I'm trying to build my own battery pack as part of an electric scooter project I'm working on. The motor I have is 200W 24V and is rated at 11 A. I've explored using six 18650 Lithium ion cells for this, but it still doesn't produce 200W because these cells are 2600 mAh and there aren't any 18650s that will give me 11 A. Also tried using three 9V batteries but those are 8 milliamperes which does not work at all. Any suggestions? How should I go about powering this motor? What kinds of cells should I use? I'd like to make my own battery pack instead of buying them.
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1\$\begingroup\$ Well, the fact that the motor is rated for 200 W, 24 V, and 11 A doesn't mean that you actually need that much power, voltage or current in order to spin it. Have you investigated how much voltage, current, and so forth your motor actually needs? Also, is it a DC motor or an AC motor? \$\endgroup\$– Sophie SwettCommented Jul 6, 2021 at 17:22
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1\$\begingroup\$ You probably want to assume an average consumption of 100 W and at 24 V that will be 4 A. Next figure out how long you want it to run for. If you want a one hour run time then you'll need 4 Ah plus, say, 50% extra, so go for at least 6 Ah. \$\endgroup\$– TransistorCommented Jul 6, 2021 at 17:24
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\$\begingroup\$ You will need a method to balance the pack. Since this is only 6S, you could use a balance plug instead of a BMS. It also sounds like you may be confusing mAh (capacity) and amps (output current). The output current of cells is usually described as a C rating. You may need multiple cells in parallel to reach 11A output. \$\endgroup\$– DrewCommented Jul 6, 2021 at 17:24
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2\$\begingroup\$ 2600 mAh is not a measurement of instantaneous current ... it is a measurement of how long the cell can provide some specified current before the output voltage drops to some specified level ... 2600 mAh could be 10.4 A over a period of 15 minutes, or it could be 130 mA over a period of 20 hours ... read the cell datasheet \$\endgroup\$– jsotolaCommented Jul 6, 2021 at 17:49
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1\$\begingroup\$ You keep mentioning batteries with low mAh ratings. 9 mAh is enough to light an LED for an hour. It won't run a scooter. \$\endgroup\$– TransistorCommented Jul 6, 2021 at 19:34
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2 Answers
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I've explored using six 18650 Lithium ion cells for this, but it still doesn't produce 200W because these cells are 2600 mAh and there aren't any 18650s that will give me 11 A.
18650 cells that can supply more than 11 A are available, but you will be limited by capacity. Larger cells generally have higher current rating as well as higher capacity, so I suggest using 26650 size cells. Six 5200 mAh 26650 cells in series would provide the power you want with about twice the run time of 18650 cells. Assuming you don't use more than 80% of the battery capacity and the average current is 7 A, you should get ~5.2 x 0.8 / 7 = 0.6 hours or 36 minutes.
However in the comments you say that you can't spend more than $30 on the batteries, but 6 quality cells (not eBay fakes) will probably cost more than $30. I suggest increasing your battery budget by a few dollars, rather than compromising performance and wasting all the money spent on this project.
What kinds of cells should I use? I'd like to make my own battery pack instead of buying them.
Li-ion cells are probably the best for you because they have higher power and energy to weight ratio than Lead Acid or NiMH, and are safer and more robust than Lithium-polymer.
answered Jul 7, 2021 at 23:12
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\$\begingroup\$ Thank you all so much for your help! I've learned so much and I think I can do it now! Just to make sure I got this, 26650 cells are about 3.3V, so 6 of them in series gives me about 20V and 5200mAh. This means, at 11 A, it will provide 200W to the motor and will run for about 20 minutes right? \$\endgroup\$ Commented Jul 8, 2021 at 22:42
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\$\begingroup\$ To connect these, I'm planning on getting a mini spot welder for about $40. Is there any specific kind of nickel strip (dimensions, thickness ,etc.) that would be optimal? Again, planning on making the battery pack 20V 5200mAh and I want it to supply about 10A, so my motor is receiving 200W. \$\endgroup\$ Commented Jul 8, 2021 at 22:51
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\$\begingroup\$ Also, my Liyafa BMS should be good for this right? I'll post link for the BMS and for the power adapter I have. For safety, could my batteries explode just by using them? Will they overheat or something? If that is a risk, should I get an aluminum or steel box to put them in? \$\endgroup\$ Commented Jul 8, 2021 at 23:01
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\$\begingroup\$ BMS: amazon.com/gp/product/B07RRTMMZT/… Power Adapter: amazon.com/gp/product/B00GO8LJ6Q/… \$\endgroup\$ Commented Jul 8, 2021 at 23:23
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If you can find a supply of old laptops, then it's possible to disassemble the battery packs. But you would have to test each cell individually, to work out which ones are good and which ones are failing. Six 18650s isn't going to be enough. You would need more of them, in a series-parallel arrangement.
Or buy a lead-acid battery.
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\$\begingroup\$ How many would I need if I were to buy the batteries from this link: liionwholesale.com/collections/batteries/products/…. I've figured that I need a 24V 9Ah battery. \$\endgroup\$ Commented Jul 7, 2021 at 15:12
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\$\begingroup\$ @LearningEngineering Each of those is 2.5Ah. So six of them in series gives you 24V 2.5Ah. If you have four parallel sets of six, that would give you 24V 10Ah. That's 24 batteries in all. They would have no trouble giving you 11A peak. Of course, that gives you 24 batteries to charge up again when they go flat. \$\endgroup\$– Simon BCommented Jul 7, 2021 at 16:25