The vast majority of 2.5" SATA USB HDD enclosures work without a power supply. Instead, they draw their power from the USB cable and have a few things to watch out for. Besides reducing the laptop's battery runtime, some laptops don't provide enough current to power the enclosure since SATA HDDs typically draw 1A or more to spin up and a USB port is only required to output 0.5A. This is also why very few (if any) USB-powered 2.5" hard disks have a USB certification logo.
To use a battery, what you will need is a USB HDD enclosure that has a separate USB power input cable and a battery powered USB port such as a cordless USB charger. This way the battery powers the HDD without any issue with the laptop's runtime or USB port power limit.
Most USB HDD enclosures show what's included, so what you'll need to look out for is a 2.5" SATA USB enclosure with two separate USB cables. Here's an example (hover your mouse over the second image thumbnail). For the battery, look for the largest capacity (in mAh) cordless USB charger you can find. Here's an example.
One thing to watch out for with using a battery is that it does not give any sound or warning message of when it's low, however, with a large capacity unit (especially 5000+mAh), as long as you remember to charge the battery when you recharge the laptop, it's likely that your laptop battery will run down long before the HDD's battery runs out.