powering 12v dc motor from Lipo for respirator.

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AnonEmiss

New Member
So,,,
I have decided that I need a powered respirator for varnishing (moisture cured polyurethane contains isocyanate, not a good smell). I made one using a full face snorkeling mask and air blower (thanks Youtube) tested it using a bench power supply and it works great (so far, so good).

Now I want to make it portable:
I have a good supply of tested 18650's that while not new, are good enough for this job (as per photo). I want to make 3 power packs using a 4s3p battery layout (I have a home made spot welder to make the packs up). I could also go to a 5s3p layout, but would prefer not to.
I was intending to use the following:
- BMS (1 per pack): 4s BMS
- Option to charge via USB (only need one): 4s USB charger
- Battery level indicator (really only NEED one, but I might put 1 per pack): Power level indicator
- I already have a DC-DC buck converter to get the required 12v DC-DC buck converter
- And finally a motor speed controller (PWM I assume): PWM speed control

During testing I found running the pump @ 12v it was too powerful, so need to reduce the airflow (hence = speed controller)
I realise that I can use the DC-DC converter to reduce the voltage, but the adjusting screw is REALLY small, and I want to use an easy-to-get-at control, plus there is a risk that I might go above the rated 12v to the motor, so just use the buck converter to set a regulated voltage (12v) and use a motor controller after that.

My Question(s):
If I am going to use the above speed controller do I even need the DC-DC converter, and will there be any 'interference' using a PWM motor controller 'Downstream' of the buck converter?

The preferred BMS has a discharge cut-out at 2.55v - what if I wanted to set the cut-out at 3.25v (14v for the pack), how could I do that?
I know these are basic questions (and yes I am totally noob) - but that should mean they are easy for you knowledgeable people to answer (I hope).

I have attached some pictures to assist with 'context'
 

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It would be safer to still use the DC-DC converter even with a PWM speed controller installed. This is because the motor's windings are built to run on 12v. A 4s Li-Ion battery will put out 16.8v when fully charged. This can potentially damage the motor or reduce it's lifespan. Normal PWM controllers (not chopper types) cannot change the peak voltage going into the windings. That being said, some 12v motors can withstand higher/lower voltages than the rated voltage (especially ones meant to run on batteries) but if you don't have the datasheet then I would stay on the safe side and use the switching converter since you already bought it. If voltage and current limits for both boards are observed, there will be no issues using a PWM controller on the output of the buck converter.

For the BMS, the board you linked doesn't seem to have a user-adjustable cutoff voltage. You will have to find another one or find the datasheet for the chips on the board and try to see if the cutoff voltage is set by some resistors on the board which can be replaced.
 
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You could just use the buck regulator directly for speed control, without the PWM module.

The speed of the motor is proportional to supply voltage; it's set by the motor back EMF matching the supply voltage (less resistive voltage drops etc).

However, use a regulator rated far higher (3 to 5 times) than the motor current. Two reasons - a motor can take several times its rated current when starting up, and many cheap power modules are advertised with their short term peak ratings rather than their realistic continuous current ratings.

It should be quite easy to add an external potentiometer if you need to vary the motor speed in use.


To modify the BMS, it would need reverse engineering and to work out how it functions, then change components to suit the new requirements. Just not practical.

I'd suggest a small relay as a latching on/off control with couple of buttons to control it.

A single transistor and a few components could be used to enable the relay circuit when the battery voltage is above a set value, so the whole thing turns itself off totally when the voltage drops below the required minimum level.
 
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rjenkinsgb said:
You could just use the buck regulator directly for speed control, without the PWM module.
Click to expand...
AnonEmiss said they don't want to accidentally go above 12v which is why they don't want to do this.
AnonEmiss said:
plus there is a risk that I might go above the rated 12v to the motor, so just use the buck converter to set a regulated voltage (12v) and use a motor controller after that.
Click to expand...

It may be possible to add a fixed resistor in series with the pot though to prevent this.
Also, correct me if I'm wrong but since OP has already bought the buck converter they could still use it as long as they always ramp up the power slowly. Also, depending on how the pot is wired into the buck converter, would it not be possible to slow down the output response by having a capacitor connected between the wiper and ground to prevent the power from being ramped up too quickly? This would still not protect the buck converter if the motor stalled for any reason though.
 
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Explore using the same battery pack your cordless tool uses, be it a 20V, 18V, 14V, 12V,... You have the charger already. and can be a simpler tailored solution. Plug-in your cordless screwdriver battery pack to your respirator air pump.
 
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Galgso said:
It may be possible to add a fixed resistor in series with the pot
Click to expand...
Use the removed multiturn preset in series - or leave that on on the board, set it to 12V, and connect the external pot (using two wires) across the upper side of the preset; it should be possible to reduce the output to minimum value with the external pot at minimum.
 
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I assume if your motor is high efficiency, the 50W will be more than 500W on startup, which is easier for batteries than SMPS to supply. Whereas a portable smart BLDC motor might use constant current controlled low to high RPM commutation. (like the modern hand-held turbine blowers)

Since you don't need high RPM airflow but low RPM higher pressure, I would test it with 2S2P, 2S3P or 2S4P. You may be satisfied with the results with 50% reduction of RPM with Vbat in at 6V. and Vbat max = 3.7*2+ 7.4V. Then use a 7.4V LIPO charger for maintenance or make two such packs and use a 14.8V charger with some BMS added since they may not be balanced.

