How to make a custom UPS using an old PC power supply and 18650 batteries?

[ghowriter]

Yes, this is a question. I already converted an old PC power supply (350 Watt) to replace the bower bricks for my Internet modem and router. What I would like to do now is install a 2-3 hour mini-UPS in the mix so I can remove it from my computer's UPS.

I have spent days online scouring the web for info on this. I've learned a bit but truth be told, most posts on this topic detail how to build a router UPS but limit the output amps. My modem (Motorola MB8600) adapter outputs 12 v 2.5a and my router's is 12v 5a. This was not an issue for the PC power supply as it was a single rail with 20a available.

Most laptops batteries are 6 cell (18650) and run for between 2 to 3 hours. I assume I would use between 4 to 12 cells here. What I don't know and can't seem to find is how do I ensure the amps required by each device. I want to keep my batteries in groups of 4 because my protection circuit boards each protect 4 cells.

I understand that unless I use a DC to DC step up circuit, 4 batteries in series would provide the 12v (4x3.7=14.8) and, if my understanding is correct, the mAh would be what one battery is (my batteries are recycled from laptop batteries and I've yet to determine their capacity) which, ideally, is 24mAh but may be as low as 1350.

So I read that most laptop batteries are 6 cell and use a standard of 4s2p. I do not understand this. I know it's 4 batteries in series and 2 in parallel, but how would this configuration double the mAh?

So, there it is. Comments, hints, tips, tricks are all welcome.

Thank you for your time.

 

[jedishrfu]

I believe this is a dangerous undertaking primarily the possibility of a battery explosion from heat buildup and short-circuits although I'm no expert in this matter and this may be why the builds you saw limited the battery size to fewer cells.

I found this article on the hazards:

https://app.croneri.co.uk/feature-a...ist through the,hazards due to short circuits.

Perhaps @berkeman can respond here.

 

[berkeman]

When it comes to battery powered projects (especially if you are not a battery and power supply expert), it's best to use as much off-the-shelf stuff as possible.

Consider using this DC UPS controller and maybe a good-size lead acid car battery:

https://www.powerstream.com/12V-backup.htm

 

[ghowriter]

Should have know someone would try to sell something rather than help. Please delete my account, I won't be returning.

 

[hutchphd]

Don't be silly. The 4s2p means two complete 4s batteries in parallel. So 14.8V at twice the amps. But these batteries must be treated carefully or you will create a fire.

 

[berkeman]

Should have know someone would try to sell something rather than help.

LOL, yeah, I own a lot of stock in PowerStream (well, and Google, which helped me find the product).

We are trying to help you to avoid setting your house on fire, among other things.

 

[Vanadium 50]

but how would this configuration double the mAh

mAh for a fixed voltage is a unit of energy. Twice as many batteries in parallel, twice as much energy.

 

[jim mcnamara]

OP seems to be gone. Closed.