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Battery Pack, 48 Volt 20 Ah LiFePO4 cells

LewTwo

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Apr 8, 2014
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Houston, Texas
I really do not want to build a battery pack but the Shark battery pack I was going to use on my current Giant Revive E-Bike Build is “Tango-Uniform” (you children that do not know what that means should ask your grandfather). I might be able to fix it but it is likely too close to end of life to be worth the effort. I will salvage the case and Daly BMS and try to find someplace that recycles Lithium Ion battery cells. Moving on.

A new 48 volt 20 AH "downtube” battery in a one of the various hailong battery cases would cost anywhere between $350 to a $1000 depending on the vendor, quality and brand of cells. A generic flat pack battery (made with 18650 cells) of similar capacity would cost $270 to $700 … again depending on the vendor, quality and brand of cells. A generic flat pack battery (made with LiPo envelopes) of similar capacity would cost $250 to $500 … and depending on the vendor I would also need a couple of large ABC fire extinguishers.

I decide first and foremost I wanted a battery made of Lithium Iron Phosphate (aka LiFePO4, LFP) cells. Now I know most folks look down on E-Bike battery packs made with this chemistry due to their lower power density and generally higher initial cost. I feel that they are safer and in the long term less expensive. Rather than rehash this subject I am going to refer the reader to the web page:
https://www.goldenmotor.com/batterypack.htm
Scroll down to bottom where the heading “Innovation in Li-ion Battery” appears.

They are also very hard to find …. especially at a reasonable price. For sometime I have been looking at various brands, sources and form-factors for LiFePO4 cells including Headway, A123, LithiumWerks, Lishen, Topband, etc. I especially liked the Headway cells because they do not require any welding and the battery pack can be readily disassembled for service or repair. But they are expensive and the cylindrical form factor is not overly space efficient. Another alternative is “prismatic” cells. These are rectangular metal cased cells with positive and negative terminals on the top/end (often screw terminals) .The disadvantage is that they are more commonly available in large capacities (50 to 200 Ah) more suited to “Off Grid” home power systems. A few manufacturers do make smaller capacity prismatic cells. One of those is Topband. I found some of their 20 Ah cells listed on FleaBay (they are also available via AliExpress and Alibaba): https://www.ebay.com/itm/184860165804
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I ordered 16. I also just happen to have a Daly 16S LiFePO4 30 Amp BMS. Not the fancy ones with WiFi, Bluetooth, etc. but a dumb common port one. I did some layouts and figure the fastest, most covenant way to assemble a battery is four rows of four cells. I will need to make some copper bus bars and acquire M6 jam nuts. That will give a rectangular block that is about 4-1/4 inches thick and 12 by 6 inches. The weigh will be about 19 pounds. This can be attached to the rear rack via some 3 Inch wide nylon webbing. Ultimately I would prefer to have two slabs of 8 cells attached to either side of the main frame. In the 16S1P configuration the avail continuous power is 60 amps ... way more than the peak 20 amps I am likely to draw.
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Thoughts, comments or flames invited …

Giant Revive BBS02 Mid-Drive
https://endless-sphere.com/forums/viewtopic.php?f=6&t=117124
 
There are times in one's life when one should acknowledge that they have well and truly screwed up. This is one of those times for me.

I ordered these batteries because they were 5 Ah smaller than the more common Topband 25 Ah LiFePO4 cells. I though that they would be a tad smaller and bit lighter. I was wrong. They are actually the same physical size as the 25 Ah cells and only about 50 grams lighter. Credit where credit is due, they are testing to be in excess of 21 Ahr capacity. However the voltage sag at 10 Amps is an immediate drop to from 3.5 volts to 2.75 volts. I am not confident at running these cells at 15 to 20 Amp current draw.
Discharge Curve Cell 04.png
To be fair I should note that my EastTester ET5410 Electronic Load measures the voltage across the load (vs across the battery terminals).
TB-022770180D-FE-20AH Cell (Dims).png
The Fleabay vendor “Nextgear” did not ship these cells in accordance with DOT regulations. He shipped them via Fedex Ground in a single layer cardboard box without any “Hazardous Materials” labels on them. Said box was about to split open when I received it. There was also no data-sheet. I think that these cells are either 10 years or 25 Ah cells that have been derated (due to age or quality problems) … or maybe both. I will not being doing business with this vendor again. He will also not appreciate my Fleabay feedback.

In summary, these cells are big, heavy, of questionable quality and age. US Department Of Transportation regulations make it impossible for a consumer to legally ship Lithium-Ion cells back to a vendor. As I have warned others, “If you receive a set of Lithium Ion cells or a Lithium-Ion battery from an online vendor then you own it regardless of all other considerations”.

Time to cut my loses and move on (Putin should consider that). I am going to use these to build 4S1P, 12 Volt batteries for use with some computer UPSs. They should be adequate for that.

For my E-Bike, I am going back to my original concept of using 16 Headway LiFePO4 cells.

Reference:
Code of Federal Regulations, Title 49 Subtitle B Chapter I Subchapter C Part 173 Subpart E § 173.185, "Lithium cells and batteries".
https://www.ecfr.gov/current/title-49/subtitle-B/chapter-I/subchapter-C/part-173/subpart-E/section-173.185

For anyone interested in LiFePO4 batteries there is an extremely good youtube video of a lecture by Professor Jay Whitacre of Carnegie Mellon University.
https://youtu.be/QlDd3jkcxoQ
 
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