HMC 150 - Firmware Upgrade - Has Anyone Done This Using Battery Power?

Just curious, have any of you done a 1.21 firmware update using battery power (instead of AC power)?

I ask because I need to do an update to 1.21 (presently i have 1.19) but after reading the instructions, it specifically states: "This Update can not be done with battery"

I don't have the DC cord for the AC adapter, so i was thinking of using the battery (my battery is 5 hrs. fully charged) - but is there a specific reason why they insist on AC power?

I know that sometimes AC power is required for my macbook pro updates (even if the battery is fully charged) - so now I'm very hesitant to do this using the battery, thinking that maybe there is a specific reason why they say "cannot be done with battery"

any input is appreciated - thanks!