It is recommended they all be the same, but don't have to be. Even the same labeled 2S could have different characteristics. The biggest part is monitoring cut off if one is a smaller mah. If they are all the same mah but diff C ratings you just have to worry about balancing. If you are just doing speed runs then you should be fine. You will have to balance charge each 2S or build a balance plug if you want to charge/balance as a 6s.
Less warning labels... more Darwin!
EZ Customs Paint / Xtreme RC
ON ROAD: XRAY T4 '15 USVTA/USGT X1 F1 , OFF ROAD RB6
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Dave Cates Founder of www.GO-RCing.org