Isteiner, might be able to help you here; Virtuali is quite right in saying this is a Battery problem, but as you may know the APU on the default FSX 737-800 is not simulated, and therefore you have a limited time before your Battery will die unless engines are running.
Here is a way around it, but you'll probably not like my Post because of the costs involved. I am using a program called FSUIPC made by a Mr Pete Downson. You may well have heard of this program, as many developers use it. I am using what is called a 'registered' version, i.e. I have had to pay for it as I use it in a net work set-up running six P.C.s Anyone can download FSUIPC and use it on one P.C. unregisted, but it does not offer many options. The registered version offers many options to do with wind settings, etc etc, and on one of the TABs available (I forget which, but easy to find) you would see a Heading 'Magic Battery=1' This of course refers to the Battery, and the No.1, is a limited time period. By changing this to M.B. = 0, your Battery life is extended basically forever.
If you want to have a look at this go to www.schiratti.com
, and on the top left side click on Pete Downson. This will take you to Petes page all about FSUIPC, and there you will see a Link to download from Simmarket if you want a registered version.
Here's the down side; it costs EUR 28.56!! You might think this is worth while, but only you can call that one. For me I can't live without it, but that's just my set up. If you do decide to purchase, once installed, start FSX, then under 'Addons', on the top line Menu, having presssed ALT, you will see FSUIPC. Hope all this might be of help?