The short run times of most radios today can severely limit even the average hobbyist. Many of the radios ship with something like a 500-800mAh 9.6V NiCd pack in them and that will only power the radio for a few hours at the most. Then it takes 12 or more hours to recharge. This shortcoming can become very annoying to those using their radio on a simulator, as you can not charge it while you use it. This has lead many people to wish for much more run time from their radios. Read on to find out you can increase your radio's run time.
The first and most logical option is to change the battery to something with a larger capacity. There are in fact many choices for higher capacity batteries, some are direct battery replacements and others will require a bit more.
Many stock tx packs are NiCds. They work fine but are very limited in capacity. One option for extending run time is to have several packs. Another option is to charge these packs more quickly on a proper charger in order to speed up charging.
Some radios comes with NiMH packs but they can be used in others as well. They offer much greater energy density and that allows them to come in sizes upwards of 2600mAh, or 3-5 times greater than many stock packs. They are the same physical size as the NiCd packs and will charge with stock charger. Of course they are not with drawbacks. One is their high self discharge rate, which is said to be 5-10% in the first day and 0.5-1% for each additional day. This is usually not a problem for most but some fliers may not like this. They can also have bad memory effects and this can wear them out far quicker than other types of batteries. One fix for the high self discharge rates is to run low self discharge cells such as Sanyo Eneloop cells.
Some people have used regular round A123 cells in their radio but I can not see how they would fit. And if they did, they would not contain as much energy as other options. So even though this has been done, I can not see it as an attractive option.
LiFePO4 flat cell pack
There aren't very many LiFePO4 packs out there other than A123 but recently some flat cell packs have become available. These look like standard lipos but do not share their chemistry. Hyperion has released several packs made for use in radios and they have a couple of very cool features. First they are 9.9V nominal packs, so they match up will with the voltage of the stock NiCd and NiMH packs. Secondly they have built-in circuit to balance and limit their voltage. This allows them to be charged in the radio with the stock radio charger. Of course you can also remove them and charge them with a proper charger at much higher rates.