Converting cheap China SMPS into Li-Ion charger - Page 1
Forum section: electronics / general

author: ym58
author_rank: Regular Contributor
timestamp: October 02, 2022, 02:36:55 am
content: I am currently trying to hack aMeishile SMPS S-200-36to make it charge a10S4P Li-Ion batterypack (880W).I first tweaked theRV1pot to tune Vout up to 42V (10S4P/Li-Ion) : quite easy, no need to adjust the TL431 bias resistors to offset the adjustment, RV1 was sufficient to obtain 42V.Then I tried to tweakR1(Isense resistor) to make the SMPS stop hiccuping once it's connected to the battery pack (the initial current demand surely is in excess of 5.5A, resulting in the PWM controler to hiccup for protection).From the initial 0.1R value, I tried 0.2R, 0.5R and also 0.05R down to 0.02R ... to no avail, still hiccuping.Meanwhile, I reverse-engineered the PWM controller and output stages to make everything quite clear in my mind ...---But now, I am stranded ... I don't know what to test or do to get rid of the hiccuping.I am pretty sure the initial rush current demand from the Li-Ion BMS is notthathigh (certainly >5.5A, though) and is definitelytransientI also think that the SMPS mosfets couldtemporarilycope with this transient inrush ... well, that's my guess ;-)Has someone any clue ?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 02, 2022, 03:48:40 am
content: Quote from: ym58 on October 02, 2022, 02:36:55 amYou don't want it that high, use 4-4.1V/cell and you'll get substantially better service life with only a small reduction in usable capacity.What you would want to do is to add a soft current limit in some way. What is the PWM controller? Some of them can be operated in current limit by clamping the voltage on one of the pins.

author: ym58
author_rank: Regular Contributor
timestamp: October 02, 2022, 04:06:29 am
content: > What is the PWM controller?>

author: tunk
author_rank: Super Contributor
timestamp: October 02, 2022, 10:55:16 am
content: Does the battery handle the constant current and cut-off phases?

author: ym58
author_rank: Regular Contributor
timestamp: October 02, 2022, 11:04:58 am
content: Yes, it has a sturdy and smart 'pro' BMS.Actually, it's a scooter battery.

author: ym58
author_rank: Regular Contributor
timestamp: October 03, 2022, 04:43:54 am
content: After spending hours on this issue yesterday, I realized that the problem maybe comes not fromHiccupinggenerated by the PWM controller but from thefeedback loop(TL431 and opto-isolator FL817).I noticed that when I connect a well discharged battery (e.g. ~35V) on my 42V-tweaked SMPS then the feedback loop will immediately start toOSCILLATE, hence the variation of the voltage output that I thought was hiccuping but that is maybe not !The output voltage first decreases to ~35V (to match the battery voltage) but soon after a sort ofLVP(low voltage protection) kicks in from the TL431 making the whole PS oscillate between zero-volt and 35V (protection ?).Reversely, if I set the output voltage of the SMPS to say 38V (instead of 42V), this protection willNOTkick in, and the charging process will start : that isNO OSCILLATIONandPROPER 5.5Acurrent (CC) flowing thru the battery !As if the TL431 voltage detection cell was not correctly configured for 42V, which is probable since the SMPS was intially designed for 36V (S-200-36). I tried to tweak R15 and R12 but to no avail, so far.Any clue ?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 03, 2022, 04:52:06 am
content: It could be the open loop protection being triggered. It could possibly be worked around by adding a resistor across A-K of the TL431 so there would always be some feedback signal, will require experimentation to tell what value to use (start high as in 100k or so, the actual value that would work is likely much lower) and not the best as it can drift with temperature.

author: ym58
author_rank: Regular Contributor
timestamp: October 03, 2022, 06:28:20 am
content: I tried what you suggested@NiHaoMike, starting from 100K then down to 10K, but still no improvement though.Should be noted that once reached 50K and then down on, the SMPS starts oscillating even with no load (no batt).But in any case it's still oscillating, no matter what.

author: f4eru
author_rank: Super Contributor
timestamp: October 03, 2022, 07:17:33 pm
content: To convert this PSU, you probably need to add a current shunt and a current feedback loop, to have a decent, permanently usable current limit.You need to make both loops stable independantly.An example :https://www.homemade-circuits.com/how-to-make-variable-current-smps/There are probably more things to add for battery safety, like a fuse to limit the current in case of rectifier diode short failure.

author: ym58
author_rank: Regular Contributor
timestamp: October 04, 2022, 05:02:03 pm
content: @f4eru: you're wonderful, it works like a charm !No more oscillations, the ouput current is nowstable @5Aor so once connected to the 880W Li-Ion battery (I've used for the shunt a 4W/0.15R resistor that I had somewhere handy).0.7 / 0.15 ~ 4.7A---I am now considering hacking a bunch of modded Meanwell SMPS that sit on my shelves (they are all set to 14.6V/20-30A). These SMPS are all 'hiccuping' and I suppose they won't hiccup anymore with your'current limiting' hack!But since these SMPS are supposed to deliver more current (25-30A) than the one I have just fixed (with your help), I guess that finding shunts of such low value will be difficult (0.7V / 30A = 0.02 ohm), so I was thinking of using aHall current sensorwith a proper Op-amp comparator/amplifier to replace the BC547 that I used for your hack.---What d'you think ?

author: f4eru
author_rank: Super Contributor
timestamp: October 06, 2022, 09:00:49 pm
content: 20 mohm shunts are common, but I would recommend to go lower with an opamp to lower the losses, hall effect is a good alternative also.Just make sure that hte extra delay does not make the current loop unstable.

author: ym58
author_rank: Regular Contributor
timestamp: October 07, 2022, 07:38:08 am
content: Quote from: f4eru on October 06, 2022, 09:00:49 pmYou're right, I'll finally use this kind of smd resistor for the current detection (just ordered different values on AliExpress) :