Battery for WS2812 Strip? (Something’s not right)

Hi all
I have posted this in a few places, so sorry if you have seen it somewhere else
Im working on a battery powered led strip project, iv put the project together and have been testing with a mains power supply
Iv run some tests and found at max I'd be pulling 4.5 amps
Averaging around 2 ish

I rounded out to about 3.5 amps, and said for 90 mins run time id be looking at the 7ah battery
7/2 = 3.5 so 2 hours runtime

I got a 7ah SLA 12v battery, running though a 12v to 5v converter to power the led strips
(Arduino is running off a usb battery pack)

However after about 45 mins I tested the battery and it said 10.9 v
Which I understand to be too low for SLA batteries (11 should be the minimum?) so I shut it off

My question is did I mess up somewhere and get my math wrong?

Many thanks in advance

Touch here for the full post on the Arduino Apprentices tumblr

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