I have a project to decorate the family "Little Red Wagon" with
Christmas lights. So I stated experimenting with LED light strings and
battery combinations. I finally decided to use an Inverter and not
cut up any light strings. The LED light set that I found has 80
lights and the specs state it uses 12 watts. So as a test I connected
two 6v lantern batteries in series and ran the leads to the inverter
(rated at 100watts), connected a lamp with 40 watt bulb.

The inverted started up and seemed to be working, but when I connected
the lamp, the green led on the inverter turned to red and lamp glowed
at 60% of what it should. The voltage from the battery dropped from
12.6 to 11.5v. I need a battery with less internal resistance.

What would make a better 12v source ? Most NiCad drill batteries are
up around 18v+
What is the voltage of battery in APC backups ?