OK, so our new Transit is going to have dual alternators and dual chassis batteries. This option is capable of delivering very high continuous currents (at least 150-200 amps) without any modifications. At the high end, there is a load-shedding signal that you are supposed to use to remove the external load if the chassis power system is overloaded.
The "traditional" way to utilize this system to charge a large lithium bank is to use a battery-to-battery (B2
charger, which is basically a charger that is powered by 12VDC rather than 120VAC. The bad news is that these devices tend to have relatively small current capacities (30A is popular). Larger ones are starting to show up, but are currently pretty pricy). The good news is that these devices can be used in parallel, so you can use multiple 30A devices to increase the capacity, though only in coarse increments. Some people have used as many as four of these in parallel.
The alternative, which seems to be gaining in popularity, is to forgo the B2B units, and instead use the alternator DC output to power a largish 12VDC to 120VAC inverter. These are readily available in 1500 - 2000 watt capacities and are much less expensive than the equivalent capacity in B2B. The idea is to use the 120VAC output of the "engine" inverter to power the "shore power" input of your coach's normal inverter/charger.
At first glance, this seems a little silly--going from DC to AC just to immediately turn it back to DC to charge the battery. But, the more I have thought about it, the more sense it makes. To wit:
--This is certainly less efficient, since an extra conversion is involved, with its associated inefficiencies. However, the penalty for this is only paid when power is coming from the engine, and (at least in the case of the Transit) is not at all in short supply. So, except for a tiny fuel consumption penalty (which seems insignificant), this really isn't much of a negative.
--If you position the engine inverter near the engine and the inverter/charger near the battery, you avoid having to run a huge DC cable long distances, since that part of the run is 120VAC, which needs only a much smaller cable.
--As I mentioned, the total cost of a high-current system is quite a bit lower, as is the required space.
--Most importantly, you have much more control of current consumption. A B2B will happily provide current to the battery right up to its rated capacity. With a large, empty battery, this could easily be unacceptably large. This is prevented by choosing a B2B setup with a total capacity within the safe zone of your alternator. This is fine, except that you only get very coarse adjustments (say 30A increments), and there is no easy way to throttle it back if, say, your alternator is getting overtaxed. With a second inverter, however, you can have very fine control. The reason is that most good chargers have parameters to allow you to adjust the maximum current it will attempt to get from shore power. So, for example, if you are plugged into a 15A outlet, it will not attempt to draw 30A to charge the battery. This very same feature works perfectly as a throttle to the alternator draw, since the engine inverter will not draw more current than is being demanded by the charger. This is a very appealing feature, and can easily be automated if you have a "smart" charger.
--What really tipped the balance for me is the fact that this approach centralizes the control of the charging profile at the main charger where it belongs. With a B2B, you have to coordinate the behavior of two (three if you have separate solar) chargers, each with different profiles. This can certainly be done, but the ability to centralize these smarts is very appealing to me.
So, I am close to being sold on this approach. Anybody see any major downsides that I have missed?
