Serial charging how-to
In a nutshell serial charging allows multiple same capacity packs to be connected in series in order to create a single higher cell count pack that retains the same capacity as the individual packs.

What does this mean in simple terms for say charging a pair 450 3s packs that you run in series on your TRex500 at the same time? You would take the pair of packs, wire them in series, set the charger to charge a 6s 2200mAh pack and poof you will have both packs done in a hour. Assuming you can charge at 1C and your charger can charge a 6s pack at 2.2A.

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Serial charging diagram
On the right there is a diagram of a typical serial charging setup. In this case there are 2 wiring adapters used, one for the main discharge leads and one for the balance leads. The adapters simply wire all the leads in series. The resulting pack is effectively a 6s1p 2200mAh pack where the main leads are tied together and all the balance leads are essentially stacked into a single connector, in this case a 6s connector.

Requirements and recommendations for serial charging

• Serial wiring adapter for the main leads that matches your charger's output capabilities. Because serial charging effectively creates a larger cell count pack, the capabilities of the charger will limit the number of packs that can be charged in series. For example a 6s capable charger can charge 2x 3s pack in series. Charging 2x 4s packs in series would require an 8s capable charger.
  
• A wiring adapter for the balance leads is basically required and needs to have the same number of connections as the main leads adapter.
• All the packs you charge in series must be the same capacity. For example you could charge a 3s 2200mAh lipo, a 2s 2200mAh lipo and a 4s 2200 mAh lipo in series together, but you can not charge a 3s 2200mAh lipo in series with a 3s 3200mAh lipo. 
• All the packs to be charged in series together need to be very close to the same voltage per cell. If I had to put a number on it I would say within .05V per cell.
  
• Serial charging is best suited for someone looking to charge flight packs made up of pairs of packs like 2x 3s 2200mAh packs used in a 500 sized heli.  

Potential benefits

• Huge time savings. Not only do you get multiple packs done at once but you also only have to hook everything up once.
• Allows multiple pack flight packs to be left connected and charged as if it was a single pack. 
• Can allow for faster charging than parallel if charger has limited amperage output but plenty of output wattage. 
• Unlike parallel charging, each individual cell is monitored and balanced separately by the charger.
  

Potential drawbacks and warnings

• If something were to go wrong during the charge cycle it would effect multiple packs. For example if a pack shorted internally and went off, it could set the other packs off as well.
• Packs must be connected in the locations on both adapters or a direct short can occur.
  
• If the packs connected in series are at different voltages, then the charge time may be greatly lengthened as the charger attempts balances all the cells.
  
• If the charger is not connected to the balance connector on each pack and the pack voltages are not exactly the same, this will result in one or more of the packs being overcharged. 
  
• Number of packs that can be charged at once is limited to the max cell count of each charger. So for example a 6s capable charger can charge a pair of 3s packs in series but not a pair of 4s packs. This comes into play when trying to charge pairs of 4s, 5s or 6s packs, as there are fewer models that will charge 8s, 10s or 12s lipos.

Sparky-sparky (warning) Serial balance charging introduces a wiring situation which can ruin a perfectly good set of balance leads or balance board. Great care must be taken to connect the proper leads to the proper places. If you switch any set of them, it will cause a direct short. This has nothing to do with the charger, it is all in the wiring after the charger and will occur both with and without being connected to the charger. What does this mean for the average guy serial charging? It means that he must understand exactly how to connect the packs properly and must be very careful how he connects them each and every time. What is the proper order? Looking at the diagram labeled "Typical serial charging setup". Notice how the upper pack is connected to the (neg)  output lead of the charger? That means it must also be connected to the (neg) side of the balance lead or board. Likewise the other pack is connected to the output (pos) lead and must be connected to the (pos) side of the balance lead or board.

Serial charging diagrams