Modern Vespa

OK - I wonder if someone could offer me some help on this.

I have a 2021 Vespa 150 GTS. I'm now on my fourth battery. I bought the scooter 6 month's old from a main dealer and the battery was playing up not long after I got it home (say a week). So I thought bugger it, I'll get a new battery.

So I purchased a Katana Motorcycle Premium VRLA Battery - YTX14-BS FA. This lasted for about 3 months and when I went 5 weeks without riding it, it was flat. So I charged it up on a fancy charger and it went on like this for 6 months, until the charger said the battery was faulty - it wouldn't hold any charge. The auto shop changed the battery free of charge and 6 month's later the same thing happened again. Fortunately, they changed the battery again and 6 month's on, it has started failing again.
I took the scooter to a Vespa specialist and they checked everything and said that the electrical system was ok, but I should go to lithium.
My wife has a similar Vespa and she can go a couple of month's without riding it and it starts perfectly every time.
Where should I start looking for a fault? (Apart from ride it more )

if your scoot is the same as what's in your avatar, then I'm assuming that there's a bunch of tat on it.

start with removing anything that is electrically extraneous.

if it's not stock, take it off. then add back while monitoring a DMM to see what component is drawing current.

then decide if you want to keep the culprit. and if so, just add an inline kill switch.

or just ride the bike once a week.

What Greasy said, or keep it on a battery tender when not in use.

Id add one more quick check.... in addition to the parasitic drag that they are suggesting above...it could also be that your stator isnt charging your battery... Id apply a multimeter to the battery and check what the leads are putting out while the engine is running, its suppose to be more than the volts of the battery.

See below .

Nothing that is attached to the scooter was on when the battery issue first started. The lights are wired, but are directly wired to the battery, thus not going through the wiring loom. They have a separate switch too.

The issue began well before any thing additional was added. So not really much help .

It's been checked out by the local Vespa specialist and they could find nothing wrong with it. They also serviced it at the same time, but held onto the scoot for a week to make sure there was no additional drain on the battery.

have you tested the stator output? what are the voltage readings cold & hot? before the regulator, after the regulator, and at the battery.

what are values are you getting at the stator itself (FSM has the procedure and values)?

beyond taking that deep of a dive, I'd just suggest a good quality battery and a tender if you're not going to ride it at least once a week.

Cheers Greasy. The Vespa specialist has recommended a lithium battery. The only issue I have with their advice, is that my wife has a 150 Primavera 2022 and hers is used the same as mine, but there is never an issue with her battery. I have to jump start mine from hers when it's flat .

The business I work for has a motor car racing division and the main technician said he'd do some diagnostic tests for me.

Thanks again.

Lithium can happily be recharged from near flat and not really suffer any damage, whereas lead tends to degrade quickly if the battery ever drops below 50% charge.

If a lead battery drops too low, and is damaged, it will never again return to a full charge.

Lead also self discharges, even with nothing on. They're not really made for often leaving for 4 weeks between uses.

Lithium is definitely the way to go for your use case, but I'd also try and increase how often you're using the Vespa, or install a trickle charging cable.

Cheers Jake .

And did they find any additional drain?

Nothing, they said everything was alright. 2 weeks later, the battery is flat.

have you thought about disconnecting the battery when you're not using the scooter? and ideally measure the voltage of the battery when you 'put it away'
just to make sure the battery is pretty much fully charged then you park your scooter.
It looks like you either are not charging the battery enough before parking it, or the charging circuit is overcharging the battery and killing it that way - unlikely if the Vespa shop has tested the charging circuitry and gives it a clean bill of health.

Switching the ignition off with the key, or using the 'kill switch'?

Hope you get to the bottom of this soon enough

fully charge battery
put a vm on battery

start pulling fuses 1 at a time till you don't see any more voltage loss.
now you know what circuit is causing the drain.

fix it