10 replies to this topic

[HelloDave]

Yes, this is very off topic, but I know there are a fair few electronic engineers on this forum. I've got roped in to repairing a friend's laptop because he's managed to break the power socket off the motherboard. It was an absolute nightmare getting inside it, but fortunately I found a service manual, and as far as I can see one of the solder joints from the socket to the main board has broken, and just needs re-soldering. (That'll be interesting - I haven't soldered in a while!)

Anyway, to the problem! When I plug the power supply into the wall its green light doesn't come on, and i'm only getting ~.2v out of it. Obviously I can't check it becuase the laptop is in bits, and once I repair the motherboard if it still doesn't work I won't know if it's the PSU that's faulty or my dodgy soldering. So...does anyone have an HP laptop and if so do you know if the led on the PSU comes on when it's plugged into the wall but not the laptop? I'm only asking becuase possibly the PSU is working, but won't deliver full voltage unless its under load (which I can't test); the fact that it puts out a small voltage tells me it can't be totally bu**ered. Normally I would expect it to work, but I'm not sure if I accidentally shorted it out during testing  :oops:

If I can't repair it then that means a new motherboard; £200 becuase a 50p socket has come loose!

Thanks for any help  

[Monolithix]

From experience, most laptop PSU's i've seen due draw some (but not much) power when plugged in but not connected to the laptop.  This is from seeing Dell and Sony laptops.

Don't quote me on that though, might not hurt to give HP CS a call and ask them to get their tech team to verify it.

[HelloDave]

Thanks for that, its not looking good though. I think the PSU is broken, but sadly so is the laptop (still); motherboards were not designed to be soldered! After a tour round the electrical shops of Birmingham we eventually found somewhere willing to let us try an HP PSU, but with no luck, so it looks like new motherboard time :cry: (and the end of my career as a laptop repairman!).

[Monolithix]

:/

How old is it?

Could you not buy a 2nd hand one cheaper than the repairs and cannibalise it?

[HelloDave]

I'm not sure exactly; it's a Celeron 900 (chip says made in '01) so not that old I don't think. Good idea about a secondhand one, though it still seems a pity to replace the whole motherboard! It's annoying really because there's continuity between the socket and the board, unless something else is fried as well; probably very easy to do by taking a soldering iron to a multilayered, double sided surface mount PCB! The power LED does flicker if you try and turn it on using the battery (which is now discharged) so that's a good sign that the whole thing isn't dead at least!

[Monolithix]

So your obvious solution is to rig up some dodgy wiring and charge the battery directly form the PSU

[HelloDave]

I had thought of that as a solution too, but had to abandon it as an option becuase -

a) my friend is too picky and couldn't deal with charging the battery, using it, then taking it out to recharge etc (I know he could buy a second battery, but still...)
the PSU is now totally broken (not my fault!)
c) I can't find a pinout for the battery!

It could be a workable temporary fix to get data off the thing though. Thanks for all your ideas - if you get any more let me know!

Dumb question from someone who's meant to be learning this stuff at uni but what voltage/current would you charge a 10.8v 4000Ah Ni-Mh battery at? That's assuming I work out the pinout of course. Why can't it have nice big obvious (and labeled) pins for the + and - terminals like my old Toshiba?!

[Monolithix]

What confuses me is why laptop and phone batteries have 3 contacts.  Positive, negative and....other?

As for the V/I ratings, are they not specified on the PSU?  Most have I/O values on them don't they?

And if the worst comes to the worst just rip out the hdd and hijack someone elses laptop

[HelloDave]

I don't know about phone batteries, but laptop ones are "smart"; they have several contacts so that the computer can accurately monitor the amount of charge in them. The PSU has V/I ratings but it isn't as simple as that; I spent some time looking round the 'net and it seems you have to charge an Ni-Mh battery at 1/10th its capacity and at around about the voltage of the pack otherwise you risk it heating up too quickly and frying. I think I need to charge my battery at about 400mA, 12v, but that's assuming the "smart" battery lets me if it isn't connected to a proper charger!

[Monolithix]

Heheh, well have fun melting your battery

As for "Smart" batteries, maybe this is implemented in the phone batteries too, as theres a battery monitor on the title bar...maybe a build in voltmeter or something?  Or am i making that up :/

Incedentally, my now defunct MD player has a batery monitor as well and that runs of normal AA batteries, so 'm probably talking out my arse.

Nevermind

[HelloDave]

You can estimate a battery's capacity by looking at its voltage I think, but "smart" laptop batteries actually have a circuit inside that monitors its current capacity and charging status. Without this the battery monitor in Windows would be a rough guess at best (like old laptops) and it would be very easy to blow the battery up by overcharging it (especially with Li-ion batteries). This is only becuase phone and laptop batteries are normally fast charged though (at a rate of about half the capacity of the battery); if you trickle charge them its much harder to have an "accident". (I hope!)

Well i'm glad my time on the internet looking at batteries last night was well spent!