I bought a targus ac adapter, it worked the first time I charged my Dell 5100 Notebook computor, now a message comes up saying it doesn't recognize the ac adaptor, how can I fix this?

It sounds like the port not the battery. You should take it someone to repair it. They are not expensive...

As you say the system is 4 years old update the bios
your download link is here!
IMP. Be sure to plug in the battery and it should be charges above 10 % and the system should be connected to the mains power supply also ensure there will not be a power outage during the update process
this will resolve the issue 100 %
Dont forget to say thanks!

the problem is the BIOS of the Dell system is looking for an ID signal from the AC adapter so it knows what adapter is connected. If the BIOS cannot ID the power supply, then battery charger is disabled.

You will need to get a Dell AC adapter.

http://www.discountelectronics.com/index.php?l=product_detail&p=1459

If the AC adapter is not recognized by the system, the battery charger circuit is disabled. Things you can check:

1. In the barrel of the connection to the computer, the small wire in the center is the sensing wire. If bent or broken, then the adapter will need to be replaced.
2. If the pin is not bent or broken, try a different AC adapter (Any D-Series Dell adapter will work) IF that adapter works, replace the AC adapter.
3. If a known good/new adapter doesn't resolve, then a motherboard needs replacement.

-check the system battery information and charger information of the computer; restart the computer, while the computer is booting up, press F2 repeatedly; check "battery info"
-if it stated ac adapter = unknown device ; update the BIOS (-visit support.dell.com
NOTE:other computers may have different BIOS, if it is not the same you may have to consult manufacturer
-click on drivers and downloads
-click or choose by service tag
-download the BIOS and install it on the computer)
-check information after updating; if same message AC adapter = unknown device; call manufaturer for a replacement AC adapter, if status has changed it is a battery problem or motherboard, contact manufacturer to test battery

the Dell AC adapter has a sensing pin so the bios can determine the wattage of the adapter. If the system cannot ID the adapter, the battery charger circuit is disabled. You'll need a functional DELL ac adapter to get the battery to charge

On the long term, a power adapter with a bigger amperage can damage battery and battery charger.

Better to get an adapter with same specs as the 65/70W one , or an original replacemant:

Dell Inspiron 1525 New AC Power Adapter Supply $14.29 - Buy.com Dell Inspiron 1525 65 Watt Original AC Adapter $25.00 - parts4nb.com Dell Inspiron 1525 Power Cord PA-12 $16.95 - AmeriMax Advance


DELL Inspiron 1525 DC Power Laptop Adapter ... £25.23 - B2C Shopping UK DELL Inspiron 1525 Laptop AC Adapter power adapter £38.99 - Very Good Battery Dell Inspiron 1525 Ac Adapter Charger Power ... £16.50 - eBay UK

AU/NZ - DELL INSPIRON 1505 E1405 E1505 AC Adapter Power Supply

Dell : Lind Electronics 70-Watt DC Power Adapter for Dell Inspiron

It is either the adapter or the connections to the motherboard. It would definitely feel better about telling you which one it is if you could try another adapter. Last thing i can say for now at least is have your ac adapter checked and make sure it is putting out the required power.

The Dell 1501 ac adapter (possibly other Dell’s as well) has been known to be problematic because of its lack of a plug to jack secure fit. You will notice that the plug can all too easily be pulled from the jack, as opposed to other systems where the plug fits more securely onto the jack. The plug and jack combination is poorly designed so that over time, the plug will come unseated depending on how much movement the machine is subjected to. This will result in power error messages or outright power-up failure. So before assuming you have a faulty adapter, make sure that it is not just a loose plug that is causing the problem. If you look inside the jack, you will notice that there is a 3mm region of non-conductive material nearest the surface of the jack. Use a strip of standard electrical tape 3mm wide and long enough and wrap it around the base of the plug (you may need more than one layer of tape to get a good fit). I have used this method to secure my plug and it has served me well. If the tape comes undone over time, just simply replace it. A more “elegant” method would be to use an “O” ring of the appropriate size (about 6mm) around the base of the plug in lieu of the tape.

The blinking orange battery light generally indicates that the battery isn't working properly, usually going out and not charging properly. In this case, since the adapter will not power the machine without the battery, and it should, I'm going to say that the adapter is going out and not getting the proper amount of power to the system. Try replacing the adapter. Be sure to replace it with a Dell replacement, the Targus brand universal adapters do not work well with Dell machines, and in most cases won't work at all.