APC ES500 switches off and the red light blinks. The problem started after I changed the the old battery. It had worked flawlessly with the previous battery.

Are you getting any warning lights? Your post seems to state that this unit is not very old? If still under warranty contact APC, they have fantastic customer service.

Check battery and charging voltages. APC says 90% of faults are a dead battery. My experience also

there are 2 parts of a UPS .
the charge side
and (battery is obvious)
the inverter side.
if the charger fails, all is lost. (are the batteries in backwards?)
if the charger works (volt meter tells you that)
then the inverter is bad. (this unit is not full time UPS , but part time, this means it switches over to inverter at power fail.
(on full time sys. inverter runs full time, like the name states)
the unit, you mean the load right.?
the UPS side plugged into a load. and the load is now dead.

the Backup plugs should work if the mains are hot.
if you kill mains, the inverter will fire off, (hear the squeal?)
and give you 500 watts for some minutes, sucking the batteries dry. (see load run time curves)
I used to do service on HUGE UPS systems, some with 100s of batteries. 3 to 5kw too.
on ours there is a port on the rear , we connect and can
check everything "BEST brand" even charge voltage.
did you look on APC site for special software and connections?

the manual does cover trouble shooting, ever read that?
RTM? start there, its online, for sure.

your battery is damage,need to replace it with a new one.
your ups is overloaded. . .

short circuit may be happen to your UPS. try to open your UPS and look for the short circuit or try to call a electronics service technician... :))

Contact APC.com
Here is the page for your model , it looks like it qualifies for the tradeups program. Or you can purchase a battery for it and fix it yourself.
http://www.apc.com/tools/upgrade_selector/index.cfm

APC Smart-UPS 1500VA has an error. All it's pannel lights blinking. what is the best possible solution to this?

Hello,

Probably the battery is drained. You should charge it first. And if the problem still persist, it could be that the UPS battery needs replacement.

It means it has an internal fault. if still under warranty period, just call APC and ask for replacement. if it out of warranty bring it to service center or better if you have to buy a new one because as far as i am concerned repair cost of this unit is much expensive than a new unit

apc ups fully dead what can i do