Charging/Discharging circuit for multiple 18650 batteries

[Raiz]

Hi all,

Basic information:
I am planning to power up Raspberry PI3 with 3 or 4 Latium iron (18650) batteries (batteries will be connected as parallel mode) which will have 3.7v and 3800maA in each cell. To turn on a PI, you need to provide 5v and 1A current.

What I need:
I realize that, I need to have a step-up convertor to change the voltage from 3.7 to 5v. This can be done using a step-up or buck convertor. Can someone suggest me a circuitry design for this or perhaps good step-up convertor module, where I can purchase from eBay or Amazon.

I also need a battery charging indicator (2 or 3 LED bulb will be sufficient) circuit to indicate the battery power, along with auto discharging circuit. I also need a way to figure out if the voltage drops below 3v, the Raspberry Pi should turned off. This can be done using GPIO pins of PI but I need to know how to find out whether the voltage has been dropped below 3volts.

Can someone help me on this please?

Thanks in advance.
Good day
Raiz

 

[BobK]

Just buy a power bank. It already does everything you need. And it is likely cheaper than you can make it.

Bob

 

[Raiz]

Hi Bob,

Thank you for the idea. Power bank is a good solution but I need to put all of these items (Raspberry Pi3, cooling fan, batteries, Ethernet connectors, etc) in to one enclosure for my final year project. I can't have the power bank separately. Any other ideas?

Many thanks,
Raiz

 

[BobK]

Take the guts our of the power bank's enclosure?

If you can't do that, you can get the controller boards for these things on Ali Baba Express.

Bob

 

[hevans1944]

Hide the power bank inside your project box?

 

[Raiz]

Hi

Thanks for the suggestion. But I am unable to find a charging module with LED indicator (I need at least 2 to 3 LED's to indicate the battery level). Could you please suggest me a Power bank module ? (charging board with LED indicators or a circuit diagram where I can create an indicator for the charging module)

Many thanks,
Raiz

 

[Colin Mitchell]

Charge each cell individually with a $1.00 controlling module that operates from USB port.
That's $1.00 spent.
Put 2 cells in series and use diodes to get 5v. That's another $2.00 spent.
Or use a 7.4v to 5v module for $2.00
Put everything in a plastic box. That's another $35.00

 

[(*steve*)]

Most chargers just have a single led. If you need a display of charge level you can either but one separately or build one yourself. Do you know what varies with charge level?

I'm also concerned that you don't know the difference between a buck and a boost converter.

 

[BobK]

Hmm. Pretty much all of the power banks I have seen (I own 3 of them) have 4 LEDs to indicate state of charge.

Bob

 

[Raiz]

Charge each cell individually with a $1.00 controlling module that operates from USB port.
That's $1.00 spent.
Put 2 cells in series and use diodes to get 5v. That's another $2.00 spent.
Or use a 7.4v to 5v module for $2.00
Put everything in a plastic box. That's another $35.00

I do have $1 charging module with me, but it will charge only one cell right? if I am not mistaken. If so, we need to have 2 of those modules and charge the each battery separately?

 

[Raiz]

Most chargers just have a single led. If you need a display of charge level you can either but one separately or build one yourself. Do you know what varies with charge level?

I'm also concerned that you don't know the difference between a buck and a boost converter.

Yes, I would love to design my own circuit for this. If you have any circuit diagrams, please do let me know..

I don't know the difference between a buck and a boost converter..Sorry, I am not really good at Electronics but I would love to learn things..

Thanks

 

[(*steve*)]

Hmm. Pretty much all of the power banks I have seen (I own 3 of them) have 4 LEDs to indicate state of charge.

I was thinking about the cheap charger modules.

Charge each cell individually with a $1.00 controlling module that operates from USB port.

If the cells are in series you need to disconnect them to charge them this way.

If they are in parallel you only need a single charger module.

I do have $1 charging module with me, but it will charge only one cell right? if I am not mistaken. If so, we need to have 2 of those modules and charge the each battery separately?

No, if you have your cells (not batteries) in parallel, a single charger module can charge them both at the same time. It will take twice as long as a single cell though.

... for my final year project.

..Sorry, I am not really good at Electronics but I would love to learn things..

Hmmm, final project in an area you're not good at...

This is homework, and we're certainly not thing to do it for you.

