Im looking to boost single 18650 to 24v 1.7-2A, is this possible?
I can get to 12v 2A, but 24v would be better.
Wanting to run one stepper motor from single 18650 at max speeds.
Looking for alternatives and ideas.
1 Like
For how long can you draw 14 Amperes from an 18650 cell?
alternatives and ideas
A standard bench power supply.
Keep in mind that with a proper stepper driver, the average power supply current drops as the power supply voltage increases, for the same motor output power.
2A @ 24V = 48 Watts.
48 Watts @ 3.7V = 12.97A.
Assuming perfect conversion efficiency.
Also suppose you can use 80% of the full charge. Thats around 8Wh.
8Wh /48W = 0.16 hours or 10 minutes of running time.
Maybe you need something like this?
https://www.amazon.co.uk/Original-Battery-Lithium-6000mAh-Electric/dp/B08LQ4P119
And will the end caps of the cell withstand it for as long as it has enough charge.
To explain the above figures.
Your load is 24V @ 2A so the power required is;
P = V x I = 24 x 2 = 48W.
The converter output has to supply 48W of power.
This means that on the converter input, the battery has to supply at least 48W with 100% efficiency.
P = V x I
I = P / V
So
I from the batt = 48 / 3.7 = 12.97A.
With say 90% efficiency of the converter.
12.97 / 0.9 = 14.41A
Tom.... 
Yes, 10 minutes would be sufficient. As this is not used continuously, but in short burst. And the full power only rarely. But i was not asking how many watts it is 
but if there are step up converters that could do this?
Also, single 18650. Not large battery packs.
Some 18650 cells can do at least 25A and peak at 35A. So that should not be the issue, the issue is are there suitable converters to do this job? Basically anything over 12v would be a plus. As with stepper motors i can get higher speeds with higher torq the higher i can get the voltage.
Currently im able to reach 15v 2A, but 18-20v would be great. 24v is the max.
And this 2A can be peak, i dont need it continuously. Actually the peak is only 1.75A.
1 Like
Hi,
What is the project/application?
Tom...
To run a stepper motor from a single 18650 cell.
This might work at 18v 1.75A from a single cell : TPS61288 data sheet, product information and support | TI.com
I did a search at DigiKey.com and there were no DC-to-DC converters with 24V 48A output that didn't require at least 8V input.
Yes, 24v might be too much to ask.
But i'm also starting to doubt that i need that much current. I have hard time figuring out how much current does stepper motors actually pull. I have only seen confusing posts about it.
Now i tested it by limiting my bench power supply's current, until it affected motors torque.
At 18v, i got 0.35A. It runs perfectly with these settings. While the stepper driver is set at 1.75A and reporting exactly that. So i'm probably not understanding something here. It cant be feeding it 18v & 1.75A, but way less power. As it only draws 6.3W from the supply.
A very simplified way of looking at steppers is to think of current as what provides the torque and voltage as the thing that gets the current up to the value needed. Current will provide the heating of the coils and so this is what needs to be limited.
Current will rise with increasing load on the motor.
The driver is limiting the current to the maximum value tolerated by the motor.
But still, it does not draw even nearly what it should from the power supply. This is what i'm not understanding. Why does it draw so little power from the supply? While the driver reports back that its running at 1750mA. And this is a TMC2209, so i get real time current data from it.
Is your motor under load?
What current does it draw when you increase the load?
Power = current x voltage
Yes, its under load. I did this test under max load & speed. I tested the max limit, when it stalls with this load. Set the speed a bit under, and just dialed the current dial until it stops running and fine tuned it until i'm just over the point.
18v @ 0.35A and its running perfectly fine. Thats 6.3W.
The motor is rated at 1A, and i'm running it at 1750mA. As i don't need continuous operation, so heat won't be an issue. And i have dynamic current control via TMC2209, so i can then adjust the current based on need.
This motor has a resistance of 5.7ohm, so at 1A that would be 5.7V.
And 1.1A @ 5.7V is exactly 6.3W. That's the only way i could make sense out of this.
At 24v, that would mean only 0.26A.
This does make sense, as this is a small Nema11 motor, and how could i dump the same power into a tiny motor as i could into a large Nema17 or 23.
Yes, 6.3W is correct. 5.7ohms 1.05A 5.99v is 6.3W. So its drawing only a tiny amount more than the specs say.
What i now do not understand, is the motor i'm considering switching to has higher power output according to the torque curves. But it also has much lower resistance and according to this logic, only 1/3 of the power draw. How can it generate much more power while drawing much less power.
I believe this logic is not correct.
The power out from a motor is equal to the power demand on the motor, assuming equal efficiency of the motor and power train.
Imagine a small child lifting an apple and a grown up lifting an apple 10 cm up: who is doing more work on the apple? Neither, since both are moving it the same distance against the same force. Who will be slightly less efficient in the real world?
Similar logic can be applied to a motor too. The larger motor and smaller motor will draw the same current x voltage if they move the object the same distance in the same time. Assuming equal efficiency, the power draw will be the same. Note, it is the power which will be the same; the individual voltage and current may be different.
BTW, NEMA 11, 17, 23 etc designations only represent the size of the faceplate (in inches x 10). They say nothing about the power rating, etc. Now, clearly a larger motor will be able to dissipate more waste heat, but that depends entirely on the manufacture of the motor, e.g. the length of the motor, quality of construction, etc.
The larger motor may have larger maximum output.
Yes, i know that nema standards are about the faceplate. Everyone knows that But i assumed that reasonable person understands that we would be talking about the same length of motor if we compared them.
Sure the power needed is the same for the same task, but i was talking about the max power. You can dump more power into a larger motor, and it can do more work.
Exactly what i said about how it was no wonder i could not dump the same watts into this tiny motors as i could into a larger. Meaning if i run both at 24v @ 1.7A, they are not drawing the same amount of watts from the power source.
What i would be interested in, and was trying to figure out.. is what is the actual power draw from the power source.
I now tested this, and the max current draw seems to stay the same across the voltage rage. In my case it maxes out at 0.4A. But i dont understand how to predict this from the motor specs.
If that is per winding (which the driver current setting formula assumes), the motor will overheat.
i assumed that reasonable person understands that we would be talking about the same length of motor if we compared them.
Yes, everyone understands about the NEMA faceplate dimensions. But that is completely irrelevant to the discussion. What is actually important is the motor current rating, which you appear to be violating.
The power supply current is not a useful measure of the motor current during operation, which varies extremely rapidly as current limiting kicks in.
Yes, it would overheat if i let it run continuously for a long time. Which i explained i'm not doing.
I'm not sure how me "violating" current ratings is the issue Modern stepper drivers have things like coolstep, that exactly are designed to do this.
And its exactly the power supply current draw that is the question here. We already know the motor current, as that is reported back by the driver in real time.
The actual question is, how to calculate the power supply current draw. I have tested it, and now know it.. but i dont understand how it could be calculated aka predicted from the motor specs.
Have fun, and do let us know how you get on using one 18650 battery for the project!