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

[janoc]

Seriously, OP, this sounds like you are building a backup power bank for when other power isn't available. That's really not something where you want to mess around with old/cheap/shitty ATX power supplies!

A proper charger is not that expensive and it is certainly cheaper than a house burning down because of a battery fire.

You do realize that ATX supplies are rated +-5%, right? That means your output voltage is going to be moving +-0.2V at 3.65V - and that's when the supply is completely in spec! Given that you want to use an old one, you will likely get much worse performance.

And you probably know what happens when you overcharge a lithium cell, even by a few tenths of a volt. Your "voltage alarms" are not going to help you much once the runaway starts.

The lack of current limiting has been discussed already - the supply itself won't limit the current for you. It will either shut off if you hit the overcurrent protection (assuming it works!) or go BOOM on you (if it is a dodgy supply with inflated ratings - very common on cheap supplies).

This is probably the worst possible way of going about charging lithium cells.

[fixy88]

Seriously, OP, this sounds like you are building a backup power bank for when other power isn't available. That's really not something where you want to mess around with old/cheap/shitty ATX power supplies!

A proper charger is not that expensive and it is certainly cheaper than a house burning down because of a battery fire.

You do realize that ATX supplies are rated +-5%, right? That means your output voltage is going to be moving +-0.2V at 3.65V - and that's when the supply is completely in spec! Given that you want to use an old one, you will likely get much worse performance.

And you probably know what happens when you overcharge a lithium cell, even by a few tenths of a volt. Your "voltage alarms" are not going to help you much once the runaway starts.

The lack of current limiting has been discussed already - the supply itself won't limit the current for you. It will either shut off if you hit the overcurrent protection (assuming it works!) or go BOOM on you (if it is a dodgy supply with inflated ratings - very common on cheap supplies).

This is probably the worst possible way of going about charging lithium cells.

I understand and appreciate that people are worried I will cause injury by doing something stupid here. I'm the first to admit there are big gaps in my knowledge when it comes to switching power supplies, but I do know how to charge these batteries safely. It is my understanding that LiFePO4 cells are actually quite tolerant to overcharge and are not likely to go into thermal runaway if overcharged by a few 10ths of a mV as you assert.

This isn't for charging a DIY powerwall (the fact that people are stockpiling old LiCo cells and charging them in back sheds to power their homes terrifies me a little) - it's for bulk charging a 70aH pack to use in a boat occasionally. It can be topped off using a standard hobby charger if needed.

[fixy88]

So it turns out this was really easy and works great. Added a voltage divider to the 3.3v vsense line and current can be wound up to 20amps no problem. Supplies seem fine with it. Just in case anyone cared 

[magic]

Yay for cheap PSUs that don't shutdown on overcurrent and aren't capable of producing enough current to blow the battery up

On second thought, check if the secondary heatsink isn't overheating. If the diodes short out that could be a bad day for the LiPos.

[fixy88]

Yay for cheap PSUs that don't shutdown on overcurrent and aren't capable of producing enough current to blow the battery up

Haha I know right!? They weren't just cheap, they were free  All are running under their 3.3v buss current rating, to be fair, and at ~1/3 1/4 of total rated wattage. No overheating problems so far.