I suspect you may not need speed control then with 25W and 250W est. on startup. going at 61% (7.4V) to ~ 50% 6.0V) of the RPM rated by 12.0V over the span of the battery voltage which is more stable and efficient.

The RPM is proportional to Vdc.

If you want to cut at 3.25V per cell, that is a good idea to extend battery life, while 3.4V or more is even better if you have a spare pack then later we can implement that with an LED or buzzer circuit to alert you without needing a power regulator. We want more in parallel to lower the ESR compared to the DCR of the motor. It would be useful to know if you have a DMM or analog Ohmeter, to measure motor DCR. For 500W stalled @ 12V, I expect DCR to be R=V^2/P = 0.29 Ohms which isn't much different than the DCR of a weak 3S2P array, which is why you need lower series, more parallel. The wider the RPM changes with normal use, the more unbalanced, (the weakest cell gets weaker faster) and the shorter life expectancy of the pack, even with a BMS charger.
 
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Ahhh, now that is a simple idea, and I think it would work nicely.
My adding a new (externally mounted) pot, and putting it in series I can limit the max voltage on the buck converter to 12v and use the external pot to go for (for example) 6v-12v.
I would need to do some work with a multi meter to find out the pot resistance @ 6v and @12 V and then get a pot with a matching max resistance to the difference between the two measurements (or at least close enough,,,,,,)
Thaks for the good idea
 
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Another good idea, I can easily enough make a low voltage cut-out circuit separate to the BMS and add it 'up-stream' of the buck converter so it is reading the voltage directly after the BMS, I will continue to look for a better BMS that has adjustable cut-off option inbuilt (any recommendations?) as having it as part of the pack is preferable (IMHO) to having it as part of the respirator.
This is defiantly a good idea if I can't find any other option.
 
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Externet said:
Explore using the same battery pack your cordless tool uses, be it a 20V, 18V, 14V, 12V,... You have the charger already. and can be a simpler tailored solution. Plug-in your cordless screwdriver battery pack to your respirator air pump.
Click to expand...
I did consider this option but there were several reasons I decided not to do this:
1 - I will probable be using my cordless tools when I am using the respirator and simply don't have enough spare batteries to go around (they are stupidly expensive for what they are)
2 - I have a very healthy supply of spare 18650 batteries that are looking for a use (some are in the photo)
3 - I can use the bench top power supply as a charger (in the photo), plus I am going to add a USB charger into the design.
 
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I can confirm that the start up draw peaks at >10amps (that was as high as I can test) and I hear what you are saying about the lower voltage pack, but (isn't there always a 'but').
I can see the odd occasion where I would want full air flow, so want to be able to still deliver the full 12v if required.
I fully expect that a 60-75% power level would be fine for 95% of the time (so a 2s of 3s pack).
I already have the buck converter, and plenty of spare 18650's just lying around looking for a project.
Somebody else suggested a low voltage cut-off circuit connected to a relay, adding a buzzer (or LED) to this makes sense.
I am not technically skilled enough to try to replace resistors on a BMS or do anything much more sophisticated than "bash it with a rock until it does what it supposed to"
 
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AnonEmiss said:
3 - I can use the bench top power supply as a charger (in the photo), plus I am going to add a USB charger into the design.
Click to expand...

Eek - no, you mustn't - a bench top supply isn't suitable, Li-Ion batteries need VERY specific and exact charging, otherwise you're likely to have a fire. A BMS ISN'T a charger, and relies on a proper charger been used.

You can use the bench supply to power a charger - but not use it as one.
 
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The BMS linked in the original post has overcharge protection. The stated overcharge protection value is 4.28v which seems a bit high, but in theory a BMS similar to the one OP linked should stop any of the cells from overcharging and the supply current limit can be set to a low value for safe charging. Apart from the overcharge protection voltage being slightly higher than usual, why would such a setup be unsafe? I've charged 18650 cells with power supplies before but never with multiple cells in series.
 
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It's still not a charger - and is DANGEROUS - look up how you charge li-Ion, it's not as simple as that, and the BMS is pretty irrelevant.
 
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Sadly, we never hear feedback on theories about lithium batteries, or ignorance about lithium batteries. It's unlikely that family members know they asked a question here to give us the feedback.
 
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Nigel Goodwin said:
It's still not a charger - and is DANGEROUS - look up how you charge li-Ion, it's not as simple as that, and the BMS is pretty irrelevant.
Click to expand...
I am aware of how li-ions are charged.
I'm not saying you're wrong, but li-ions are charged by constant current and then constant voltage once they get close to the full charge voltage, which is exactly what a power supply would do if you set the voltage to the correct value (4.2v for a single cell) and set the current limit to a safe value. Of course balancing is required for multiple cells in series, but if that is taken care of why should one not charge batteries with a power supply? I have done this before several times without experiencing any issues. Please tell me what I'm missing here.
 
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Obviously not, google it and see where you're misinformed - but basically how are you detecting full charge?.
 
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Nigel Goodwin said:
Obviously not, google it and see where you're misinformed - but basically how are you detecting full charge?.
Click to expand...
Unlike lead-acid batteries where the terminal voltage is not necessarily a good indicator of state of charge, li-ion batteries can have their state of charge determined by terminal voltage alone when the current through the cell is low enough to render voltage drops across the internal cell resistance negligible. The supply will stop putting out power when the cell reaches 4.2v (fully charged) since it will be set to that voltage and there will no longer be a potential difference.

This article seems to agree with me:
 
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