My 04 Lowriders battery drains down when sitting for a month or over the winter. It drains the battery so it won't hold a charge. When I hook up the battery tender I can hear the fuel pump hum with the switch off.

try a different battery sounds like the battery is no good not holding charge long

Battery weak. Doesn't hold charge. Replace it.
http://electronicshelponline.blogspot.com/

the battery can be disconnected if you let it sit for more than a week. most common drain that i encounter is in the audio system. disconnect the negative battery cable and put a test light (wired light bulb) between battery post and cable end. if the light is on or even dim, start pulling fuses one at a time. if the light stays on, replace the fuse and move on. if the light goes out then you have found which circuit the draw is in.

Dealer should have been able to figure this one out!

Best bet to trouble shoot is next time it is going to sit for a couple of days disconnect the battery. After the couple of days pass hook up the battery and if it's dead then the battery isn't holding a charge, if not then something in the vehicle is draining the battery.

If you live in a colder climate weak batteries tend to be more of a problem as temps get colder.

If the battery is more then 3 years old it is a prime suspect.

Hope this helps!

You can tell if the battery is weak while it is cranking. If that is the case then battery need to recharged. When a car is not running regularly, it doesn't have the charge from the alternator so it becomes weak. If there is no sign of weakness in cranking then the problem is not from the battery, but it is from lacking of two basic things: ignition and / or fuel. Need to find out what is the root cause. Good luck.

Time for battery replacement!

Batteries will lose power if they sit for long periods of time. It is a good idea to start the vehicle once in awhile to restore the charge to the battery. Batteries will lose their charge and it is not a good idea to let them sit for a long period of time, especially in the winter months. If your battery is older, it may not be holding a charge as good as a new one would.

Charge up the battery, check the battery connections, sounds like a discharged battery