 

[Raiz]

I was thinking about the cheap charger modules.



If the cells are in series you need to disconnect them to charge them this way.

If they are in parallel you only need a single charger module.




No, if you have your cells (not batteries) in parallel, a single charger module can charge them both at the same time. It will take twice as long as a single cell though.






Hmmm, final project in an area you're not good at...

This is homework, and we're certainly not thing to do it for you.

Hi Steve,

Thanks for the advice again. It is not an Electronic degree, even though I am coming up with a electronic device.

Yes, I figured out that, it will take twice the time when you charge 2 cells using the $1 module because it releases a small amount of current. Is there a better circuit other than a power bank circuit?

I do have another question for you. How do I charge multiple batteries (18650 model) at once? How do I plan a circuit diagram for that? (when they are connected parallel mode, it should have fast charging too)

 

[Colin Mitchell]

18650 cells have to be individually monitored when fast charging. When on say 14-hr rate, they can be charged in parallel an an overall detector used.
Just use the $1.00 module for each pair and remove the cells in pairs for charging.

 

[(*steve*)]

Thanks for the advice again. It is not an Electronic degree, even though I am coming up with a electronic device

Why are you doing something for your final project that is unrelated to your course?

Why are you doing something for your final project that you hardly have the first clue about?

By the looks of it we'd need to do a huge about of the design work for you. Can you give us the name of your educational institution so we can get credit for it and not you?

 

[Raiz]

Why are you doing something for your final project that is unrelated to your course?

Why are you doing something for your final project that you hardly have the first clue about?

By the looks of it we'd need to do a huge about of the design work for you. Can you give us the name of your educational institution so we can get credit for it and not you?

Well, that is a very good question...I am doing something related to Network security, but I need this prototype to be build as a portable device. Therefore, I need a proper circuit design to power up the Raspberry Pi (which will have a cooling fan as well as other external ports) as well as an indicator circuit to show the battery status to the user. The electronic part of this project is very minimal compared to the project which I am working on. Just think of a portable router

Trust me, I am not an expert in all these subjects (especially in electronics) but I have created Transmitters and Audio amplifier circuits when I was small.

 

[Raiz]

18650 cells have to be individually monitored when fast charging. When on say 14-hr rate, they can be charged in parallel an an overall detector used.
Just use the $1.00 module for each pair and remove the cells in pairs for charging.

isn't that a hassle to remove the cells when charging?

 

[BobK]

If this is just a prototype needed to demonstrate your software, why do you have such specific requirements for it?

And, if there is a Raspberry PI in there and presumably a display of some kind, why not put the battery indicator on the display?

Bob

 

[Raiz]

If this is just a prototype needed to demonstrate your software, why do you have such specific requirements for it?

And, if there is a Raspberry PI in there and presumably a display of some kind, why not put the battery indicator on the display?

Bob

Hi Bob,

No, this device doesn't have a display. To answer your question, the university expectations are very high therefore I need to come up with a good device..

Many thanks,
Raiz

 

[(*steve*)]

the university expectations are very high therefore I need to come up with a good device..

The key word is"I". The university presumably don't want you to outsource your design, and we are not going to do your homework for you.

What research have you done on lithium battery charging requirements and cell protection?

You've mentioned "fast" charging several times. How much energy will be stored in your batteries and what is the design target for time to recharge? That pretty much tells you one factor of major importance of the charger.

What research have you done to determine the range of voltages which can be supplied to the rpi? What are it's specifications in terms of voltage range and noise on the power rails? This will tell you stuff you need to know about the output of your power supply.

What research have you done about the cell voltages of lithium cells? What voltages are available to you (with possibly multiple cells)? Are there charge and discharge limits?

What research have you done on the options for regulating the various battery voltage options to get the output voltage required? What research have you done on efficiency?

What power will be consumed during the required runtime? Given battery voltage, regulator efficiency and cell aging (and possibly a design limit on discharge), what capacity (in mAh) is required? Given this and the cells available, how will it be achieved?

How will you limit discharge?

What research on discharge characteristics have you done? Is the any change that indicates remaining capacity? Does this reverse on charging? If such a thing does not exist, how could you emulate it? And finally, what are the options for displaying the capacity?

That's just a few basic things you need to do for yourself. Have you done them?