Modern Vespa

Early stages of this plan. I'm looking to add heated grips and a heated seat pad to my ET2. Grips will draw 20-50 watts and seat will draw 15-25 watts so I'm expecting a load between 3.5-6.25 amps.

The only data I've been able to find on the stator lists the output at 1.5-2.0 amps at 3,000 rpms. I've swapped the stock 35w halogen headlight for a 20w LED so that should give me an extra 1.25 amps to play with but I'm still looking at a pretty big deficit.

Since I need to find the extra power somewhere I'm thinking about running the heaters off an auxiliary battery with an isolator to protect the primary. I figure I won't be using the heat most of the time so I should be able to gradually recharge the aux battery on warmer days. As far as capacity, I'm looking at a 6Ah battery which should get me at least 45 minutes at peak load.

Any obvious flaws?

I'm pretty sure the ET2 headlight is powered by a separate coil on the stator than the rest of the electronics, and is AC, so the LED upgrade doesn't give you more regulated DC power for use elsewhere.

So your plan to power from an auxiliary source sounds like a good one.

You will have to charge the extra battery at home every few days. It would be easier to buy some heated gloves with rechargeable batteries.
Some handlebar muffs and a lap skirt will go a long way to keeping the cold away. Tucano urbano make some great stuff.