How to a DIY 18650 battery?

[Arbol]

Hello, I am thinking about adding a kit to my MTB. For the battery, I am considering a small 12s2p battery with 18650 cells. Basically, I need a max of 15A, which is the current of the controller I want (S06S). As a consequence, it would be good to have a current of 7.5A for each cell, a figure which is delivered by a few cells in the market already (or close to it). I have locally found a technician who can spot weld (they are a shop specialized in batteries). Some questions:

1. How many cables should a battery have? From the interesting report at http://www.electricbike.com/martin-cromotor/ I see the Sony VTC4 are put in small packages, and each package has 5 wires: two red, two dark blue and one light blue. The light blue one is for balance charging. Are the two red ones to connect to the controller, and the two dark blue ones to charge the battery?

2. Balance charging: I read the Sony VTC4 do not require BMS. I guess that Martin Cromotor uses a RC balance charger to balance the cells from time to time. The article says "so he can check the voltage of each individual parallel string if he wants". This means I should have a balance tab for each 2 cells (2p) or one for each 12 cells (12s)?

3. If adding a BMS to the package (which I believe is required for all batteries but the Sonys), the balance tap is completely unnecessary, right?

4. The also interesting report http://www.electricbike.com/high-current-batteries-that-are-not-lipo/ suggests the Samsung 20R and the Sony VTC4 are the best 18650 cells in the market. However, I see other interesting cells, such as Panasonic NCR18650PF (2900mAh), the Panasonic NCR18650B (3400mAh) or the Sony NC1 (2900mAh). I have tried to find a comparison among these cells in this forum, but I have not found a complete one. Is there a ranking of those cells, depending on durability, discharge rate and capacity?

 

[agniusm]

Li ion battery should have 2 sets of cables if BMS is built in. One for charging one set for the load. Balance wires are connected to parallel groups, one per each. If you have 2P12S battery pack, it will have 13 individual wires, 1st negative 11 positive - negative series junctions and 13th positive. Even if its sony you need that for occasional balance, be it rc charger or proper one. Even on sony's bms is a good option. If they do not require balancing, bms will act as fail safe for lvc or hvc in case you accidentally overdischarge the pack or charger goes fūcked and voltage rises. BMS is always a good thing to do, it aint cost that much for advantages you get!
There are many options but you have to work out which will suit you best. I would half the max stated discharge rate. Lets say you have 3400mah cell and max discharge for the cell is 5.1A or 1.5c, I would rate that as .75C which is 2.55A there fore for your 15A load you will need 15/2.55 cells or 6 cells in parallel. That makes 20.4AH pack.

 

[spinningmagnets]

If you only need to draw 30A from the pack (as the occasional peak maximum), then you don't really need the high-current cells, so you would have a lot of options to choose from. The Panasonic 18650-PD cell seems to be well regarded and provides a verified 2900-mAh per cell, which would provide a lot of range in a reasonably small pack.

here's some handy DIY pack pics: http://endless-sphere.com/forums/viewtopic.php?f=28&t=65094#p985797

 

[CroArcher]

http://lygte-info.dk/review/batteries2012/Common18650Summary%20UK.html

 

[Arbol]

Li ion battery should have 2 sets of cables if BMS is built in. One for charging one set for the load

Sorry for my basic question, but since English is not my mother language sometimes I need to ask basic questions in order to make sure I fully understand: when you say "one set for the load", the "load" refers to the connection to the controller, right?

Thank you very much for all the insightful comments. It seems 18650 are the way to go. One needs a spot welder, though. But if one is available, a well dimensioned package is unbeatable.

I have been looking at pages that sell these batteries, and there are "protected" and "unprotected" versions. If I understand correctly, the "protected" ones have some kind of BMS embedded. So if one plans to add a BMS anyway, one should buy the unprotected ones, right?

Where are good places to buy either 18650 cells or BMS? I see prices in Europe are very expensive, in the US affordable, in China quite cheap. Which are the purchasing experiences of people that already have bought these cells?

 

[agniusm]

Yes, load is controller or basically anything that requires portable power. You need unprotected cells as that protection is good say up to 30V. If you are building a battery pack yourself i would recommend researching suitable BMS for it.
Most people here, including me purchase their cells from China. It involves some risk but that is beaten by human nature in getting stuff as cheap as you can
I have done one pack where i took my cells to be spot welded but at the moment i am trying to build my own spot welder for around 80USD. (There is a topic further down on this page) Time will tell if it was worth a shot.

 

[Arbol]

Thanks for the clarification.

From reading the forum, I see bestech BMS products named a lot:

http://bestechpower.com/481v13spcmbmspcbforli-ionli-polymerbatterypack/

Is this company a source of suitable BMS?

 

[agniusm]

I bumped in to them while i was building my Panasonic battery pack, dnmun had ordered couple to play with so he's the man who knows most about them. i bought one, then second. It is working fine.

 

[Arbol]

I have been looking at vendor prices, and apart from big spreads in prices, I see many vendors only sell packages of 100 or even 200 pcs. This suggests a different strategy: buying 100 pcs of a value cell, such as the Samsung 22p as quoted in the second link in my first post. for example, a 13s7p would give 48v and enough amperage for a s12p, since em3ev argues a 13s4p of Samsung 22p should have a continuous max of 15 to 20A, and scaling from 4p to 7p puts even the 15A above the 22A max of the s12p. Of course, the problem of weight and location may become seriousbwith this strategy. Does it make sense?

Edit: or even do a 20s5p package with a s12p of 72v being sold by BMSbattery.

 

[Arbol]

I bump up this thread. I have been learning a bit on 18650, and a summary is as follows. Please comment / correct as needed:

- ICR, ie Lithium Cobalt or LiCoO2, is the usual "li-ion"
- IMR, ie Lithium Manganese or LiMn2O4, are "safer" than ICR, have a higher C than ICR but lower Ah than ICR
- NCR, or Lithium Nickel Cobalt Aluminum, or LiNiMnCoO2 or also NMC, are the "best" cells

If we do not consider the really cheap ICR, we have two groups of cells:

- IMR ones: for example, Sony US18650 V3 or Samsung 22P. These are about 2.2Ah, about 5C / 10A, and they cost about $2 / 2.1 the Samsung vs $2.4 the Sony.

Look the following comparation:

http://www.dampfakkus.de/akkuvergle...98&akku3=353&akku4=73&akku5=106&akku6=485&a=2

- NCR ones: for example, the Samsung 29E and the Panasonic 18650PD. These are 2.9Ah and also high discharge. Cost is between $3.8 and $4.

em3ev has these two groups already in his triangle packs: he has the 22P (IMR) for the "value package", and then the 29E and 20R (both NCR) for the expensive packages (20R has lower Ah but higher C than the 29E).

If I compare prices and Ah, I believe the 22P / Sony V3 have the highest price to quality ratio.

I do not know about life expectancy between the two groups.

 

[Arbol]

What do people think about the 26F?

According to:

http://www.dampfakkus.de/akkuvergleich.php?akku1=99&akku2=498&akku3=353&akku4=120&akku5=73&akku6=106

the 26F are almost as good in capacity as the Panasonic PD / PF and Samsung 29E, at 2A. Even at 3A and 5A, they hold up well.

I guess 5A is already too much for the 26F, but I guess that if one buys a big number, that should not be a big problem.

Does anybody have real experience with 26F? What about life cycles in comparison to other cells? and sag?