Well, yes and no… I don’t think that an LFP battery will take more than say AGM during bulk. But the AGM will quickly begin to limit the current when it hits absorption, and this is why high output alternators are much better with LFP. It’s not that the LFP can take higher current, it’s that it can take whatever current until it’s full, and not start limiting current at 65-85% SOC and make the alternator run for hours at low output. So if the B2B is sized right, you still gets this advantage with LFP. The B2B only charges when the alternator is supplying sufficient voltage, so the lead doesn’t charge the LFP really, it all comes from the alternator. Now of course the best setup is an LFP compatible smart regulator, but if you keep the lead start battery and a dumb alternator the B2B from lead works well if it’s sized to about 50% of the dumb alternators output (that’s all it can reasonably do anyway).Interesting, indeed. I'm new to the topic of LFPs, but I have some thoughts. I thought one of the great benefits of LFP was that it could take advantage of all of the output of high-output alternators, and in so doing charge up very quickly, reducing engine run time for charging. If you're charging the AGMs with the alternator you lose this advantage. If you're charging LFPs with AGMs, what advantage is there at all? You're just shuffling energy from one store to another with the consequent transfer inefficiency.
If you have a high output alternator with a smart regulator then yes, your alternator should be fine and you could use the B2B from the LFP to the AGM. Dumb alternator not so much…Well, the system should be designed to protect against alternator overheating, especially with a high-output alternator; the smart regulator should do this; and, the BMS will protect against LFP overcharging, which is where the BMS disconnect comes from, no? In fact, I'm told some BMS's, or systems built with them, can avoid the disconnect by reducing or ending the alternator output (via field current control, presumably).
Not so?
BUT one very common misconception is that the BMS will protect against overcharging. The VAST majority just protect against overvoltage NOT overcharging. Protecting against overcharging is up to you in your system setup. Unless you have a tricked out BMS setup like a fully integrated Victron system. And yes SOME BMSs have a Field Control Circuit so protect the alternator before disconnect, but that is not for normal control… more like an airbag… the alternator regulator should handle charge control in most setups.