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Modding old ATX supplies for 3.65v output at high current for LiFePO4 charging
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Siwastaja:

--- Quote from: fixy88 on June 29, 2019, 12:48:31 am ---Can I ask specifically why it isn't feasible? Thanks!

--- End quote ---

Because battery charging requires a current-limited supply. Even though a modern switch mode PSU does have current limiting, their control ICs implement, on purpose, modes that prevent the supply from running in the current-limited mode. It's typically either a "hickup mode" or "latch mode", but either one prevents it from charging: the power supply provides the constant current for a few milliseconds, and then just errors out.

If they did use a control IC with disableable over-current error modes, it would do the trick easily, but this is almost never the case: even though allowing the continuous constant-current output mode is trivial for a control IC manufacturer, the demand for such controllers is low enough that these ICs are fairly rare. The ability to do constant current is a huge "value add" for a manufacturer, even if it comes for free.

This is sad, since the power stage electronics might be able to do the trick easily, but given they are built for a specific purpose, i.e., supplying constant voltage to a load with limited capacitance, they won't handle constant current loads (big capacitors, batteries, directly connected LEDs, big motors...) for more than a few milliseconds.
fixy88:

--- Quote from: Siwastaja on June 29, 2019, 10:37:13 am ---
--- Quote from: fixy88 on June 29, 2019, 12:48:31 am ---Can I ask specifically why it isn't feasible? Thanks!

--- End quote ---

Because battery charging requires a current-limited supply. Even though a modern switch mode PSU does have current limiting, their control ICs implement, on purpose, modes that prevent the supply from running in the current-limited mode. It's typically either a "hickup mode" or "latch mode", but either one prevents it from charging: the power supply provides the constant current for a few milliseconds, and then just errors out...
....., supplying constant voltage to a load with limited capacitance, they won't handle constant current loads (big capacitors, batteries, directly connected LEDs, big motors...) for more than a few milliseconds.

--- End quote ---

Thanks for the detailed explanation! I have have powered leds from very small SMPS supplies in the past and just let them run at max current which was close to the led rating and that worked fine. I shouldn't have assumed it would be the same for an ATX supply though...
fixy88:
It seems like a few people have successfully defeated OCP/UVP on the control IC http://planetimming.com/atx_mod/atx_mod.html http://burnt-traces.com/?p=159
tooki:
I don’t think it can be emphasized enough how dangerous lithium battery charging can be. There are many purposes in electronics where jury-rigging something works fine, but lithium charging most decidedly is not among them!

FYI, PC power supplies are designed to provide large amounts of current at low cost. They do not provide particularly clean power nor precise voltages, because the assumption is that any computer part that needs clean, precise power will use local regulation to produce it from the “bulk” power from the PSU. Just bear this in mind whenever you’re tempted to use a PC PSU for any other task.
tooki:

--- Quote from: fixy88 on June 29, 2019, 11:00:52 am ---It seems like a few people have successfully defeated OCP/UVP on the control IC http://planetimming.com/atx_mod/atx_mod.html http://burnt-traces.com/?p=159

--- End quote ---
For the love of god don’t do this, especially not for lithium charging!
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