Question about making a battery


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beylian

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Dec 31, 2021
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2
I am currently testing cells from old laptop batteries, I want to make a battery for an electric scooter. I have the smallest batteries of 1000 mAh and 2100 mAh have the most amperage.Is it true that those under 1500mah should not put them,the batteries are from different laptops and different brands is this a problem?if in one package there are 4 batteries of 2000 mAh and 1 cell of 1500 mAh this means that the whole package will be weaker because of this cell of 1500 mAh, when the current runs out bms will turn off the whole package.
 

Korishan

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Jan 7, 2017
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6,835
Quick answer Nope Nope Nope and Nope

Don't use old laptop batteries for a scooter. Unless you can make sure that per cell amp load is <500mA max
Don't use capacities <80% of original manufactured. The lower the capacity, the more degraded the cells are, and the more likely they could thermal under heavy load.
This is even more so for cells that were used abusively (face it, most laptop battery packs were probably abused heavily, unless these are new-old stock)

Please read the FAQ located on the home page for more details about mixing capacities, chemistries, brands, etc.
 

OffGridInTheCity

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Joined
Dec 15, 2018
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2,032
Congratulations on starting your second hand cell processing - watch out as it can become addictive.

I am currently testing cells from old laptop batteries, I want to make a battery for an electric scooter. I have the smallest batteries of 1000 mAh and 2100 mAh have the most amperage.Is it true that those under 1500mah should not put them,the batteries are from different laptops and different brands is this a problem?if in one package there are 4 batteries of 2000 mAh and 1 cell of 1500 mAh this means that the whole package will be weaker because of this cell of 1500 mAh, when the current runs out bms will turn off the whole package.
Good questions but they show that you could benefit from some basics. For example - checkout the FAQ section of this site (https://secondlifestorage.com/index.php?forums/frequently-asked-questions.42/) and maybe start with S and P - https://secondlifestorage.com/index...-xsyp-with-x-and-y-being-numeric-digits.6379/ - as this will help communicate in a way to get you useful answers.

Your question is mainly around building a balanced pack but you don't mention the power aspect - e.g. laptop cells aren't designed for hi-amp and not typically suitable for scooter motors.

So let's start with a specific example and discuss balance and power. Then perhaps you can extrapolate from there.

Let's say you have room for a 28cell battery pack and you do 7s4p. That's 7 sets of 4 cells in parallel. You want each set of 4 to have the same mah. One could be made up of 4 cells = 2100 + 1000 + 2100 + 1000 = 6000mah. The other 6 packs must be 6000mah as well for things to stay balanced. So you could do a pack of 2100 + 1900 + 1000 + 1000 = 6000mah. Generally - you want to evenly distribute the capacity and cell types among the packs that make up the battery.

Tip1: typically you choose cells closer in range - e.g. within 85% or higher compared to the original specs.
Tip2: Use cells closer together in capacity - say 400mah max difference.
Tip3: If you can use all the same cell type - things go easier because they have the same 'chemistry'
Tip4: Check the IR and if you use proper (lower) IR then things go better.

OK - now we have a 7s4p (28 cell) battery! Now you need to think about POWER - will this have enough power to run your scooter motor.
Let's say the scooter motor needs 500w of power.
A 7s4p laptop battery would be capable of 1a/cell (typically) which is 4a (4 in parallel) * 24v (7 in series) = 96w as you're max power. **Won't run your scooter!!! as @Korishan says** You'll have to make it at least a 7s21p - 21a * 24v = 504w. That's 7 x 21= 147 cells - and even so, a surge of power above 1a won't work well and the scooter would be sluggish.

This is why they make hi-amp scooter (EV) cells as apposed to laptop cells. A hi-amp cell can do 10a/cell. So if we do a 7s4p of 10a/cell - we have 10a * 4 = 40a * 24v = 960w of power. For example - here's a reputable source for second hand cells and here's a 10a listing - https://www.batteryclearinghouse.com/products/30-cells

Conclusion -
1) Figure out the voltage you need to achieve - 24v or 36v or 48v.
2) Figure out how much power you need. 500w motor needs at least a 1000w surge? - so battery capable of 1000w of power.
3) Figure out the series portion of the battery design - e.g. 7s = 24v, 10s = 36v, 14s = 48v.
4) Figure out how many cells you have room for - the 'p' part.
This will give you the 's' and 'p' and then you can find the minimum amp / cell you need.
 
Last edited:

Redpacket

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Feb 28, 2018
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1,392
+1 to Korishan & OffGrid's posts above.
Also note that the higher you make the system voltage, the lower to current you need to pull from the batteries.
But even with a 48V system, laptop batteries (as explained well above) likely won't work well for you.
You need "high discharge current" rated cells.
Eg "vaper" or "tool" type cells with a discharge rating of say "5C" (ie 5x the nominal maHr number) or more.
 

italianuser

Member
Joined
Feb 25, 2020
Messages
478
Hi there Beylian, welcome.

How @Korishan said, for heavy loads like a motor you need power cells, cells that can give for e.g. 20Ah. A laptop battery doesn't need to give huge current and normally uses so called energy cells.

If you go in the "Cell Data & Specification" forum you can find datasheets (yes, do open it!) for all kind of cells and see what that maximum discharge current is.
This is a high drain 15Ah cell: https://secondlifestorage.com/index...8650-20q-cell-specifications.1747/#post-63167

Keep us updated with your project :giggle:
 

beylian

New member
Joined
Dec 31, 2021
Messages
2
I currently have about 120 cells I'm thinking of making a 36v battery 10s10p half of the cells are from a laptop the other half from hoverboard batteries
 
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