Hi, I believe this is my first post here, so hi !
I've been enjoying the technical articles on this site for the last few months. Excellent, great work. They bring together a lot of technical knowledge in one place. In contrast to searching information on some of the big forums where really useful snippets of information are buried in threads that are many 100 pages long your well-written articles are fun to read with all the useful information in one place.
So, now a question: I have a 450 size electric helis and about 6 flightmax 3s 2200mAh (some 20C some 25C) batteries for it. Now I just finished building my first quadcopter with 4 DT750 motors. The first prototype frame I knocked together is made from solid wood and fairly heavy so the 4 motors draw a lot of current. In order not to have to buy new, large capacity lipo batteries I fitted the Quad with 2 XT60 connectors in parallel so that I could attach 2 of my 2200 mAH at the same time. I'm starting to wonder whether this is a good idea after all. Here's why: if I pick any two batteries out of my 6, they never will exactly have exactly the same voltage - even when straight out of the charger. So I have a slight voltage difference between the batteries (say up to 0.5 V) - I guess this will in effect cause one battery to charge the other one - until they're at equal Voltage, with a current between the batteries that will depend on the internal resistance of the packs. I wonder what Voltage difference between the packs is acceptable in order not to damage the LiPos. I know that the internal resistance of the packs will limit the current between the two packs, but I assume that this internal resistance is fairly low - otherwise the LiPos wouldn't be able to provide such large bursts of current.
Now, I have seen references to people using LiPos in parallel, but I haven't found a good technical discussion on how to handle this. Do you have to balance them during charging in a similar fashion as serial cells are balanced ? Any tips ?
BTW. I notice that there a three "Electric Power" sections in the forum, two for airplanes, one for helis. I would like to suggest that they could be merged into a single subforum as most things electric apply to both rotorcraft and fixed wings.