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DIY Powerbank 40000 mAh

So here is my project: Building a 40 000 mAh powerbank 

So this is the materials to be use:

1.6 or 12 pcs of 4.2v 6800 mAh 18650 Li-ion Battery

2. Charging module - 2 USB output ports 5V 1A and 5V 2.1A and 1 micro usb input port

 

I am finding suggestions at:

1. Best DIY Case is made of

2. Charging module which supports fast charging (input and output)

3. should i buy 12 pcs li-ion battery and 6 are parallel and those pair of 6 battery in serial (8.4V) and buy step down module?

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Step down would always be better than boosting voltage, you get higher efficiency.

 

There's no such thing as a 6800 mAh 18650 battery - the highest capacity in a 18650 is around 3400mAh.

Here's what Panasonic offers and they're rating them at ~ 3250mAh : NCR18650B.pdf

 

If you buy something from eBay or some crappy site advertising 6800mAh, most likely they're batteries half filled with sand or only partially filled with "battery material", giving you maybe 1000-1500 mAh of actual capacity.

If you want huge energy storage, it would make sense to connect 2 batteries at a time in parallel and then have 4-5 of these pairs in series to get a high voltage like 3.7..4.2 x 5 = 18.5v .. 21v

It would allow you to use smart chargers that can do cell balancing like those sold on HobbyKing, here's an example: https://hobbyking.com/en_us/imax-b6-ac-v2-charger-discharger-1-6-cells-genuine-au-plug.html

A charger like that one uses standardized JST-SH connectors (easy to buy, which you can use to set up cell balancing for your battery pack) and charges batteries at up to 50w, so you charge such a big battery in an hour or so, safely.

 

dyi case ... first thing that comes to mind would be repurposing an old laptop power adapter brick, but it may make more sense to have something with a lid you can unscrew, and something in which you can easily drill some holes to allow for ventilation when charging the batteries (maybe add a 40mm fan to one end to blow air while charging)

lots of project boxes at stores that deal with electronic components, here's an example https://www.digikey.com/products/en/boxes-enclosures-racks/boxes/594

 

such stores also have other fun bits, for example battery holders: https://www.digikey.com/products/en/battery-products/battery-holders-clips-contacts/86?FV=ffe00056%2C8000bd&quantity=0&ColumnSort=1000011&page=1&pageSize=25

 

You can grab a couple of these 4 x 18650 battery holders and solder wires on the bottom pins (individual pins for each battery) to make two sets of batteries in parallel , then connect the sets in series (or whatever combination you're thinking of) etc ... saves you from having to solder directly on the batteries or to spot weld tabs on batteries.

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1 hour ago, SpecsTacular said:

So here is my project: Building a 40 000 mAh powerbank 

So this is the materials to be use:

1.6 or 12 pcs of 4.2v 6800 mAh 18650 Li-ion Battery

2. Charging module - 2 USB output ports 5V 1A and 5V 2.1A and 1 micro usb input port

 

I am finding suggestions at:

1. Best DIY Case is made of

2. Charging module which supports fast charging (input and output)

3. should i buy 12 pcs li-ion battery and 6 are parallel and those pair of 6 battery in serial (8.4V) and buy step down module?

Like @mariushm there said, no such a thing as 6800mAh 18650. Do not buy anything over 3400mAh, and do not buy any Surefire, Firestrike, etc. brands -- they're literally all fake.

 

As for how to write the batteries and everything else, I'm going to disagree with @mariushm there: I'd just wire them all in parallel -- when the cells are wired in parallel, there is no need to balancing, which simplifies the setup greatly, and you can just literally buy a fast-charging capable powerbank from your nearest shop, open it up and hook your cells into it. Then just make a custom case for it and POOF, you're all set.

Hand, n. A singular instrument worn at the end of the human arm and commonly thrust into somebody’s pocket.

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Any better charger and discarger module that can charge the batteries faster and discharge faster?

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You have to charge the batteries at up to the specified charge rate, otherwise you'll degrade them prematurely and you'll overheat them, risking them catching fire.

 

For example see datasheet I attached in my previous post ... it says typical capacity 3350 mAh and charging current 1625mA ... that's basically 1/2 C

I would say for good batteries I would not charge them at a current higher than 2A , provided there's a fan blowing air over the batteries to keep them cool.

datasheet even tells you how a charger should charge the battery ... look at the green and see on the right how the maximum is around 1.5A and once the voltage goes over around 4.1v, the charger is supposed to limit itself.

 

charging.png.93766aa1e6f1dc4d056cd70c43e0620d.png

 

It's up to you how you parallel and put in series batteries... assuming 1.5A..2A per battery and 8 batteries you're looking at 50w..67w already. I'd aim to charge such a power bank with a 12v or 16.5-18.5  65w laptop adapter as these would be super common and cheap.

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