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3P charging

BroomJM

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Apr 7, 2021
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I have placed three 18650 cells into a plastic housing that creates a 3P pack, with roughly 7800mah worth of capacity. I have connected a male 5.5x21 jack to this pack, to power a ~3.5v motor.

I bought a 110v power supply for Lithium batteries (4.2v output) with a female 5.5x21 end on it, so I can plug in this little pack. Is this a safe way to recharge this pack? Is there a better way to know what state of charge this pack is at?
 
I should have mentioned these are Li-NMC cells, but I believe the charger I got is technically for LiFePO4 ?
 
I should have mentioned these are Li-NMC cells, but I believe the charger I got is technically for LiFePO4 ?
lifepo4 max per cell voltage is 3.65 volts.
NMC is 4.2 volts.
NMC as you probably know is a "vent with fire" chemistry so it pays to be careful.
Please consider a protection circuit.
 
lifepo4 max per cell voltage is 3.65 volts.
NMC is 4.2 volts.
NMC as you probably know is a "vent with fire" chemistry so it pays to be careful.
Please consider a protection circuit.

OK, I have verified that the output from this charger is exactly 4.2 volts. What I don't know is if this charger is configured in some way to know when the cell is fully charged, and stop supplying any current.

It came with very crude "Operating Instructions", which state that it has over voltage protection. It says, "The charger will cut off automatically if battery is fully charged (1/10c) and indicate a red LED when charging and green LED when battery full."

So, it may actually be that this is designed for Li-NMC and NOT LiFePO4?
 
OK, I have verified that the output from this charger is exactly 4.2 volts. What I don't know is if this charger is configured in some way to know when the cell is fully charged, and stop supplying any current.

It came with very crude "Operating Instructions", which state that it has over voltage protection. It says, "The charger will cut off automatically if battery is fully charged (1/10c) and indicate a red LED when charging and green LED when battery full."
The 1/10c bit sounds like it terminates charge on tail current.

So, it may actually be that this is designed for Li-NMC and NOT LiFePO4?
4.2 volts is absolute max for NMC and far exceeds the 3.65 volts that is the max for lifepo4.
Still recommend a protection circuit.
 
The protection circuit typically goes with the cells not with the charger.
The use case for this battery pack calls for it to be light and portable. It's in a 3P arrangement. Is there a BMS for that or are you talking about something else I don't understand?
 
The charger you have is designed for a Battery with a BMS --they make all kinds of chargers for raw cells

So, if I bought that hobby charger, I see it has a place to connect a 3-cell lead. If I'm understanding correctly, that would be like the wires a BMS uses to balance cell voltages, in a series configuration. Is that right?

Here's where I think I might be confusing myself: These cells are in a plastic housing with built-in leads for a 3P configuration. So, they are constantly wired in parallel...never in series. When you balance cells before placing them in series, don't you wire them in parallel to essentially make one large cell, to help ensure that each of the cells are at the same voltage? Aren't the cells I'm using always in balance, by virtue of always being in parallel? I get that they can have different internal resistance (they're all 2200mah cells, but 2 brands) but aren't they constantly going to be balancing across the cells, because they're in a 3P arrangement?
 
The use case for this battery pack calls for it to be light and portable. It's in a 3P arrangement. Is there a BMS for that or are you talking about something else I don't understand?

Think of a protection circuit as a bms without cell balancing.
Since your battery has no series component it doesn't need the balancing function.
It does need the protection circuitry to keep the cells within the safe envelope though.
 
The important thing about any Cell Charging is to stay within it's voltage range --The bad thing about Li-ion is if the charger keeps going well over 4.2v they can go BOOM --- also something to thing about is how low will you be discharging these cells
They have several charging circuit boards that use a simple 5v USB connection also

 
The important thing about any Cell Charging is to stay within it's voltage range --The bad thing about Li-ion is if the charger keeps going well over 4.2v they can go BOOM --- also something to thing about is how low will you be discharging these cells
They have several charging circuit boards that use a simple 5v USB connection also

OK, so this is getting more complicated, but if I placed the chip you have shown above into an 18650 3P battery pack, I would have both over-charge and over-discharge protection, correct? I presume the micro USB port can be used for discharging as well as charging the batteries in the pack.

The dual-speed motor I'm driving is rated for 3 volts @ 500ma, or 1.5 watts. On the lower speed setting, the motor "sounds" like it does when powered by the 3 volts that two fresh D-cell batteries provide. On the higher speed, it sounds like it is running noticeably faster than it normally does with D-cell batteries. I presume this is because the 18650 cells operate at a higher voltage. When on the lower setting, the higher voltage, combined with lower amperage, "sounds" like it's pulling around the 1.5 watts you would normally only see on the higher setting.

The charging board/protection circuit listed above would require me to have a DC-DC buck converter to bring 5v down to 3v, nominal, for the output. I am certain I would not be able to run this 3v motor at 5v for very long, even on the lower of the two settings. I would also have to find a way to go from micro USB out, from the battery pack, to 5.5x21mm in, to power the motor. I am not finding any connectors/cable with that particular combination.

I did find a similar board (below) that has both a standard USB-A port and the micro port. Either could be used for charging and the standard port could be used to power the motor, with a readily-available USB to 5.5x21mm adapter cable. I still have the problem of too much voltage, though.

I'm starting to think this isn't going to work with the 3P Li-NMC pack I built. I have shipments coming with 21700 and 32650 cells. I'm thinking either one of those, in a single-cell configuration, will do what I'm wanting to do. If I can buy/make a container for them that makes it easy to remove the battery and place it in a dedicated charger, it might be a lot better than what I'm currently trying to do.

 
If you are running your motor directly from your battery pack right now it will work fine with the USB circuit board --- he micro usb is only an input for charging the cells --- changing to a different battery number with the same voltage range will be the same as what you have
 
I've been running a capacity test with these three 2200mah 18650 cells and after running the motor for 13 hours yesterday, it's been running for almost 9 hours today. If it makes it until 6PM today, that will be a total of 24 hours and I'll call that good enough.

The pack has 6600mah, in theory. If it makes it 24 hours, that means the little 3v motor is pulling no more than 275 milliamps, on the low power setting. So, the pack is 3.7v and it's drawing 275 (or less), which means right around 1 watt is all it's consuming. No wonder it's taking forever! :)

This means I could use a single 2600mah cell and expect at least 8 hours of run time from that, at 275ma. I'm starting to think two cells is the most I would really need or want. Also, a single 32650 cell, with 6500mah, would be equal to what I'm getting from these three 18650's. Considering the D-cell batteries the motor is meant to run on take two to get ~3 volts, and those weigh about 4 times as much, to get maybe a little more capacity, I'm thinking it's still worth the time I'm putting into figuring out how to make this work. :)
 
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