From AR Drone Wiki
The battery is a 3 cells high grade Lithium-Polymer battery, with a capacity of 1,000 mAh at 11.1 volts and a discharge capacity of 10C (10 Amps). It weighs 105 Gram and it includes a PCM (Protection Circuit Module) which protects the battery against over charge, over discharge and short circuits. The battery is protected by a rigid casing, and complies with UL2054 safety standards.
It has two connectors: one discharge connector to provide current to the A.R. Drone(mini-Tamiya connector), and another smaller one for balance charging(JST-XH Plug). The balance charging ensures an optimal charge of each cell to provide a better capacity and longer lifetime. Refer to the Parrot logo hologram on the side to check authenticity. If you buy any non Parrot battery the two connectors is what you need JST-XH and mini or small tamiya. A normal Tamiya will not work.
Parrot states "The A.R. Drone works only with Parrot A.R. Drone battery" this is not quite true but as LiPo batteries are dangerous you should be very careful when using other packs in the A.R. Drone.
Our own Symon posted a battery safety 'note' Battery Handling
The battery pack is built from 3 battery cells, the plug for the charger has 1xGround and 3xPositive connectors.
The charger is a balance charger as the batteries are LiPo. The lights marked 1, 2 and 3 are the internal individual cells in the battery pack.
You'll find the lights will; over time flash in different orders depending on which has the lowest voltage and this lowest will be charged first, then the second lowest etc until all charged and the Status is green. Sometimes you will see 2 or all 3 LEDs lit indicating all cells are at the same voltage and are all being charged together yet separately in the Pack. This will continue in varying stages until status is a steady green which is indicating a charged pack. Should you get flashing Green and Red LEDs then this means the battery cell in under maintenance/conditioning. This is normal for LiPos to keep their lifespan.
The charger has to charge each cell in the pack separately as not to overcharge a single cell in the battery pack which for LiPos are very very dangerous as the cells will expand when overcharge then burst and catch fire.
If you notice any swelling whatsoever on the plastic then stop the charging immediately and get it outside in a fire proof bucket on on bricks/patio etc as you don't want to be holding the pack when it's at the swelling stage, in case it does burst.
The best way to pick it up at this point is to unplug the power from the charger and use the charger as the handling point. Try to keep the handling of the pack to a minimum as possible, a balance charger is easier to replace than a hand.
Should you have a fire from a Pack you will get multiple bursts as there are three cells in total and the battery may jump upon bursting, do not throw water over it that makes it a lot worse. The best home solution is sand but if you have a fire extinguisher if your at work etc then the best is a dry chemical extinguisher but this may only work well if the battery casing has broken apart.
Never leave the cells unattended to charge, ensure you charge them on a fireproof platform, Do not puncture the cells for any reason.
If you crash your A.R. Drone very badly, then remove the battery and sit it somewhere fireproof for at least 20 minutes with supervision and have sand or a dry chemical fire extinguisher to hand.
3rd party batteries
When using a 3rd party battery the issues are cost, weight, power, size, connector, quality.
Size - If the battery does not fit in the existing hull location you must do some warrantee voiding cuts. If it is taller it may change the flight characteristics and center of balance.
Weight - any non Parrot battery must compare its power verses the weight with the standard. How much extra flight time do you get for a specific weight. Does the weight effect flight performance unacceptably.
Power - how many volts for what weight. If too powerful does it warm the engines significantly?
Connector - Does it have a mini-tamiya ( also called small tamiya ) connector. If not are you replacing or using a conversion from that connector to the drone. Some people convert all their connectors to Deans or Xt-60 connectors.
Quality - Some low cost chinese batteries are not very well put together.
Cost - Hong Kong batteries are approx. $10. dhr will put more here after they finally arrive on their cruise from the orient. I hope they are well rested.
Real life tests
In Jan 2012 seadoodude posted this to the forums
It's easy to change the connectors or make an adapter. MaxAmps are good high C batteries that can charge quickly with a good charger. Best upgrade for the drone is a good charger and extra batteries. I got their AC/DC 6amp charger so I can charge out in the field from car battery or on house AC.
There are many brands to choose from. Best to find ones that will fit in the battery bay, 1000 to 2200 mAh and within 50 grams of the stock 110g stock battery with a high charging C rating. The absolute max is the 2600 mAh MaxAmp, but at 210g, it's on the heavy side and barely fits under the hood. It will fly, but noticeably sluggish. And with the extra weight not that much extra flight time. I posted test flight times in a Battery thread and it looks like the sweet spot is somewhere between 1300 and 1800 mAh if you can find one under 150 grams or thereabouts. If you want to carry other payloads, then stay closer to 100 grams, which are up to 1300, perhaps 1500 mAh.
Tests were conducted using the outdoor hull, inside at a steady hover.
Parrot 1000mAh, 110grams - 11:03
Bastens 1300mAh, 102grams - 14:26 (30% more mAh, 31% more time)
MaxAmp 2600mAh, 210grams - 22:39 (160% more mAh, 105% more time)
You usually get what you pay for in life. But it is tempting to get 4 cheapos over 1 expensive.
dhr - I will note that with a good charger I still never get 11 minutes from a stock battery.
Here is a publicly-editable spreadsheet full of batteries, with tested flight times and compatibilities. You can input batteries you're not sure will work or not and the spreadsheet will help you find out if they will fit.