MacBook Pro won't start

i have 2011 macbook pro 13". i've made regular, heavy, daily use of years - few problems , none can recall past 6 months. used last night, morning not wake sleep , not boot. i've spent day searching solutions. there's no external reason fail (it wasn't dropped, didn't have liquid spilled on it, has not been subject extreme heat or cold). in fact because use external keyboard , monitor touch it.

purposes of diagnostics, have disconnected external cables , devices - exception of power cable (as noted below).

symptoms:
when power button pressed, hard drive spins , all.
- display remains dark
- dvd drive not respond
- there no startup chime
- keyboard not light
- "sleep" light on front of chassis remains dark

have tried resetting smc , pram
smc tried booting ctl-option-shift-cmd pressed.
tried disconnecting battery , charger, pressing power button 30 seconds, reconnecting charger.
on restart, sometimes hard drive , fan run. if reset smc again, may or may not happen.

battery old, led indicator lights on left side of chassis show 8 green lights when button pushed. there no indication battery failing. charger shows green light @ magsafe connector. have tried different power supply. shows green @ connector.

if try boot power supply connected, hard drive spins continuously.
if disconnect power supply , boot, drive spins few seconds , system shuts down (much might dead battery), however, if boot power supply connected, disconnect it, drive continues spin. , if mpb shut down , connect power supply, disk begins spin.

have removed , re-seated 2 8gb ram modules , on theory it's unlikely both fail @ same time, have inserted each individually each of 2 slots. no change.

i've disconnected , re-seated hard drive connector.

i've tried booting recovery mode (both 'r' , option-r)
i've tried booting single-user mode (shift + power).

same result.

insights? suggestions?

 

try disconnecting battery, connect magsafe, , wait 30 seconds. if machine powers properly, battery needs replaced.

 

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