2s - the 's' stands for the number of cells in 'series' inside a pack; 'c' along with a number (25c for example) is the discharge rating.
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Anyway, a pair of good 2s lipos in series would be best to fit the 3905 ( I said good, not maxmaps...). If cheap and cheerful is your thing, then look into the Zippy items from hobby city, or the SMC lipos, or the TrueRC lipos even. Links to most of the big players on my website too- trucks links page. :)