For most people, the battery life on a DSLR is more than sufficient, but if you want to do a really long exposure or time-lapse, one charge might not last long enough to get the shot you want. You can always connect it to an extension cord to charge while you shoot, but that can be really annoying to work around.
Instead, if you know a little bit about circuitry and you're confident enough to crack open your battery and poke around, here are two ways you can modify it to last longer for extended shoots.
With some parts from a cheap aftermarket battery, JP Danko hacked his battery to connect to an external battery to make it last 4 times longer. You'll need a camcorder battery, adapter plate, battery charger, and replacement battery pack for your camera (you can find links to the ones JP used here). You'll also need some high-gauge wire, a soldering iron, and a hot glue gun.
Basically, you'll be cutting open the replacement battery and removing the circuit board and soldering on new leads, then hot gluing everything back in place.
If you don't want to tear everything apart, you can also just connect the leads of the external battery directly to the terminals.
The external battery is connected to the camcorder battery base plate so that it can be removed for charging, and the replacement battery goes into the camera. The door should close, but you may need to make a small notch for the wire.
Check out JP's step-by-step tutorial for more details and photos of the process.
If you're doing a really long exposure, a better bet might be to turn your battery into a power supply unit that can be plugged in to charge constantly like Instructables user andrea biffi. You'll basically be replacing the insides of your battery with a step-down adjustable power supply module, which you can find super cheap online.
First, Andrea opened up the camera battery and removed the two lithium-ion cells inside. He drilled a hole in the battery case and ran the wires for the power source through it, soldering them to the input and output terminals.
If you're interested in trying it for yourself, be sure to check out Andrea's tutorial for details.
Know of any other cheap battery hacks? Let us know in the comments below.