Standard alternator vs Balmar Marine alternator with and without Victron current limiting devices. The original source for this video was from one of my RV blogs. This is presented as a planning tool for those considering converting to lithium batteries in their RV. Be aware that changes to your system will be required to make the conversion.
I know this video is from Victron and thus it is not unbiased, but I found it interesting in that it described some types of charging that are dangerous for alternators when connected to Lithium batteries. And while the Victron DC-DC charger is mentioned it is not the center of the tests.
Typical Lead Acid batteries (SLA) tend to have their internal resistance
rise tremendously as they approach a full state of charge - which means
they accept less and less current, and hence not overdrawing the engine
But LiFePO4 batteries do not, and will accept most of the current given them when charging - which is as you noted - can be very hard on stock alternators trying to pump out all that these batteries are willing to accept. On one hand this is one of the great benefits of Lithium batteries - they charge up must faster - especially beneficial when charging from a solar system or when running a generator. But as noted if and when connected to the engine alternator for charging it can over tax it.
This is why there are DC-DC chargers made. These devices specifically limit how much amperage charge is given to the house batteries - both by design as well as by switchable parameters in the unit itself. These are made in different amp ratings. I am considering a 20 or 40 amp Renogy model -
Amazon but there are many of these types of devices from other manufacturers-
These DC-DC chargers not only limit the amperage draw on your engine alternator, but they can also be set for your specific house battery chemistry, set to differing charging modes, and can be set to a maximum voltage, at which point it greatly reduces the amount of charge coming through it into your batteries - making life much easier on the alternator.
Someone mentioned that the downside of these is that they only transmit power 1 way - which is true - as they are designed to only allow current flow from the engine into the batteries. But you can still use the dash battery connect switch (found on many RVs) which connects the house batteries to the engine battery momentarily in order for the house batteries to assist a discharged engine battery (theoretically from not being used) - it depends on how the DC-DC charger is wired in. The dash switch utilizes a solenoid to connect the 2 battery systems due to the high amounts of current the engine starter draws. But properly wired the DC-DC charger should never be connected through that solenoid at all. They have their own on-off remote switching and when off they are automatically isolated from the solenoid connection switching.
BTW - I would like to have 400Ah of Lithium batteries. They would make all the difference in using my RV as I add a solar system, I hardly would have to use the generator any more, and they would really enabled us to boondock much more enjoyably than fighting with my old SLA batteries. Also saved several hundred lbs of battery weight too.
So it is good to recognize a potential problem when using Lithium batteries which may be connected directly to the engine alternator - possibly overly stressing it. But there are simple solutions, and the benefits of Lithium batteries are many.