The LED strip needs 12v. It might work with less than 12, probably down to about 10v and it should work with more than 12v but you're shortening the life of the leds.
Each led should use about 20-40mA, and since you have about 15 leds in a strip, the total power should be about 300-500mA
If you want to power the strip directly from batteries, rechargeable batteries will be about 1.2v discharged, about 1.35v charged, so a pack of 8 batteries should be enough. You'd get about 10-11v which should light up the strip just fine.
But do yourself a favor and forget about eBay batteries, those are cheap for a reason. They say 3000mA but in reality they're probably 1000-1500mA if you're lucky and the chemical composition is so bad they're only going to survive probably less than 100 recharges.
If you want rechargeable batteries, I strongly suggest Sanyo eneloop :
https://www.amazon.com/dp/B004UG41XW/ref=as_li_ss_til?tag=tgnethgada-20&camp=0&creative=0&linkCode=as4&creativeASIN=B004UG41XW&adid=0CNCNEH65EX1G7MNVZCX&Yes, they're more expensive at 20$ for a pack of 8, but they're really worth it... you'll save money in time.
Alternatively, you can buy a boost regulator, which can take a low voltaje and boost it up to a higher voltage, so you could for example use 4 alkaline batteries (~5-6v) to generate 12v for the light strip.
Here's an example :
http://www.ebay.com/itm/DC-3-5-30V-to-4-30V-LM2577-Boost-Module-Voltage-Step-Up-Regulated-Power-Supply-/130978297634?pt=LH_DefaultDomain_0&hash=item1e7eea3722This takes at least 3.5v and produces up to 40v ... so if you use 4-6 non-rechargeable batteries (which have 1.5v-1.65v) then you have 6-6.6v at the input, and you can set this boost regulator to output 12v with no problems at all.
If the light strip uses 500mA, the boost regulator will use about 1A from the batteries which is a bit much, so 4-6 alkaline or rechargeable batteries will last about one hour until they discharge. Since you say you only need to light them up for short periods of time, that hour could last for a week or so.
By the way, with such a boost regulator, it would also be possible to just use one of those 7.5v 1A power adapters that comes with switches, scanners etc or a good usb charger that can do 1-2A (not a cheap charger, those can't give enough power)
You can improve the life of the batteries by adjusting the brightness of the leds, but now we're getting into PWM or limiting current that goes to the leds and that's another story.