Im using aux to bat I, and it usualy lasts for 10s, the buff varies on the aux power, when it was made, usualy between 55 to 66...those last aux skills on admiral level, help it a bit, but still its too random. And there is a flaw in the cooldowns.
Okay, did a bit more experimenting. I'm not positive, but as near as I can tell increasing the skill on the BOff increases the duration of the buffing effect. I could have sworn when I started the buff lasted for only 6 seconds. Now I've got maximum points into the BOff skill and it says it lasts 10 seconds -- although it could be I adjusted something else and didn't notice it.
Here are my tentative conclusions:
Increasing Aux increases the amount of time the battery will stick around in your toolbar before you have to use it.
Putting points into the BOff's skill increases the duration of the batteries buff after you use it.
Putting points into the corresponding captain's skills as determined by Dalnar above increases the amount of power boost granted by the batter.
Also, if you are running your toolbars with settings 1 or 2 where only buttons 1-8 show up and buttons 9 and 10 are hidden, sometimes the battery will show up in the invisible 9 and 10 buttons. I got around this by filling in the 9 and 10 buttons on all of my toolbars with copies of fire Quantum torpedoes, so all the invisible slots were filled.
Assigning the emergency battery to a specific place on your toolbar by dragging and dropping it from the Powers list does not consistently result in the emergency battery showing up in that same slot next time you execute the Aux to Battery power.
The max i could achieve with aux to battery II, was +85 to levels/power for 10s (thats with eng team III buff up)
That's pretty decent, I'd say, though I'd rather that the time got longer than 10 seconds.
But one thing to remember: when firing all your beams on a cruiser, it often brings your power level down from 100 (or wherever) down to 30 or something. But I have found with this battery on, my power level does not dip at all. So like, if it's boosting me over 125, then when the beams would drop it below 125 normally then they actually don't. I usually don't go below 105 power level to weapons when I have EPS + Emergency Battery + Emergency Power to Weapons, and that's with firing three beam arrays and one cannon at the same time on my cruiser. (Yes, I have a Phaser Cannon Mk II on the front of my Commander-level cruiser... yes, it lets you do that... I did it instead of turret because it's 180° but higher DPS, and having one less beam weapon makes the beams not kill my weapon power level).