My BELKIN F6C1000-TW-RK just quit working, I guess I need new batteries as I bought it in 2005? Which batteries and where can I get them?

User Manuals only. Contact them direct

http://www.belkin.com/support/dl/f6c1000-tw-rk.pdf

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Please check after removing the panel to see if the battery voltage is down or gone below the threshold limit and so the UPS has shut off.
Also if the battery connection is bad the load will not be supported. Clean and reset the connections. If not the UPS board has a fault and might need careful checking.

You can download a user manual from the Belkin WEB site.
http://www.belkin.com/support/article/?lid=en&pid=F6C1500-TW-RK&aid=8381&scid=2

For the price it will take to get it fixed, you can buy a brand new one. But if it is less then three years old, belkin will fix it for free.

This sounds like a battery issue...I would have the unit and batteries tested at your nearest batteriesplus store... www.batteriesplus.com

I think your issue is that the unit is overloaded. Try this...unplug a couple of items to get the load lower...then pull the plug from the wall...did it keep those few units running while disconnected...if not then the unit could still be overloaded...unplug everything but a small electric radio...do the same thing to see if it will keep the radio playing. If this doesnt work, go by your nearest batteriesplus store and they can help. www.batteriesplus.com

You need to put the batteries into the correct series and parrallel...go by the nearest BatteriesPlus store and they will help you with this. www.batteriesplus.com

Here's the manual with battery replacement instructions. If not detailed enough, give their support line a call.

http://cache-www.belkin.com/support/dl/f6c1500-tw-rk.pdf

Toll Free from USA: 1 800 2BELKIN

It should work, make sure each profile is set the same way. Keep in mind you should still be able to utilize your standby feature, and your screensaver as well.