Converting cheap China SMPS into Li-Ion charger

[ym58]

I am currently trying to hack a Meishile SMPS S-200-36 to make it charge a 10S4P Li-Ion battery pack (880W).


I first tweaked the RV1 pot 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 tweak R1 (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 ...

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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 not that high (certainly >5.5A, though) and is definitely  transient
I also think that the SMPS mosfets could temporarily cope with this transient inrush ... well, that's my guess ;-)

Has someone any clue ?

[NiHaoMike]

I first tweaked the RV1 pot 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.

You 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.

[ym58]

> What is the PWM controller?
>

[tunk]

Does the battery handle the constant current and cut-off phases?

[ym58]

Yes, it has a sturdy and smart 'pro' BMS.
Actually, it's a scooter battery.

[ym58]

After spending hours on this issue yesterday, I realized that the problem maybe comes not from Hiccuping generated by the PWM controller but from the feedback 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 to OSCILLATE, 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 of LVP (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 will NOT kick in, and the charging process will start : that is NO OSCILLATION and PROPER 5.5A current (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 ?

[NiHaoMike]

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.

[ym58]

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.

[f4eru]

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.

[ym58]

@f4eru : you're wonderful, it works like a charm !
No more oscillations, the ouput current is now stable @5A or 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
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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 a Hall current sensor with a proper Op-amp comparator/amplifier to replace the BC547 that I used for your hack.
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What d'you think ?

[f4eru]

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.

[ym58]

Just make sure that hte extra delay does not make the current loop unstable.

You're right, I'll finally use this kind of smd resistor for the current detection (just ordered different values on AliExpress) :