Copy and paste from what i posted on another forum:
Balancer is always a good idea for lipos, irregardless of how big or small they are.
In the case of airplanes, the lipos experience a relatively consistent amp draw, this is good in a way that they don't 'shock' the cells.
For cars, we have very different demands in comparison, our amp draw tend to have huge flactuations, going from low draw to massive surges in amps when we're exiting corners with full throttle, against the car's inertia and road resistance. This tends to put the lipos thru a much harsher 'lifestyle' which do tend to magnify the underlying micro differences in every single cell, which could promote an overall imbalance.
Another area to note, imbalance is almost gauranteed to happen when we subject the pack to over discharging. With the newer developments on voltage cutoff controls, chances are reduced, which is a good thing.
Chargers nowadays have came a pretty long way in their precision of voltage control and evaluation, which is critical for CC-CV charging, in other words, lipo. Certain chargers come with balancers built in as well, some chargers have circuitries which charge every cell individually, some requires an external balancer, some takes a feedback loop from their external balancer, etc.
Take your pick
2 packs will be a good number for a day's racing without overstressing the lipos.
Wait for the pack to cool off a bit, as long as it's not hot or spiking warm, you're ok to charge. For better longevity protection, it will be good for it cool to close to ambient temperature before charging.
You can charge your packs the day before your race, no need to repeak.
There are good cheap packs, there are lousy cheap packs, there are even lousy expensive packs, don't be surprised here. Pick something of 20C and onwards. It's better to settle for a real 20C claim than a 25C overclaim.
5 years of experience with lipos small and big, mine as well as my customers' taught me a lot about these batteries. i will share with you whatever u want to know, as long as i know it