continuous draw on harvested 18650 cells?

diplomatically · Jun 3, 2020

My e-bike kit is a 52 volts, 1500 watts system which means the controller can do up to 30 amps roughly. I imagine I'll need all that power because of the hills in my city (Seattle) and I'll have the kids in tow sometimes. I want to use 18650 cells harvested from laptop batteries (14s10p). I'm not sure where I got this info, but from what I understand 18650 cells harvested from laptop batteries are best used if the continuous current draw is 500 mA per cell or below. If so, then I'm looking at only 5 amps, 260 watts continuous from the battery. That is no where near enough power. I know others have built e-bikes with used 18650 batteries - am I missing something here? Is 500 mA really the draw I should stay within?

john61ct · Jun 3, 2020

You may be overestimating what your kit can do wrt hills and "towing kids", you need that 30A for much longer than just a short few seconds "peak", need to plan as if that is your continuous need.

Forget scrapped cells, save your money up until you can afford a quality built pack of top-notch new cells.

Unless you're just fooling around, and don't mind buying stuff over again and again in the first year or two.

Hillhater · Jun 3, 2020

I want to use 18650 cells harvested from laptop batteries (14s10p). I'm not sure where I got this info, but from what I understand 18650 cells harvested from laptop batteries are best used if the continuous current draw is 500 mA per cell or below. I....

A lot depends on exactly what cells you “harvest”... 1.8Ah, 2.2 Ah, 2.6Ah, etc
Shure 500ma would be a safe figure, but most cells should be capable of 1C continuous discharge with little problem, and more than that for 10-30 sec bursts.
So , if you are smart and manage to get some good 2.6Ah cells ( common in many recent laptops) , your 10P pack should get you close to that 30A draw you are planning.
It would be wise to test a few cells at 2A and 3A continuous draw, to at least check for overheating and voltage sag.
PS.. you can buy new 18650s that will easily deliver 3 A for less than $2.0 each !

diplomatically · Jun 5, 2020

john61ct said:
You may be overestimating what your kit can do wrt hills and "towing kids", you need that 30A for much longer than just a short few seconds "peak", need to plan as if that is your continuous need.

Forget scrapped cells, save your money up until you can afford a quality built pack of top-notch new cells.

Unless you're just fooling around, and don't mind buying stuff over again and again in the first year or two.

Hillhater said:
A lot depends on exactly what cells you “harvest”... 1.8Ah, 2.2 Ah, 2.6Ah, etc
Shure 500ma would be a safe figure, but most cells should be capable of 1C continuous discharge with little problem, and more than that for 10-30 sec bursts.
So , if you are smart and manage to get some good 2.6Ah cells ( common in many recent laptops) , your 10P pack should get you close to that 30A draw you are planning.
It would be wise to test a few cells at 2A and 3A continuous draw, to at least check for overheating and voltage sag.
PS.. you can buy new 18650s that will easily deliver 3 A for less than $2.0 each !

No, I don't mind buying stuff over again and again - I wouldn't mind the lesson first hand.

Yeah, 2.6 Ah cells (tested not rated capacity) are my cut off for this project, so I'll test those cells at 2 and 3 A for sag and heat. At what voltage drop or temp increase should I reject cells?

john61ct · Jun 5, 2020

Best to keep going until you get them to at least match as much as practical

ideally same brand similar age

start with capacity testing, timed CC loads

The power and resistance should be less critical at low C rates.

diplomatically · Jun 5, 2020

Hillhater said:
PS.. you can buy new 18650s that will easily deliver 3 A for less than $2.0 each !

Where can I buy these cells that can easily do 3A continuous for less than $2.00?

spinningmagnets · Jun 6, 2020

Cordless tool cells are designed for max range and max amps that an 18650 cell is capable of.

Laptop cells are average capacity and low amps, because they are sold based on the lowest cost to make, and sales price. Laptops are specifically designed to use as low an amp-draw as possible, so...if they used high-amp cells, it would be a waste of production cost and eat into profits.

For big hills while towing a child, you will need a big mid drive. I'd suggest a BBSHD at a minimum. Depending on the budget, maybe even a high-quality Lightning Rods drive. It would last a lifetime and never overheat.

The stock BBSHD can draw a max of 30A at 52V, so that would be the minimum goal of any battery pack design you are considering. If you are willing to give up some potential top speed, the best upgrade would be swapping-in a smaller chainring. I thing the stock unit is 48 tooth, and there are several sizes, as small as 30T.

If you are new to ebikes, then building a battery pack is a huge leap, and one mistake in construction can lead to a large battery fire. Here is an article that might help you understand what you are getting into.

https://www.electricbike.com/introduction-battery-design-1/

diplomatically · Jun 7, 2020

spinningmagnets said:
Cordless tool cells are designed for max range and max amps that an 18650 cell is capable of.

Laptop cells are average capacity and low amps, because they are sold based on the lowest cost to make, and sales price. Laptops are specifically designed to use as low an amp-draw as possible, so...if they used high-amp cells, it would be a waste of production cost and eat into profits.

For big hills while towing a child, you will need a big mid drive. I'd suggest a BBSHD at a minimum. Depending on the budget, maybe even a high-quality Lightning Rods drive. It would last a lifetime and never overheat.

The stock BBSHD can draw a max of 30A at 52V, so that would be the minimum goal of any battery pack design you are considering. If you are willing to give up some potential top speed, the best upgrade would be swapping-in a smaller chainring. I thing the stock unit is 48 tooth, and there are several sizes, as small as 30T.

If you are new to ebikes, then building a battery pack is a huge leap, and one mistake in construction can lead to a large battery fire. Here is an article that might help you understand what you are getting into.

https://www.electricbike.com/introduction-battery-design-1/

Thanks, but buying a new mid drive kit is not an option at the moment. I have a rear wheel 1500W kit (fine print actually says it's 1860W peak) I bought 2 years ago that I'm simply trying to make use of. If fam and I end up using bikes frequently and the rear wheel doesn't prove enough power then I'll look into others.

Hillhater · Jun 8, 2020

diplomatically said:
Hillhater said:

PS.. you can buy new 18650s that will easily deliver 3 A for less than $2.0 each !

Click to expand...

Where can I buy these cells that can easily do 3A continuous for less than $2.00?

In the First post..... here..
https://endless-sphere.com/forums/viewtopic.php?f=31&t=61608
He is a very reputable supplier.
....you may want to consider his fully assembled 13S, 6P packs ?

continuous draw on harvested 18650 cells?

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