Need a 12V 3AH sel battery for the above light

take it to an accredited auto electrician for a charging system check including the voltage regulator and wiring conditin
at idle the battery voltage will be 12-13 volts and at 2000 rpms it should be steady at 14.5-14.8 volts dc

The best would be to take out the battery, which is necessary to replace it anyway, (you'll nee a phillips screwdriver for that) and unplug it.

On the side there should be text similar to: "12V" and "3Ah". You should have a 3Ah battery in it, but they come in a variety of shapes.

You can then check and order on Digikey or Nextag the correct battery size and put it into the flashlight. Check that the black wire is on the black tab or the one designated by a '-'.

There is good news and bad news concerning a replacement battery for this particular model light.
The good news is that a suitable, but not exact, replacement will be available at Battery Mart on November 30, 2010.
The bad news is that it cost $29.95.
http://www.batterymart.com/p-sla-1039-12-volt-2-3-ah-sealed-lead-acid-battery.html

I purchased about a dozen of these lights in 2006 and have had them in storage since. No one can or will be more unhappy than I should all of them need new batteries.

You have to remove the cover around the gearshift, there you'll find the bulb, it's a small 12V bulb, just pull it out of the holder and install a new one.

You can order it from Black and Decker, Vector sold all their product lines to them in 2006 and they carry the replacement battery cheaper than anywhere else I found.
http://www.dewaltservicenet.com/Products/ProductDetails.aspx?pid=69262&tid=16032

I found a replacement battery at interstatebatteries.com. Search for DMU6-3, you'll find 6v 3ah SLA 187/250 Faston for $10.00.

You can order one from Black and Decker, Vector was bought by them in 2006, here is the link; http://www.dewaltservicenet.com/Products/ProductDetails.aspx?pid=69328&tid=15836

I found a replacement battery at interstatebatteries.com. Search for DMU6-3, you'll find 6v 3ah SLA 187/250 Faston for $10.00

Make sure that your battery connections are clean on contact surfaces and properly tightened. Check to make sure your battery has 12v or better in it.If not, have it charged and load tested.