Why does the display go dim when on battery? - Dell Inspiron 1720 PC Notebook

Change the batteries. LCD displays will appear very dim and hard to read except at certain angles is the batteries are low.

we have no idea what you are talking about in computer repairs

what PC, fails, or PC device, none stated, at all.

impossible answers or wrong await you.
dim screens, are cause by 2 things, or more.

if the battery is weak the OS detects that (no OS stated)
and dims the display on purpose. or a bad battery too.

or is turned down in brightness controls.
if a PC is older than 2010 , the CCLF lamps go dim (means bad)

Check power settings,display will dim on battery power by default.Settings for display brightness are adjustable for AC and battery power.

not sure about that one but there is a dimmer wheel by the light switch. move it up and down to dim and brighten the dask lights/cluster. wheel may need replacing. also work the lights on and off switch manually to see if there every thing works smooth or hopes from dim to brite.

you should first have the alternator tested. any auto parts store will test your alternator and battery for free. Also you should remove the battery cables and clean them with a battery terminal cleaner. sometimes a loose or dirty connection at the cables will cause the battery not to charge properly.

You didn't supply your Make, Model of computer or your OS? The best I can do is give a general idea. Open Control Panel, Power Options, Change to High Performance, open Advanced power settings, adjust Dim Display 100%, plugged in. Your laptop software defaults to dim the display on battery power to conserve battery life.

Open the Control Panel, Power Options, Set to High Performance, open Advanced Power settings,Change Dim display to 100%, Plugged in, Adjust to your preference. If the Laptop is on battery , the software is set to dim the display to conserve battery, you can set it to 100% , your battery time will suffer.

On Battery the Laptop dims the Display to conserve the battery. Open Control Panel, Open Power settings, Advanced Power settings, Change to High Performance, Dim Display to 100%

Open Power settings, in the Tray Notification Area or Control Panel, Power options, change to High Performance, Advanced Power Settings, Adjust Dim display. If it is on battery it will dim display to save battery.

simple get a new battery