PSU power supply

I have a few PSU’s lying around and want to use one as a charger for my 12v 3s4p 18650 pack I made. The psu is outputting a fixed 12.1v. Any suggestions how to bump these up to 12.6v? I need to additional 0.5v to fully charge the 18650 cells

The yellow wires are the 12v line.

Would it be as simple as adding a resistor somewhere?

Also the 12.6v will be fed to the battery packs bms which has a number of safe guards.

Hi Mat.
No I don’t think it is as simple as adding a resistor. I would not go down this path. This supply might be good enough to supply DC to another intelligent charger but not to connect directly to batteries. Charging LiPo batteries (individual or parallel groups) in series requires individual cells or groups to be monitored and charge currents balanced. I believe there are add on boards that will do this but not having the need I have not personally investigated. I think that commercial LiPo battery packs for tooling etc may have these “smarts” built into the pack.
Your battery pack would be 10.8V or 11.1V anyway, not 12.
Do these supplies provide all the voltage outputs these days with nothing connected? Some used to use the 5V output as a reference for the other regulators and some needed a minimum load on the 5V supply to even start and give any outputs at all.
Cheers Bob

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Hi Bob,
I edited my original post to include the voltage to the batteries will feed into the bms. A fully charged 3s 18650 pack should be 12.6v (4.2v per fully charged and good condition cell).

The psu puts out 3.3v, 5v and 12.1v I think. I have modified another pack that I use when tinkering. I just plug the terminals into the required voltage and away I go. There is no constant load required. The only thing you need to do is ground the green wire to trick the psu into thinking it’s on. They are fantastic on the bench because of the regulated supply.


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The 4.2V is the charging voltage. They won’t stay there for long. Should settle at 3.7V (or 3.6V) per cell and pretty well stay there for almost the whole discharge cycle.
Cheers Bob

4.2v is the max the cell should safely take so 12.6v is the required voltage to fully charge the cells. My pack is currently resting at 12v and the cells haven’t been charged for 6 months or so.

Hi Mat
Sorry. I am not familiar with the circuitry of the ex computer power supplies. There may be a way of lifting to 12.6 but I don’t know. Google the model number and you may come up with a circuit.
You will still need to balance the 3 groups so you will have to research what is available to do this.
Cheers Bob

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The bms takes care of that.

The penny drops. I have just realised that “bms” refers to battery management system. I have now twigged that all you want is a means to lift your PSU volts to 12.6. Without a circuit and possibly a description this is going to be difficult. Can’t help I am afraid.
Cheers anyway Bob

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it can be done but it would raise all rails by the say degree that is with respect to the start voltage of each rail for eg … most of these circuits have a secondary switching circuit witch switches all the rails by the same percentage, if you choose to contact silicon chip over this matter there were quite a few articles written about how to lift the the 12 volt rail to 13.3 to drive cb.radios and alike… i may have the articles but it would take me quite some time to find them have 100s of copies of the mag ..any way look for the switching secondary circuit it will most likely be an "tl-494" switching chip its 14 pin from mem…dome lots or switch mode repairs in the past…any way i would contact silicon chip they will point you in the right direction…


If you can find the reference it may be as simple as fitting a diode in series with it to raise the value by 0.6V.


from memory its a resistor value in the timing circuit raise or oiwer it …if you can find a tl-494 scheme it`s easy to find and adjust be careful though the diode packs usually max out at 30 amps…from mem we used to put 10 turn pots in them

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In one of the photos there a 16 pin chip. No details on it though. I assumed that would most likely control over voltage.

Photo does not tell much. It could be a board with several layers (I have seen up to 12). I think circuit is a must.
Cheers Bob


its full of surface mount almost impossible to do much with unless your really good with surface mount electronics…the earlier type power supply’s did not have very much surface mount electronics in them…
you would do better to go to an electronics grave yard hard to find these days though…the supply’s that i used to work with had much less surface mount gear inside them…old style switch modes are rare these days …almost all been scrapped for metal inside of them…you would have to browse for the circuit if you could find one…make and model and wattage you would need but even though with a schem…still may be hard also look on the p.c.b…to see if it has a revision printed on it somewhere.or any numbers at all on the p.c.b itself can often help when looking for schems for this type of gear almost all come out of china for a long time now so much flooding from there.even though that supply is old can tell its done work from the color of the board…


I’ll just hook up a dc dc boost converter then and set to 12.6v. The bms can handle the rest.


Sounds like a plan Mat,

Make sure to send through some photos of your finished product if you’d like, I’m sure that anyone with similar questions would be happy to see how you went.

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So ordered the Core Electrics boost converter and installed as per pictures. I modified the original cage. I don’t have access to any fancy 3D printing stuff. Thought I would add a volt and amp meter while I was at it. The amp meter isn’t working. I have never got them to work. Any suggestions? Any it’s not pretty but it will work. Anderson plug fitted to charging lead. The other wires are spare just in case I need a 3,5 or 12v line.

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Hi Matt,

Looks good! We’ve run out of time today, but we’ll get you answer on the current meter tomorrow.

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I did wire the meter up initially with positive for the amp meter shared with the load + and the negatives were tied together. I got zero amps.

I also cut down the shunt so it would fit. My understanding is that as long as it’s wired in series it shouldn’t matter.

I rewired using another diagram and wired the amp meter to another power source and earth.

I now get 4-5amp draw running a light bar. It’s drawing 1.6amps when tested with my multimeter.

Excuse the dodge wiring. I was pissed off and in a hurry so the ends got twitched.

Amp wires: yellow to +load, black to psu - and red to psu +

Hi Matt.
Sight unseen I will try to help out here. Photos not much use except to show some pretty thin wires. Circuit would be better.
This meter looks like one of those combination Volt/Amp meters. These things can be a real pain sometimes. The meter has to be powered (like your DMM only that has its own battery) and most times this comes from the voltage being measured. Not a problem if only measuring volts, but will not work ff the voltage gets below about 6V, just connect the voltmeter part across the voltage to be measured.

Amps are a bit different. For the meter part to work it must share a common connection with the negative side of the voltmeter which is usually the negative side of the supply. The external shunt surprises me in this day and age. Far easier to use a hall effect type of current measurement. Never mind we have a shunt.
The Amp meter is in fact a Volt meter which measures the few millivolts across the shunt and calculates the correct current to display. The shunt then should connect between the negative supply and the negative or ground side of the load. The positive side of the measuring meter then connects to this point.

You should have got a wiring diagram in the box with the meter.

Here is where you have to be careful. The wire connecting the negative supply to the shunt should be fairly robust and have good connection. There are only a few millivolts here and any excess voltage drop between shunt and negative will be seen as millivolt drop across the shunt and will be interpreted as extra current.
That is what may be the problem here.

The other problem arises if you have more than one device powered and they need to share a common ground or share a ground with anything else like test equipment. The ground is actually the point where the shunt connects to the earthy side of the load and can be several millivolts “above” supply negative.

You can see where problems could arise where multiple grounds are connected via chassis “grounds” and earth clips etc.

The simple way around these potential problems (Dare I suggest it) is use an analog moving coil meter with internal shunt in the positive side of the supply and do away with the external shunt and the co-powered meter.
If you must go digital use a meter with its own isolated power source (battery???) in the positive side. That is what your DMM is. A non-invasive method like a current pick up coil (a clamp meter as such a device but has to be constantly re-zeroed when using DC) is another way but a bit exotic and messy.

I think I am correct about the shared negative connection with this type of meter. I personally have not had much success with them and do not use in the general scheme of things.

Hope this is of some help.
Cheers Bob