powering 12v dc motor from Lipo for respirator.

[AnonEmiss]

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'

 

[Tony Stewart]

End of charge conservative at CV is I< C/10 or < 10% of CC rate. Aggressive is < 3% of CC rate. The aging rate accelerates above 3.8V during charging and more rapidly above 4.0V and even more above 4.1 as this causes self-heating which is an accelerator of aging.


There are different profiles for Lithium chemistry, safety, speed-charge and long life cycles (>> 5000) that depend on how much time you spend below 3V and above 3.8V and the rise in the junction temperature has a common Arrhenius Effect every 10'C rise above room temp.

Just beware, there are tradeoffs and safety concerns if unattended and choice of cutoff voltage for charge and discharge are big factors.

Common is 10% CC until Vbat > 3V then CC until CV then cutoff when Ibat< 3% or 5% or 10% CC (depending on OEM) with no external load! Otherwise, both charge and battery current, must be measured simultaneously.

Did you notice anything uncertain from the statement "For these reasons, the charger should control the final voltage to within ±50 mV of 4.1 or 4.2 V and be able to detect when the battery is fully charged" ? Why 4.1 or 4.2?

The capacity is slightly improved with 4.2V end of CV cutoff, but life expectancy is compromised due to the increased self-degradation when above 4.0V. I once plotted the data from the Battery University site and when only using 50% of the rated capacity and also limiting CV to 4.0V before cutoff you might have 90% capacity thus if you stop using the battery at 40% you have only used 50% of the capacity yet the total Ah times N cycles can be 10 times more energy used than following the manufacturer's specs. Most users just want the longest flight time or longest mobile time between charges and don't think about the long term. Besides, they usually don't have the option to use a conservative charge profile until iOS recently does this automatically based on your usage and I recall Lenovo laptops once had a setting for the situation where a laptop was plugged in all the time, so the capacity was kept at 66% or 75% (?) just like the factory ships them for the longest shelf life.

Now, this was anecdotal memory on conventional Lithium-ion and Lithium Polymer cells and not LiFePO4 cells.

Series battery packed cells must be extremely well-matched when new because there is a positive feedback mechanism that degrades the weakest cell faster than the rest in series. Whereas in parallel, the strongest cell with the lowest ESR supplies or receives more current, which can ages the battery until the ESR matches the others even if the Capacity in kilofarads is different. Thus, conservative 0.5C charge rates are preferred to reduce aging of the weakest series cell.

Just because they say you can charge 70% of the battery capacity in x minutes, doesn't mean you should! There is always a tradeoff with aging and aggressive charging.

I have also charged Li Ion batteries with a digital lab supply on occasions and even primary EverReady Alkaline and Lithium Coin cells in a pinch at very conservative currents to avoid thermal rise just to eke out a tiny more charge storage life.

 

[Tony Stewart]

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:

https://www.digikey.co.uk/en/articles/a-designer-guide-fast-lithium-ion-battery-charging

Lead-acid voltage can be accurate in some situations with capacity 0 to 100% from 11.5 to 12.5V if preloaded after charging to bleed off some secondary charge layer within a minute at room temp. Otherwise the voltage can float at 12.8V for a while after charging. Lead Acid also have much higher ESR. The CCA rating is a drop from 12.5V to 7.5V at 0 deg. Although your car might never demand that unless the engine was -30 'C and the battery was kept warm with a 100W blanket.

Tony from WinterPeg
( but now near Toronto)

 

[Tony Stewart]

FYI

BU-808: How to Prolong Lithium-based Batteries

BU meta description needed...

batteryuniversity.com

https://iopscience.iop.org/article/10.1149/1945-7111/ad6cbd (LiPO4)

Understanding Charge-Discharge Curves of Li-ion Cells • EVreporter

This charge curve of a Lithium-ion cell plots various parameters such as voltage, charging time, charging current and charged capacity. When the cells are

evreporter.com

https://batteryarchive.org/

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