18650 Cells

What it it?

https://www.google.com/search?q=18650+cells&client=ubuntu&hs=KUs&source=univ&tbm=shop&tbo=u&sa=X&ved=0ahUKEwj8xPLHwdLfAhULtIMKHTQ3ADUQsxgIKw&biw=1438&bih=738

General Performance

• 10A discharge, self-protection circuit inside.
• 3.7V
• 3 Ahr
• 18 minutes of 37W
• 10W for 1 hour
• 100 of these give 1 kWhr
• 45g each, so 1 kWhr weighs 4.5 kg.
• 4 kWhr is under 20 kg, and costs $1400 - see below
• Continuous power is 14.8 kW (20 hp).
• At this density, ultra light cars (500 lb total) - will have 30 miles per kwhr.
• 1kwhr ~ 4MJ - so about 1/10 kg gasoline. Given gas is 20% efficient in cars, we have 1/2 kg equivalent. Tbis gets us to about 240 mpg equivalent.
• With 1 kW solar added- that gets us to about 400 mpg equivalent.

Peak Performance

• 20A continuous is common, and max at 40A continuous.
• 20A continuous means higher peak for a short time - 40A. That is 150W per cell for a short time! Such as passing in automotive applications.
• 100 of these give 15kW peak - for 5 kg of batteries. This is high power density
• Each bat is $3, so for $900 you get 45kW peak power - sufficient for excellent performance on hybrid vehicles that have a small engine (5kW) and have much higher peak power for hills and passing (50kW).
• Solar cars as such become practical, with solar hydrogen power.

Best Cell

• Samsung 25R - [1]. 20A continuous for 6 minutes at 3V. 60W cell. 100 of these are 6 kW, which make for a decent burst power source at 4.5 kg weight.
• Practical and affordable scenario - 12kW - 6 minutes of time at a weight cost of 9kg. $560. As part of an auxiliary hybrid drive train.
• Changfa Diesel costs 180kg in weight - so is much heavier.

Discharge Characteristics

• Can 18650 cells really discharge at 10A rate? Yes! [2] - Panasonic goes up to 20 amp rate. 6S (6 cell) packs are 22v. 10 of 6s cells, if discharged at 20A, give a 200A welder for 6 minutes. See Discharge Rate.
• Crappy ones can be as low as delivering 2.5A each - [3]. With 100 of these, that's still a 250A welder which has a weight of 5 kg for the batteries. Still impressive.
• Discharge rate of 20A yields temp increase to 50C - [4]

Specs

• Note actual spec from a buyer on Amazon - [5]

Reviews

• https://thegunzone.com/best-18650-batteries/
• https://www.18650batterystore.com/Best-18650-Battery-Guide-s/142.htm

Management

• While specs say 250 cycles for 15A discharge - Multiple thousands of cycles are possible when charged to 80% and drained to 30%. - [6]

Sourcing

• Samsung 30Q - $3.75 - [7]. This says 15A rated and 50W continuous max - 15A for 12 minutes. [8]
• Sample - $1420 total with shipping for 400 batteries, 4 kWhr, 20 kW for 12 minutes continuous rating. May be relevant for a power cube application.
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Cost Analysis

• From <$1 to $7 each - so choose wisely -

• Best - Samsung 30Q - 3000 mAh - $3 in bulk - [9]

• These appear to be on spec at 3000 mAh - [10].
• Cycle life and specs - [11] - 250 cycles to 70% of rated
• 1000 cycles at 6A discharge rate bottoming at about 60% capacity for 2000 cycles? - [12]. So these last pretty long.

• From lifetime design perspective, 2000 cycles is only 3 years, though. If used daily - batteries last only 3 years. In practice - 10 year max is likely. Not a good solution environmentally.

20A Continuous

• This one says it's 20A continuous - [13]
• See ratings link for 30A continuous
• 20A continuous and best seller- Samsung 25R 18650 - [14]

Links

• Ratings of 18650 batteries - [15]
• 21700 Cells
• Arduino Battery Chargers
• 18650 Battery Packs
• Battery Pack SMEs

External Links

• The Wikipedia Page on 18650 Cells
• Fantastic forms for recycling old 18650s into DIY Power Walls https://secondlifestorage.com/index.php
• Best rated battery and everything you need to know about 18650s - [16]
• A Video by the YouTube Channel "Teaching Tech" Titled "Beginner’s guide to building safe 18650 batteries" (Goes over basics of 18650 Cells, and design process guidance from their experience (Also lots of useful links/mentions) )