Step 3: Battery and Charger
20mAh LiPo cells are available from a number of micro flying specialist shops, but they weigh a whopping 0.8g.
I was happy to find 10 and 8mAh cells at only 0.35 and 0.325g. They are available from Micro Radio Flier and MikroAntriebe.
I added some short leads with female Molex 1.25 pitch battery connectors. This connector is commonly used on batteries for Minium Kyosho, Blade and Vapor micro fliers, but I used an extra lightweight version as available from a.o. Aether Sciences. I used a precision soldering iron thin soldering wire and worked fast and decisively in order not to overheat the battery. I put solder about halfway the battery strips. I also pre-tinned the stripped wire ends before soldering them on. Then I cut of the excess strips ends, taking care not to short circuit the battery by touching both strips at the same time with the pliers. Finally I covered the bare strips with some tape, also covering part of the leads and battery to strengthen the assembly somewhat.
Rated at 10C, these batteries are not meant to deliver more than 80 to 100mA, so I was curious to find out how well they would perform. As described in the further steps, three motors proved to be too much, making the battery voltage drop too much and making the receiver cut off. But with two motors these batteries give 4 to 7 minutes of flying time.
Having no 20 mAh batteries at hand, I also experimented with the 10 and the 8 mAh batteries soldered in parallel (see last image). Flight time was well over 10 minutes with two motors. But with three motors the flying still came to an end quickly as soon as the three motors were used close to full power simultaneously.
Note on the charger: LiPo batteries should typically be charged at 1C, this means with a charging current equal to the mA value of their capacity (in other words, taking one hour to charge them). I used an Eggy Universal Charger from Aether Sciences with a minimum charging current of 10mA. I expect no problem charging the 8mAh battery at 10 mA. Actually I charged these batteries a couple times at about 60 mA (by accident) and it did not destroy them. I could however not fully charge them and their lifetime might be shortened.
Further lowering the mass: The 0.35 and 0.325g cited above are the masses when trimming the battery strips and sides. As delivered they are both close to 0.4g. I did not dare to trim the sides yet. I also did not shorten the strips on the 8mAh one to much yet, as soldering on short strips was rather hard on the 10 mAh one.
Plug-and-play alternative: order your battery with connectors already mounted. A number of specialist shops will deliver the 20mAh cells with 5 mm Bahoma magnetic connectors (also see Step 5 on the receiver).