Sorry for the year-after necrobump, but I feel like this thread would be more appropriate to post in rather than making another spammy one asking the same question.
With Thermal Expansion stuck on Minecraft 1.5.2 for now, what BuildCraft energy storage devices would people recommend for 1.6.4? I've considered using IC2s and mixing it with Power Converters, but that seems like an insanely messy method.
if it is just bc, mj storage isn't energy... a bc battery is storing liquids or burnables and making them on demand.
Pretty much what DREVL said. Refined fuel in a steel tank is among the most dense energy storage solutions known in Minecraftia - at over 19 million MJ per block. Well, before the recent nerf. But even with that it's still almost 5 million MJ, which is considerably more than any battery from almost any mod stores. Only going via power converters into IC2 exerimental with its 40 million EU MFSU's and back would beat it. But that thing does cost you 40 diamonds, among other things.
So you should make use of the "power requested -> redstone signal" gate trigger attached to wooden pipes on engines. Each wooden pipe buffers 1,500 MJ, and any time that number falls below I think 750, the gate conditional will turn on the attached engine to try and refill the buffer, stopping when it gets back to 100%.
As a result, if you have 20 combustion engines with refined fuel set up like so, then you have 20 wooden conductive pipes storing up to 30,000 MJ for you. That MJ is instantly available to you, as if it came from a battery. But while you are in the process of consuming this energy, the engines are turning on one by one as the pipes' internal buffers drop. With all of them running, you're getting 120 MJ/t. And once you stop requesting power, they will keep running until the 30k MJ buffer is restored, then turn themselves off and stop eating fuel.
And if you have a setup where the engines never turn off, like a Railcraft boiler? Spam it with small engines. Use commercial (or even hobbyist) instead of industrial steam engines. Overload the boiler with more engines than it has output to run, so the engines run slower than they could. All this allows you to have many engines, and as a result, many wooden pipes that provide you with a big combined energy buffer.