Not sure if this is the right place to post, kind of looked like it.
Don't know where else to ask.
I have a small 7" LCD monitor that takes a DC transformer adapter,
with a barrel connector. The LCD screen is rated 9v-12v DC
Get a car lamp bulb (a 27W one will do nicely), solder a couple of wires
to it and put the wires on your battery terminals. It should light,
brightly. If not, charge the battery.
If it lights brightly there, put the wires (carefully) on to the new
plug. It should light brightly - if not, your wires going to the battery
are too long and/or too thin.
If you have a test meter, put it on volts across the battery, with the
new plug not plugged in - then plug it in and attempt to switch the
monitor on. The voltage should initially be >12v, should drop, but not
much. If it isn't 12v, even after a good charge - it's duff. If it drops
very quickly -it's duff.
Good question on the barrel connector. I'll check on that. Although I
am pretty sure it is the same size. As far as the battery goes I know
it is good. I have several that I have tried, all good, different
sizes. A 4aH and a 8aH, neither worked. The wires are not that long,
only about 12", and 18 awg. Im guessing the error is in my
calculations for amps/watts etc. But it looks pretty straight forward
for what I am dealing with. Calculating these things is not my strong