title_body: How can I safely parallel charge my LiPo batteries? I have multiple LiPo batteries that I want to charge up so I can get out flying as fast as (safely) possible. 
  
  With my existing LiPo charger, how can I parallel charge multiple batteries at once?
upvoted_answer: Only parallel-charge batteries with the same number of cells.
  
  Safe parallel charging requires that the voltages on individual cells be close to each other. A difference of 0.1V per cell is considered to be the maximum safe difference (I found it online, and have used ever since, although I have no scientific proof of that).
  
  One way to compare the packs before parallel-charging is to compare their total voltages and make sure that the maximum difference between any pair of packs does not exceed 0.1V times the number of cells. However, this may be unsafe due to the fact that some packs may have more unbalanced cells than others, so it is better to compare each pair of cells that will be charged in parallel.
  
  With many packs of uneven charge, it may be possible to divide them into groups whose cell voltages are close enough to be safe to parallel-charge and then charge each group separately.
  
  A parallel-charging board with enough protection is recommended, with fuses limiting the total current of each pack being the minimum. Some boards also have fuses or current indicators for each cell of each pack for more safety.
  
  Choose an appropriate charge current, which equals the sum of charge currents of all parallel-charged packs. Charging at 1C times the number of packs seems to be reasonable and safe if there are no large differences in capacities and cell degradation between packs.
  
  Regular safety rules of charging single LiPo packs apply, for example never leaving the batteries unattended while charging, keeping away from flammable materials, etc.