The other plug is the balance connector. This is used when charging on a 'balance charger' to ensure that the cells all remain at the same voltage level as each other, to help safeguard the pack from one cell dropping below the 2.9V minimum voltage.
The life time of these packs is hundreds of charges, way more time overall than about 3-4 seasons worth of race use.
A Mamba Monster set-up would be better, and if you go for the 2200kV combo, your current batteries (assuming you buy decent batteries like the ones I mentioned above) will work perfectly with that too. The 2200kV motor will want to run on 4S too, though you may have to change your gearing.
1 14.8V 5000mAh pack should get you 15 minutes run time on a track, or about 25 minutes when mucking around in a field.
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