Awesome! Glad things are coming along.
Lithium Polymer (LiPo) batteries are funny things. The individual cells aren’t hard cylinders or rectangles, so much as vaguely rectangular foil sacks. Each sack is a cell, and each cell in a pack adds 3.7V to the nominal voltage. The cells come in a variety of sizes, and the size is generally proportional to the power capacity (mili-amp-hours) of the pack.
Below is a picture of three different pack sizes used by my lab (on a 1/2 inch grid):
Each pack has two cells stacked together (you can see the split) and therefore produces a nominal 7.4V. The power capacities of the three packs are (top to bottom) 2,000mAh, 1320 mAh, and 730 mAh.
You can use a 2 cell 7.4V or a 3 cell 11.1V pack with your motor and ESC. 3 cells will give you higher voltage, and increase the top speed of your motor, but won’t necessarily give you the longest flight time/weight. You may find that the limiting factor in your battery selection is the high current you need to draw.
The size (power capacity and physical dimensions) of the individual cells in the pack, not the number of cells, determines the safe discharge current. The picture above shows three different 2 cell packs from the same manufacturer, and the middle one can (safely) continually supply 17A, while the small one can only continuously supply 9.5A. In general, the larger the power capacity of the individual cells, the higher the current they can supply.
Now, if you do end up getting LiPo batteries they will come with a HUGE safety sheet, but here are some of the highlights:
Be nice to your batteries. Don’t bend/tear/puncture the cells. Don’t charge the cells too quickly, charge them higher than 4.2 volts/cell, or discharge them lower than 3 volts/cell. Doing any of these bad things can cause the batteries to swell up, leak, or (maybe) explode in a horrible fireball of death. You can and should get packs with protection circuits that will cut the cells off if they are being charged too high or discharged too low. You can also get separate low voltage cutoff circuits for your blimp.
Don’t short the cells! Aside from the danger of battery damage and explosion, there is the heat involved. LiPo’s can discharge huge currents when shorted, and as my 11’th grade physics teacher famously said “It’s the amps that make things hot.” I have heard of people shorting these things to watches or bracelets and getting horribly burned. One set of batteries I bought came with a warning that if you shorted one across a gold ring, the ring would vaporize and sever your finger. I don’t believe that for a second, but it would suck.
You will need a special charger for LiPo batteries, and even so, you’re not entirely safe. Depending on how “smart” your charger is, you may have to set the number of cells in the pack, and the charge current. Setting either of these two high can cause the afore-mentioned horrible fireball explosion.
You can also get special packs with extra leads that go to each individual cell, and a “balancer” (in addition to a charger) that will, well, balance the charge on each battery. I may get one of these for the helicopter robot I’m working on now, because they let you safely squeeze a little more power into your pack. I haven’t really felt the need for them yet though, and the special batteries and balancer are more expensive. Actually, everything having to do with LiPo batteries is expensive, but sometimes worth it.
Don’t let me scare you off of LiPo batteries with all this, they really are awesome. My lab has been using exclusively LiPo batteries in our robots for over a year now, and we have had some brief shorts and bent/dented a few cells without incident. The worst thing that has happened is that we ruined one pack by leaving it plugged in and switched on over a weekend. It drained down to zero (no cutoff circuit! Bad!) and got all swollen. We followed the safe disposal procedure of soaking the cells in salt water for a few weeks, then threw them away. That’s it, no big fireball.
This seems like a lot of power for a “small” blimp. What is “small” to you, or is it just going to be an incredibly fast, small blimp?