Battery upgrades are very common. The best upgrade for you depends on what battery(s) you now have and what space you have to add new battery(s).
You can't add one 6v battery. Your battery system is 12 volt, either a single 12v battery or multiple 6 and or 12v batteries.
If you have a single 12v battery you can add another (or several) 12v battery in parallel with it. Parallel batteries work best if they are identical. If you add a new battery to an old battery the new battery will discharge into the old. The same is true if you add a larger or smaller 12v battery.
You can also replace your single 12v battery with 2 6v batteries in series. This is perhaps the most common battery replacement. If you decide on this, make sure you have enough room. The 6v batteries, golf cart batteries, are taller than the 12v batteries.
If you have two 6v batteries in series, you can add 2 more 6v batteries in series, in parallel to the 2 batteries you have.
Lots of options.
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