Modern Vespa

I apologize in advance for the lengthy post that follows.

I have a Derbi Boulevard 150 a Vespa LX150. Both have the same Piaggio engine.

I recently noticed that the Derbi battery was not charging as quickly as the Vespa battery when both were put on the battery tender. A test of the voltage at the battery terminals while the bikes were running indicated that the Derbi was not charging the battery, reading 12.4 volts (essentially, the normal battery voltage) for the Derbi vs. 15.3 volts for the vespa,

I brought the Derbi into my local Vespa dealer in Denver for troubleshooting and repair. Initial diagnosis was a bad stator, which they replaced with a new Vespa stator. After installation, additional testing also indicated a bad regulator, which they also replaced with a new Vespa regulator. So far, the diagnosis sounded reasonable, as they needed a functioning stator output before they could determine that the regulator was also bad.

When I called to see when I could pick up the bike (still in the shop, work will be completed today) the service manager said that I had installed the wrong type of battery in the bike. He said that because the battery compartment has a vent tube, the bike requires a maintenance-type battery, rather than the maintenance-free battery that I have currently in the Derbi (I've had several batteries over the life of the bike, and the last couple have been maintenance-free). When I explained that my current battery was working well (I keep it on a battery tender between rides), the fact that it was not the original maintenance-type battery should not make a difference. I asked that he measure the charging voltage at the battery while the bike was running (and presumably charging the battery after the repair).
I was told that the charging voltage at the battery with my current (maintenance-free) battery was about 13.4 volts with the bike running, but that the voltage using a maintenance-type battery that they substituted for testing was about 15 volts. He suggested that I replace the battery for the charging system to work properly.
As an engineer, this explanation makes absolutely no sense to me, and I declined the offer to sell me a new battery. If a battery is the correct voltage and amp-hour rating (and it is, according to the recommendations for this bike), it should make no difference whether it's a maintenance-free battery or not, and the charging system output should remain the same for either type of battery.

This is a reputable dealer (I bought my Vespa from them), but I'm losing confidence when I hear this kind of explanation.
Am I wrong or is there something I'm missing?

Stupidest thing I've ever heard. Most of my clients, now run on Motobatt or Yuasa AGM batteries. Even Piaggio supplies AGM batteries (Yuasa, FIAMM, Piaggio) as replacements for their scooters that initially had lead acid ones...

Thanks for the reply, SaFis.

I'm picking up the bike tomorrow (without a replacement battery), so I'll get some first-hand measurements at that time (yep, bringing my meter with me!).

But as of my last phone call (3 hour ago) the tech is sticking to his guns that the charging voltage with my battery is only 13.4 V (he calls it "anemic"), but about 14.7 V with the YB9B maintenance-type battery that he says I should use.

My battery doesn't seem to have any problems. It's on trickle charge when I'm not using the bike, but it easily supports at least a full day of riding with multiple starts and stops, even with the bike's charging system not working.
Can you think of any battery conditions/problems that might cause the low charging voltage? If so, then I might spring for a new battery. Otherwise, I can't explain the "alleged" discrepancy.

Thanks again for your reply.

There's no difference chemically between AGM and flooded plate batteries. Your old battery may be getting weak, if it's more than a few years old or if it's led a stressful life lately because of the charging system problems, but there's no reason to replace it with an old fashioned unsealed battery. If your dealer is reluctant to sell you a maintenance free AGM battery, buy one on your own. There are lots of places that sell them. I buy mine at the local Batteries Plus store.

Fixed that for ya.

Comparing the on-charge voltage of an old battery with a new fully charged one is completely meaningless. You should put the old battery on your charger and observe the voltage as it charges. If it never gets above 13.4 volts while on charge then it does need replacing.

OK, so the Derbi needed stator/regulator - but the Vespa voltage appears way too high. A normal low-maintenance battery should have 14.4V as its bulk charge - an AGM is OK with 14.8V. Anything above 14.8V and you are looking at a bad regulator. So double-check that voltage!

As for any bike needing an old-style flooded battery just because a vent tube was originally fitted - that's bullshit. Be suspicious he's trying to sell you something you really do NOT need!

roadster, thanks for the info. I don't fully understand the physics of why a fully charged old battery (that appears to be working well) would charge at a lower voltage than a new battery, but I'll probably install a new AGM battery when I get the bike back, just to satisfy my curiosity.

jim, thank you for your reply. Your point about the Vespa charging voltage being too high is interesting. In fact, several years ago I replaced the battery with a larger capacity battery than the original (including the larger battery tray that the later Vespas came with), and I had to replace that battery again at the beginning of this riding season (even though it had been on a maintenance charge all winter). I don't know if the higher Vespa charging voltage shortened its life, but in any case, I could replace the battery at least five times for what it cost to get the regulator replaced at the dealership, so I think I'll just leave it alone for now.

Understood - but if and when your CEL comes on every time you rev above idle you'll know it's time for that new regulator (and yet another new battery).

Yes I did notice that your reading on the vespa was about 1 volt too high but you may just have an inaccurate voltmeter so I would try it on another vehicle first. My voltmeter is of reasonably good quality but if its internal battery is flat the readings go all over the place.