Why does my battery keep dying when lights are turned on

"Hi,

Check the output from the alternator.

Connect a DMM (Digital Multimeter - voltmeter function) across the vehicle's battery terminals when the engine is running. BE CAREFUL when you do this - the engine is running, watch what you’re doing.

There should be at 13.8 -14.5VDC across the battery terminals.

If not then there is a problem with the alternator.

If you do measure this voltage, how old is the battery?

It may be that it is failing to hold its' charge.

Measure the voltage across the battery terminals without the engine running. A good battery should measure at least 12V DC. (or very close to it if it is under this value, certainly not less than 11.5V DC).

If you haven’t got the gear to do this, take it to a place like AutoZone and ask them to do it for you."