Modding old ATX supplies for 3.65v output at high current for LiFePO4 charging - Page 1
Forum section: electronics / projects

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 12:36:04 am
content: I have a 4s LiFeP04 pack that I hope to use very occasionally in short notice. It is stored ~1/2 charged. Unfortunately the pack was originally supplied with mismatched cells - at the I was unable to return them. This makes charging very slow as the cells have different charge curves and a standard charger reduces the charge rate before some cells are close to charged as the inbuilt balance shunts can only handle very low currents.I'd like to build a charger with 4 independent channels capable of around 20-25 amps per channel at 3.65v and am considering using modified ATX supplies. Is this feasible and is the modification likely to be more complicated than changing a few resistors? Thanks for any ideas!

author: wraper
author_rank: Unknown
timestamp: June 29, 2019, 12:47:54 am
content: Modifying PSU for this purpose is not feasible. But it's completely suitable for powering proper charger, though there might be issues with minimum load requirement for some ATX PSUs.QuoteYou'd be an idiot if used dodgy charger at such current.

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 12:48:31 am
content: Quote from: wraper on June 29, 2019, 12:45:10 amCan I ask specifically why it isn't feasible? Thanks!

author: wraper
author_rank: Unknown
timestamp: June 29, 2019, 12:53:59 am
content: First of all, constant voltage won't work to begin with. It needs current regulation with current dependent on battery charge. Also it needs to detect when battery is fully charged, overcharging is completely unacceptable.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 29, 2019, 01:04:14 am
content: Have you checked how many banks are high/low? Most likely the easiest way to solve the problem is to build a temporary balancer with a comparator switching a load (such as a car light bulb) using a MOSFET, to be manually connected to a high bank.You can also modify the BMS to have a higher balance current. I like to use LEDs as the loads to give a visual indication of balancing activity.http://www.kerrywong.com/2017/09/10/modifying-a-4s-100a-lifepo4-bms-module/comment-page-1/

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 01:04:59 am
content: Quote from: wraper on June 29, 2019, 12:53:59 amIf the voltage is set at 3.65v, and the supply max current is ~25 amps, then I can't see any issues with the basic charging requirements - it's CC to 3.65v and then CV. Overcharge will be monitored. My uncertainty is around the modification being possible at all.

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 01:07:50 am
content: Quote from: NiHaoMike on June 29, 2019, 01:04:14 amIt's mainly 1 cell that is very different from the others. I'd need ~20amps of shunt to charge at a decent rate.

author: wraper
author_rank: Unknown
timestamp: June 29, 2019, 01:26:40 am
content: Quote from: fixy88 on June 29, 2019, 01:04:59 amAnd 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.

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 01:38:36 am
content: Quote from: wraper on June 29, 2019, 01:26:40 amMy (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.

author: digsys
author_rank: Unknown
timestamp: June 29, 2019, 01:51:18 am
content: 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.

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 01:55:47 am
content: Quote from: digsys on June 29, 2019, 01:51:18 amThey are able to be charged at least 25 amps safely. Cheers :-)

author: wraper
author_rank: Unknown
timestamp: June 29, 2019, 02:12:57 am
content: Quote from: fixy88 on June 29, 2019, 01:38:36 amATX PSUs don't CC, they shut down if current limit is exceeded. Or fail, if PSU is crappy and without proper protection.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 29, 2019, 02:53:05 am
content: Quote from: fixy88 on June 29, 2019, 01:07:50 amConnect 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.

author: magic
author_rank: Super Contributor
timestamp: June 29, 2019, 06:05:20 am
content: ^do it outside and far from civilization if you go that routeQuote from: fixy88 on June 29, 2019, 12:36:04 amYou 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 amReducing output voltage is a few resistors. CC mode would take adding shunts, comparators and driving the optocouplers yourself.

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 09:34:17 am
content: Quote from: magic on June 29, 2019, 06:05:20 amThanks 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 tryhttps://weekly-geekly.github.io/articles/257427/index.html

author: Siwastaja
author_rank: Super Contributor
timestamp: June 29, 2019, 10:37:13 am
content: Quote from: fixy88 on June 29, 2019, 12:48:31 amBecause battery charging requires a current-limited supply. Even though a modern switch mode PSUdoeshave 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.

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 10:49:05 am
content: Quote from: Siwastaja on June 29, 2019, 10:37:13 amThanks 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...

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 11:00:52 am
content: It seems like a few people have successfully defeated OCP/UVP on the control IChttp://planetimming.com/atx_mod/atx_mod.htmlhttp://burnt-traces.com/?p=159

author: tooki
author_rank: Super Contributor
timestamp: June 29, 2019, 11:04:48 am
content: 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.

author: tooki
author_rank: Super Contributor
timestamp: June 29, 2019, 11:07:19 am
content: Quote from: fixy88 on June 29, 2019, 11:00:52 amFor the love of god don’t do this, especially not for lithium charging!

author: magic
author_rank: Super Contributor
timestamp: June 29, 2019, 03:50:15 pm
content: Quote from: tooki on June 29, 2019, 11:07:19 amLol, indeed.You need additional current regulation reacting and slowing down the PSU before OCP activates, not to disable OCPBut that's no longer a trivial mod. Output voltage adjustment is simple, but constant current regulation doesn't exist in those PSUs and would need to be built from ground up.

author: fixy88
author_rank: Contributor
timestamp: June 29, 2019, 10:19:08 pm
content: Thanks for all the thoughts :-) Fear not, even when I'm doing things somewhat on the cheap, I'm very aware of the safety issues around lithium cells. If I manage to get something working, it will have over volt alarms and cutoffs.

author: wraper
author_rank: Unknown
timestamp: June 30, 2019, 03:21:48 am
content: Quote from: fixy88 on June 29, 2019, 10:19:08 pmIf you want to get something really working, build proper charging circuit powered from PSU. Not deciding adding 6th feet to a dog if it is still moving after adding 5th feet.

author: fixy88
author_rank: Contributor
timestamp: June 30, 2019, 09:21:15 am
content: Quote from: wraper on June 30, 2019, 03:21:48 amFacepalm seems a bit harsh - it is common in commercial chargers and batteries to include these safety measures.

author: wraper
author_rank: Unknown
timestamp: June 30, 2019, 10:07:10 am
content: Quote from: fixy88 on June 30, 2019, 09:21:15 amIt's not the point. Point is that adding those things to ATX PSU is like adding 5 and 6th leg to the dog. It won't be easy, and most likely won't work properly anyway. Especially considering you didn't even know they don't have CC.

author: janoc
author_rank: Super Contributor
timestamp: June 30, 2019, 11:47:02 am
content: 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.

author: fixy88
author_rank: Contributor
timestamp: June 30, 2019, 10:07:48 pm
content: Quote from: janoc on June 30, 2019, 11:47:02 amI 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.

author: fixy88
author_rank: Contributor
timestamp: September 07, 2019, 09:10:16 am
content: 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

author: magic
author_rank: Super Contributor
timestamp: September 07, 2019, 09:16:49 am
content: Yay for cheap PSUs that don't shutdown on overcurrent and aren't capable of producing enough current to blow the battery upOn second thought, check if the secondary heatsink isn't overheating. If the diodes short out that could be a bad day for the LiPos.

author: fixy88
author_rank: Contributor
timestamp: September 07, 2019, 09:19:15 am
content: Quote from: magic on September 07, 2019, 09:16:49 amHaha I know right!? They weren't just cheap, they were freeAll 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.