The Trex 500 was actually built with the intention of pilots with T450's to utilize their existing 3s 2200 lipos in series to fly the 500. A great marketing idea for those wishing to have a larger electric helicopter, without the need to purchase new lipos or a new charger.

Many pilots have done this without the need for bigger batteries and chargers. However with this hobby many always want more power. So many guys use larger 3s 2650+in series, while others opt to use just one 6s 2650+ lipo. The price of chargers that can charge 6s lipos have come down since the first 500's had been released, so many just use one single battery.

If you decide to use 2 batteries in series, it is recommended to use batteries of the same specs, same brand, same capacity, disharge rate etc. Ihave added a couple of photos of series and parallel adapters for the use with 2 batteries.

For batteries wired in series, you increase voltage, and parallel you increase capacity.

Hope this helps.

Aloha,
Les