I have a portable LCD screen rated at 12VDC 8W. I measured the current with an amp meter and it came in at 400mA. I assumed I would be able to power the screen with 10 rechargeable AA 2400mAh batteries for about 5 hours.

Wrong! The batteries last no more than 40 minutes before the appliance switches off and the power supply is exhausted.

I then tried 12 AA batteries and measured the voltage at 16.4 volts. The appliance only lasted 50 minutes before switching off. Measuring the batteries afterwards I was still getting 14 volts but there still isn't enough power to switch the appliance back on.

Can anyone explain what is going on?

I've just hooked up a 12V 4.7Ah C cell racing pack to the same screen and it's still going strong after 80 minutes.

 

How many forums are you going to post this in?

 

This is the second forum I've posted the question in Mike. Why?

 

You need only eight 1.5V batteries to get 12V.

Have you thought how much current these batteries can supply continuously?