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I'm trying to build an UPS for a device that needs 5V and 3A. But I'm not sure if this block diagram (using 3 3.7v 18650 cells) is safe and would work. Can anyone give me some feedback? I hope my writing is clear enough. ups

GB5
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    That isn't a build it's a block diagram. It's also a tiny block diagram on a big sheet of paper. The devil's in the detail. – Andy aka Sep 30 '20 at 17:30
  • Please crop your image. – JYelton Sep 30 '20 at 17:35
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    Also you may want to review https://electronics.stackexchange.com/q/105136/2028 – JYelton Sep 30 '20 at 17:36
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    No, you can't stack single cell chargers like that. This is a far, far, far more complicated project than you think it's going to be and a very inappropriate subject area to pick for learning purposes.. Buy a solution or maybe at the outside after doing a lot more research than you have so far try to DIY something with a gel cell and switching regulator. – Chris Stratton Sep 30 '20 at 17:36
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    That's not a schematic. There is literally no details to assess will it work or not, or if it is safe or not. Except that it seems that Chris already saw from the block diagram that it will not work. – Justme Sep 30 '20 at 17:41
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    @ChrisStratton may be due to my lack of skills to do a proper schematic, my idea with the cells wasn't clear. The idea would be to do this with the cells https://www.youtube.com/watch?v=ySXtBuawf70 Could elaborate on why it wouldn't work, so I can learn :) ? – GB5 Sep 30 '20 at 18:46
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    @Justme what type of details would you need to provide feedback? (honestly asking) – GB5 Sep 30 '20 at 18:47
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    Youtube links aren't a meaningful way to present technical information. People have asked many versions of the multiple tp4056 and lithium cell question in the past, typically in ignorance of the series issue, but it's simply never a good idea even if that is cleverly overcome. On-line power sources are *hard*. **Realistically, abandon this whole idea, it is a fundamentally bad choice for a project**. Buy something, or at most try to work it out with a simpler gell cell. – Chris Stratton Sep 30 '20 at 18:54
  • @GB5 Honestly, a schematic. The blocks opened up, filled with capacitors, resistors, chips, and connections between them. In case you are unaware, lithium batteries, when improperly connected, overcharged, etc, can cause a fire and explosion, which may cause injuries and burn houses, so you need to be extra careful with them. – Justme Sep 30 '20 at 19:57

2 Answers2

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For your circuit to work you need 3 AC to DC isolated supplies because the output of TP4056 isn't floating, otherwise it is safe and can work.

Moty
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    Are you seriously proposing this? As an actual, viable solution? – Wesley Lee Sep 30 '20 at 22:00
  • What don't you like about it? – Moty Sep 30 '20 at 22:04
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    3 separate AC-DC adapters, with dodgy battery chargers in series, to power idk.. a RPi? I guess if you have literally nothing else it will technically work but... tbh it just doesn't sound like an actual proposed solution, more like a tongue in cheek answer.. – Wesley Lee Sep 30 '20 at 22:25
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    You are right my friend. – Moty Sep 30 '20 at 22:40
  • @Moty thanks for the suggestion, but with 3 ac-dc adapters it would get too big and expensive for it to be worth for me. But thank you anyway :) – GB5 Oct 01 '20 at 08:09
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Your problem is that you are connecting 3 cells in series to produce 11V. But to charge them from a 5V supply, you would have to connect the 3 cells in parallel. While it's possible to reconfigure them on-the-fly, it makes the whole design far more complicated.

USB power banks typically use one cell, or several cells in parallel, and a boost converter to step the voltage up to 5V. That makes things a lot less complicated.

An alternative would be to use a charging circuit designed to charge 3 cells in series, at about 13V. But beware, LiIon cells tend to catch fire if charged wrongly.

Simon B
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  • Ok, so if I connect them in parallel and then use a DC-DC Step Up Boost Converter it will work? The device I want to power is the Edge TPU and it really needs the 3A to keep working properly. – GB5 Oct 01 '20 at 09:40
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    @GB5 Should work. You may well be able to buy off-the-shelf modules to do it. – Simon B Oct 01 '20 at 12:59
  • Thank you :) I will look into it. – GB5 Oct 01 '20 at 13:48