New Battery

One thing that people might not know about non-acid batteries is that their amps an hour are much, much, much lower.

Example: YTX20-BS is 18Ah. The lithium I think go to 6Ah max and mostly 3-4Ah. When you hold the lithium battery, you think, "Oh my God, it's so light. The new technology must be marvelous." On the other hand, for some reason, they just don't include as much energy capacity in those batteries. So if they added all 18 amps an hour there, the battery probably would have been very, very expensive and also heavy, and it probably would burn really, really well. Therefore, there are always all kinds of trade-offs, and new batteries, unfortunately, are not a magic solution for everything.

So if you, for example, turned off your engine and kept the lights on, it means your lithium battery can drain really quickly. If you had, for example, an acid battery instead, that would not be a danger for a long time of having accessories on while the engine is off.

While I understand that maybe not many people run the accessories while the battery is off, all I'm saying is that 18 A/h provides a really good safety margin for energy supply compared with those newer batteries.

I think the idea here for this post is that sometimes people think a newer battery is better because it's newer, but they forget that it just doesn't hold as much energy in the first place.

A1A Rider, you’re right about the raw label numbers, but quoting 3Ah to 6Ah lithium batteries is a bit disingenuous for big bikes. Those tiny capacities are meant for 250cc dirt bikes or small commuter twins, not a Gen 2 Rocket 3. No one should be throwing a 4Ah battery into a 2.5-litre triple engine IMO, you're just asking for trouble.
For a 2.5L engine of the Rocket 3, you need proper sizing. For example, I run the Motobatt Pro Lithium MLX16UHD-HP. It’s a 10Ah battery that pushes 600 Cranking Amps (CA).
When you scale up to a properly sized 10Ah LFP battery, the original argument about missing a 'safety margin' completely falls apart because of usable capacity vs. nominal capacity:

• The AGM Reality: While a standard YTX20-BS is rated at 18Ah, lead-acid batteries shouldn't be discharged past 50% if you want them to live. Because of severe voltage sag under load, you only get about 8–9 usable Ah out of that AGM before the voltage drops too low to crank a massive engine like ours.
• The LFP Reality: Lithium Iron Phosphate (LFP) can safely utilize nearly 100% of its rated capacity while maintaining a perfectly flat, stable voltage curve right until it's empty. The 10Ah Motobatt actually gives you a true 9.5 to 10Ah of usable energy. There are other brands too that are suitable, Antigravity for example.

So, in the real world on a Rocket 3, a proper 10Ah LFP starting battery actually gives you more usable capacity and a better safety margin than an 18Ah AGM, all while dumping 600 Cranking Amps instantly.

The flaw in your argument is treating the AGM's 50% discharge guideline as a hard limit while assuming lithium remains usable down to nearly 100% (actually around 80%).

The 50% recommendation is about extending AGM battery lifespan over a long time use, not the point at which it stops working or should never approach.

Likewise, a lithium battery discharged to nearly 100% has little or no reserve left to start the engine.

Motobatt explicitly warns that lithium starting batteries have relatively limited amp-hour capacity and are unsuitable for applications involving substantial accessory loads. Motobatt positions itself as a racing battery, and race bikes would not have all these accessories it needs to power. Information - LifePO4 Lithium