Learning power supply and battery basics

Learning power supply and battery basics
Posted by ullbeking on 11 Dec, 2020 14:44
Hello,

I am commonly faced with problems that are caused by power supply defects (bad design, breakage, etc).

For example, let's say I purchase a cheap PSU on Ebay for a second-hand laptop computer, which I also bought on Ebay. In my experience 90% of "weird problems" are caused by power supply problems.

How can I test the power supply so that I can be confident that it's up to specifications? For example, that it's not a cheap counterfeit with an OEM label. Moreover how can I test the "health" of a battery, and understand the relevant characteristics that are required to power the laptop computer?

Is there a "theory of load simulation" that electrical engineers use to determine the following example things:
- Can the PSU handle the load (laptop) without dipping or having dangerous transients in current or voltage?
- Similarly can the battery can handle realistic demands placed on it by the laptop, and to measure how long it can sustain this?
- How can I replace bad cells in the battery?
- How can I safely teardown a PSU to determine why it's faulty
Thank you for any advice!!

ubk

#1 Reply
Posted by bob91343 on 11 Dec, 2020 19:03
You ask a battery of questions that have no simple answer. If these topics interest you, perhaps a few books would be helpful. Batteries are a black art that is constantly in progress. Power supplies less so; a used power supply that puts out the correct voltage whether loaded or not is probably a good one. The engineers of the computer took great pains to design the charging circuitry so I wouldn't worry along those lines.

As for repairing batteries, just replace the defective cell or cells with new ones of the same type and rating. Or close.

Nothing is ever as simple as we want, so the more you know the better the job you can do.

#2 Reply
Posted by opampsmoker on 11 Dec, 2020 23:26
Its hard without full schem and bill of materials.
If an output diode is getting overvoltaged by LC ringing of the leakage and eventually blows from it, that could do it.
Or just a MOV that eventually blows due to mains transients.
Or a bad electrolytic capacitor.....or el'ic caps that arent rated for the ripple.
If you have a fet thats not rated for the flyback voltage (in a flyback), and the fet is Chinese and you cant find its datasheet...then youre not going to be able to check it to find out.
Is there enough inrush limitation to stop the lytics from getting eventually blitzed from high inrush.

A thermal cam can be good to see if anythings heating up too much...beware emissivity concerns.

But if you load a 48V PSU with 90W and it sinks to 47V, then its not rated for 90W.

But usually if that happens it would shut down on its overload protection...if it has any.

Suppose there are damping chip resistors across the filter inductors...but then they fry cuzz they are too small...so then theres no damping on the LC filter and something gets overvoltaged by the subsequent overvoltage ringing.

etc etc..but without full schem and bom...your up against it.

Ive seen MOVs placed downstream of an input filter..and the natural on-at-mains-peak literally puts a overvoltage spike (from input LC filter ringing) into the mov which eventually kills it.

#3 Reply
Posted by atmfjstc on 12 Dec, 2020 01:26
Quote from: ullbeking on 11 Dec, 2020 14:44
How can I test the power supply so that I can be confident that it's up to specifications? For example, that it's not a cheap counterfeit with an OEM label.

You can't, not easily. All you can do are very basic tests like, if the supply is rated for 500W, pull 500W from it and see what happens - if it goes into protection, explodes, output voltage is 25% below spec... that pretty much settles it. But more elaborate tests like, how reliable is it over time, whether it's genuine, etc. require extensive experience. Testing products thoroughly is hard, a whole field of engineering in itself.

Which is why it is often worth it to pay more and go to a reputable manufacturer (and distributor!) rather than muck around on eBay, unless you know exactly what you are doing and have the experience to tell apart gems in the rough from garbage.

Quote from: ullbeking on 11 Dec, 2020 14:44
Moreover how can I test the "health" of a battery, and understand the relevant characteristics that are required to power the laptop computer?

Same applies for batteries. If the label says 2500mAh and you only get 1500mAh out of it after a full charge-discharge cycle (with the proper C), it's either toast or the label is a lie. But there can be many other things wrong with it which aren't so easy to detect - high internal resistance, internal shorts, unsafe protection circuitry, etc

Quote from: ullbeking on 11 Dec, 2020 14:44
How can I replace bad cells in the battery?

Cells in a battery pack should be replaced with some that are as similar as possible to those remaining - same chemistry, same voltage (of course), same capacity, same current rating, ideally same manufacturer. And same state of charge.

Quote from: ullbeking on 11 Dec, 2020 14:44
How can I safely teardown a PSU to determine why it's faulty

Best not to, if you're not sure of your skills. But if you must, the main danger is the capacitors. Make sure you understand how to ensure that they're discharged and stay discharged.