Replacement battery for VTECH IP 811 cordless dies fast

If you mean the VTech CS5121 then you may buy a replacement battery at http://www.batteriesplus.com/product/28276-RAY19-Battery/549-0/5325-Cordless-Phone-Batteries/142532-Vtech/CS5121.aspx

You could try leaving it on charge a bit longer but if it keeps happening it either means dock doesn't charge the battery any longer (check for the charging symbol on the phone when in dock), or the battery in the phone has had its days.

Try to fully charge them first and check for how long do they keep charge. But, yes, for some batteries 250-300 recharge cycles is a life span.

You can download the manual at
http://vt.vtp-media.com/ecp/documents/product_Product/791/UserManual/2095/mi6896_manual.pdf

One source for batteries is
http://www.batteriesplus.com/products/549-0/5325-Cordless-Phone-Batteries/127618-Vtech/mi6896/1.aspx

Open your phone and
check it, but I think this
is the right battery.
Batt-6010Vtech 89-1326-00-00
Replacement Battery For The Following Vtech DECT 6.0 Phone Models 6031
6032
6041
6042 6053
DS6111
DS6121
DS6121-3 IS6110
DS6121-4
DS6121-5

Almost certainly you will have to spend $50 or $60 on a new battery. Assuming it has a removable battery, which most cordless phones do. Take the phone to Radio Shack and see if they have the proper replacement battery.

Your phone may have a faulty sensor. If the only notification is that your screen displays low battery, and there is no interruption or disruption of service or quality to the product you can most likely leave it as is. If your phone is still under factory warranty, do not replace the battery, ask for a replacement for a faulty product. I would personally test the lifetime of your phone by leaving it off the charger for a day, and if it lasts the entire day without dying, then it is a problem with your phone's sensor / battery indicator. If the phone dies after a few hours of use or standby, then there is a problem with your battery and your battery tester.

that happens when the battery's a low get new rechargeable lithium battery's

Have you tried removing and replacing the batteries.

buy a new battery from ebay.
just type in the search box, 80-5808-00-00
(the battery number)
You should be able to get a replacement for around $6 incl shipping.
This worked for me anyway. (Same symptoms you described)