Electronics > Projects, Designs, and Technical Stuff
Modding old ATX supplies for 3.65v output at high current for LiFePO4 charging
fixy88:
--- Quote from: digsys on June 29, 2019, 01:51:18 am ---You need to find out what cell type specs and it's MAX charge currents - both boost and top (as others have mentioned).
You will HAVE to do the current limiting yourself, don't rely on the P/S, or you could possibly shorten / kill the cell life.
--- End quote ---
They are able to be charged at least 25 amps safely. Cheers :-)
wraper:
--- Quote from: fixy88 on June 29, 2019, 01:38:36 am ---
--- Quote from: wraper on June 29, 2019, 01:26:40 am ---
And how you do you get CC by simple modification? And for final 10% you need to reduce charging current a lot or not charge at high current to begin with.
--- End quote ---
My (possibly incorrect) understanding was that the supply would limit the current to the maximum it could supply at the set voltage. If the voltage is set at 3.65v the current will fall off as the battery reaches that set charge voltage.
--- End quote ---
ATX PSUs don't CC, they shut down if current limit is exceeded. Or fail, if PSU is crappy and without proper protection.
NiHaoMike:
--- Quote from: fixy88 on June 29, 2019, 01:07:50 am ---It's mainly 1 cell that is very different from the others. I'd need ~20amps of shunt to charge at a decent rate.
--- End quote ---
Connect that bank to the 5V output of the PSU through a high current diode and a length of wire to act as a current limiting resistor. Plug the PSU into a one shot timer (available at home improvement stores or salvaged from an appliance) so you won't accidentally leave it running for too long. Keep a watch on the voltage with a multimeter.
magic:
^do it outside and far from civilization if you go that route :-DD
--- Quote from: fixy88 on June 29, 2019, 12:36:04 am ---4 independent channels capable of around 20-25 amps per channel at 3.65v and am considering using modified ATX supplies.
--- End quote ---
You cannot change the ratio of output voltages on one PSU because it's determined by turn ratio of secondary windings on one common transformer. You also cannot current limit each rail independently, for the same reason. So you are looking at converting one PSUs per channel.
Each needs to have a rail rated for 25A or you are looking at upgrading secondary diodes, chokes and capacitors, pretty much everything.
The channels will all have earthed grounds, hope you are OK with it.
--- Quote from: fixy88 on June 29, 2019, 12:36:04 am ---Is this feasible and is the modification likely to be more complicated than changing a few resistors?
--- End quote ---
Reducing output voltage is a few resistors. CC mode would take adding shunts, comparators and driving the optocouplers yourself.
fixy88:
--- Quote from: magic on June 29, 2019, 06:05:20 am ---^do it outside and far from civilization if you go that route :-DD
--- Quote from: fixy88 on June 29, 2019, 12:36:04 am ---4 independent channels capable of around 20-25 amps per channel at 3.65v and am considering using modified ATX supplies.
--- End quote ---
You cannot change the ratio of output voltages on one PSU because it's determined by turn ratio of secondary windings on one common transformer. You also cannot current limit each rail independently, for the same reason. So you are looking at converting one PSUs per channel.
Each needs to have a rail rated for 25A or you are looking at upgrading secondary diodes, chokes and capacitors, pretty much everything.
The channels will all have earthed grounds, hope you are OK with it.
--- Quote from: fixy88 on June 29, 2019, 12:36:04 am ---Is this feasible and is the modification likely to be more complicated than changing a few resistors?
--- End quote ---
Reducing output voltage is a few resistors. CC mode would take adding shunts, comparators and driving the optocouplers yourself.
--- End quote ---
Thanks for the input - it is helpful :-) I have found this voltage adjustment and current limiting circuit which is not too complicated so I might give it a try https://weekly-geekly.github.io/articles/257427/index.html
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