's. A little 13s4p pack could put out 120A. Do you really need more than 120A?

The whole beauty of this system is you can choose from dozens and dozens of different types and chemistries of 18650 cells. With lipo you've got turnigy, zippy, multistar, and... that's almost it before you start scraping the bottom of the barrel.

With 18650's you've got tons of great Samsung, Panasonic, LG, Sanyo, Sony and other cells, each of which have many individual unique cells with different power and capacity combinations.

 

[wesnewell]

You mean like this.
http://lygte-info.dk/review/batteries2012/Sony%20US18650VTC5%202600mAh%20%28Green%29%20UK.html
They're certainly better than most, but at what cost.
And even then they will sag below 3.5V at 30A.
I'd need 96 of them to match my 24s lipo pack rated at 200A, and they would only be rated for 120A. What would that cost? I'd wager a lot more than the $275 mine cost. And then the nightmare of assembling the pack.

 

[teslanv]

Here is a feasible Harness design, using PP45 connectors at the Pack Level and 4mm HXT connectors at the Serial And controller Level for 90A Current.

View attachment LiPo Harness.pdf

 

[docnjoj]

I like the idea but it would then depend on price/ availability/quality. My old Sony Fatpacks are still alive and at about 90% after a gentle 3.5 years paralleled with a 10ah Ping, also still running. Limit to about 25 amps and all is well. The Sonys are "v" or "VT" and never ran a tool in their lives. Bought when each Fatpack was $45 delivered.
otherDoc

 

[teslanv]

You mean like this.
http://lygte-info.dk/review/batteries2012/Sony%20US18650VTC5%202600mAh%20%28Green%29%20UK.html
They're certainly better than most, but at what cost.

What is the cost of a catastrophic failure of a LiPo battery? A burned Bike. An injured human?

I will readily admit to using LiPo for it's power and price, but I cannot convince myself to recommend it to consumers, without serious warnings about it's potential risks.

 

[Eclectic]

Actually this is kinda' where flathill was going in this thread:

"open source UNIVERSAL quick swap battery"
http://endless-sphere.com/forums/viewtopic.php?f=7&t=65880