[Matt1995]

Looking for a battery for my 01 bullet 500. Is there a certain part number anyone has used from a Walmart or advanced that works pretty good?

[AzCal Retred]

There's an issue with the charging systems - the voltage regulator likes to regulate at 16V, not 14V. This boils out the water fairly rapidly.
Do you have a kickstart machine? Both of mine are KS only. I used a used Milwaukee Tools 18V Lithium battery to circumvent this issue, but a NiCad or NiMh would do the trick as well. Any 18V battery should work, it just needs 2-3 AH capacity.
I used spade lugs taped into place, with a little contact grease to preclude corrosion. The tool batteries are armored anyway, but I added some bubble wrap for cushion & tucked the whole mess away in the left hand tool box.
Now when the reg/rec parks at 16V, the 18V battery just doesn't care, it just says "well, come on..." and takes a partial charge. There's plenty of capacity for headlights, turn signals, horn & tail lights at idle for 5-10 minutes. Cooking your battery becomes a thing of the past. and, it's a bunch lighter besides being more vibration proof. Another positive is eliminating exposed terminals. I actually dropped a small tool on the (+) terminal of my old battery and shorted to frame ground. LUCKILY I had a screwdriver within reach to flip the tool free with. A coin or wad of keys getting in there while riding would create an ugly situation.
The stock reg/rec works great - at 16V - and adapting the battery voltage to it is way easier that re-engineering the reg/rec circuit. Putting a small battery in the toolbox means you get to toss the old clunky battery box, opens up the middle of the machine and eases maintenance & cleaning chores.

[mrunderhill1975a]

If you are looking for a kick start lead acid battery similar to stock, I believe you would look for a 7n-12 or a 7amp/hr 12 volt battery of the correct size. Although, AzCal method of using an 18 volt Milwaukee lithium is very intriguing, I don't have one lying around.  I wonder if a 18volt NiCd battery would work?

[AzCal Retred]

Yup. The OEM rec/reg on both mine flatlines at 16V. The 18V batteries are normally recharged at 20+ volts, so 16V doesn't chemically stress them. You lose some capacity, but mine have no other function than running the ignition & lights, so normal "engine off/lights on" times are under a minute anyway. I have some nickle metal hydrides from a Roomba that would fit right in. The tricky bit is finding a proper spade lug that fits their connectors, after that you just make sure you are on the right polarity. The lithium is nice because the weight is low; motor vibration does less harm to lighter objects. The bubble wrap helps too.