Why to start your engine with the charger OFF (a glow plug question)

[sedelange]

I had the engine manual out for an unrelated issue and noticed the details about the glow plugs.
Resistance - 1.0-1.5 ohms
Current draw - 8-9 amps
Normal energised time -  10 seconds at or above 41F, 20 seconds below 23F
Maximum energised time - 30 seconds
OFF time between start attempts 30 seconds

I believe if the starting limitations above are followed the glow plugs should last a long time.  In my case, it is never below 41F when I am starting therefore 10 seconds should be all that is needed. 

[Ron Hill]

Most of our manuals talk about a glow plug engagement of about 30 seconds - with NO shore power battery charger on. 
I agree with Steve, that if you follow the correct instructions your glow plugs will last.  Mine only lasted for 5200 hours and 21 seasons!!  AND ARE STTILL WORKING !!