Stupid question No. 438 – Battery chargers

[Zebedee]

I've just spent the day in the shed clearing some, well actually lots of sh!te, and amongst other things I found an old PC, which I ripped to bits.

Looking at the power supply, it has both 12V and 5V outputs, which started me thinking.

Why do folks spend large amounts of hard earned on battery chargers when the power supply from a clapped out PC can supply apparently supply the goods?

OK, the old power supply doesn't have the pretty LED's or volt meters, but couldn't it do the job?

Personally, I prefer to ride the bike to charge the battery, but I figure it's worth asking the question.

Cheers

John

PS I'm looking forwards ho hearing, at length, just how stupid this idea is

 

[John Armstrong]

Most chargers push out around 14v to overcome the batterys internal resistance and therfore get the battery fully charged. 12v will not get the battery fully charged IMHO, AFAIK etc etc.

 

[Rob Farmer]

Sorry to dissapoint you Zebedee but it's not a stupid question at all

John's right the PC power supply is too low a voltage to charge your bikes battery fully. Most of the intelligent chargers are actually quite good value when compared to buying a new pc power supply. The Optimates vary the voltage and current when charging to optimise the charging - not bad for something cost £30 ish.

However the PC power supply does provide quite a high current rating if you need a fixed voltage power supply for something else.

if it's an older unit you will have +12V, -12V, +5 and -5V from the power supply. Depending on your power rating the +12V will provide about 10Amps of current. however, Switchmode power supplies do not provide a stable DC voltage but have a slight variation in voltage, A "Sawtooth" - it looks like a saw blade if you view it on an oscilliscope. I found this if you want a stable supply for something else http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

Bet you wish you hadn't asked?

 

[Zebedee]

Cheers Guys,

Maybe an old PSU isn’t up to the job of a full on battery charger, but I was starting to think along the lines of a bench mounted 12 V power supply for running the portable compressor off the mains, or testing … stuff, so thanks for the link Rob.

… right then, I’ll see what other hair brained schemes I can come up with after a couple of beers tonight.

Cheers

John

 

[Zebedee]

It works!

Well, after following the link that that Rob posted, and a few more besides, I managed to build my bench top power supply.

As you can see, the output voltage falls a little short of 12V, but the white connector at the bottom of the frame is a standard twin filament brake/tail light bulb holder wired across the 5V connector, and when the bulb is fitted the voltage steps up to just over 11.6V which is close enough for me

Even with the lower output voltage, there’s enough … uumph … to power the little mini compressor for inflating the tyres, etc.

Also, with a bit of … jiggery-pokery … it’s possible to configure the device to output a range of voltages from 24V down to 1.7V

Sorry for the technical language.

One good thing about playing abut with the thing is that it’s given me some practice with the soldering iron, and I might even feel inclined to have a go with the elastictrickery on the bikes.

Get your solding irons out and have a go

Cheers

John

 

[davegs]

power suplies

If you have a knowlegble friend there is probably a small "pot" (potentiometer)
on the power supply pcb which if turned gently will possibly increase/ decrease the out put voltage. Some years ago when mobile phone chargers were larger with metal cases I "adjusted " one as a trickle cahger wich worked for years, transformer went in the end but a free cahager for a couple of years. I would be careful about runniong a compressor from it as this will take a bit of current (even a a small one) if it starts getting warm turn it off.
Good luck nice to see inovations like this.
dave (with a burnt out phone charger) GS.