I haven't checked out the specs on my factory alternator but if I get a 30A DC-DC charger then I only need 30A plus a little more to run the charger itself, and maybe a little to compensate for what I might lose due to cable length, right? Let's assume for the sake of argument that I have a 100A alternator (which sounds low - my 2004 Chevy had at least a 120A alternator) I'm pretty sure that even at idle it's going to give the 35 - 50A I need, right?
With a DC-DC charger you need to think in terms of power, not just voltage or current individually. A 30A DC-DC is probably actually specified as a 360 watt charger.
The benefit here is that the input and output are not directly related other than total power transferred. You can put a lower or higher voltage on the input and the charger will construct an appropriate output voltage and supplied current for the charge state.
Let's say you need 14.2V on the battery for a particular step (absorption for example) but the alternator is regulated down to 13.6V, be that by the car computer or just the internal regulator. But it's a 100A alternator, so you have plenty of shoulder room.
At 14.2V the charger will be capable of 360 / 14.2 = 25 amps into the battery under ideal conditions. It may do a full 30A but I'd suspect this is spec'd at it's minimum output. Doesn't really matter, it can be given different ways and you just have to read the spec on the model. If they say 30A at all possible output voltages then you need to assume the power you have is actually higher, say 14.8V (presumed max output) at 30A means the charger is rated 444 watts.
So you'll need to supply it at least 360 watts plus losses and efficiency. I'd guess the charger is probably around 85% efficient, meaning it'll put 360 watts into the battery but will take 360 / 0.85 = 423 watts input (the 15% being lost internally in the charger). In the theoretical I mentioned this would mean an input of 444 /0.85 = 522 watts. Like I say, you need to read spec closely to understand how best to use a particular charger.
For this example, including efficiency, the alternator at 13.6V will need to give it 423 / 13.6 = 31 amps to get the full capability of the charger. If you have less than 31 amps available what's going to happen is the charger will probably just current limit, using the power it gets to make 14.2V but at less current.
Main point here is you'll need 423 watts at the end of the cable.
That's where your losses in the cables will factor. Work backwards. If you have 0.5V of drop at 31A that means you'll only actually deliver 13.6 - 0.5 = 13.1 V to the charger input. You'll only being giving it 13.1 * 31 = 406 watts. The charger in this case will probably max at 24 amps (406 * 0.85 = 345 watts / 14.2 = 24.3).
The benefit here is that even at 13.1V the battery gets 14.2V. If you can reduce losses you raise voltage at the charger input for a given current. It's not a perfectly static situation. If you raise voltage or reduce drop, the current you need may very well decrease, thus reducing drop. Round-and-round you go, voltage, current, resistance/impedance and power are inter-related.
Your challenge is to find a cable size that doesn't drop excessively but is also cost effective. You might be fine with that much drop, it's lost as heat but it's not necessarily unsafe. You don't strictly have to run super fat no-loss cables with a DC-DC because drop is usually more of a nuisance and not always critical. The main issue I've found with DC-DC with a lot of input side drop is having them cut in and out if the losses put you under the shut down voltage. You want as little drop as you can achieve but you do not need to be obsessive.
Shut down is a trigger point where the DC-DC senses that the engine is not running and it should no longer pull energy from the source system. Continuing to do so with the engine off would of course run the starting battery down.
To solve this there's options. One is to lower the shut down voltage further, say 12.5V instead of 12.9V or something higher. This is not ideal because you will draw down the main battery each time you shut down, leaving it at perhaps 75% state of charge. That's enough to start most of the time but still putting unnecessary wear on it.
So you can size up cables, eliminate the drop.
Another option is to use a deterministic shut down, which is an ignition sense. If the DC-DC has this it's the best choice. What this does is tells the charger with a definite yes or no that it's OK to take power. In this case you could tolerate a drop as low as the minimum input voltage of the charger, which could be 11.something volts since you know with ignition hot the engine must be running and the drop is just in the feed, not a problematic draw on the starting battery.
The caveat here is that current must go up as voltage does down for a constant power. If you want the theoretical 423 watts delivered at 12.0V you'll need cables and a source capable of 35 amps to get 360 watts (e.g. 14.2V @ 25A) out of the theoretical DC-DC.
You don't want this if you can help it but it's technically OK. The best way is combination of ignition sense and a reasonable drop. With a 30A/360W type of charger something in the 4AWG to 6AWG will be fine. You might even get away with 8AWG but that could be pushing it on a longer run with a lot of drop. Probably safe still if you use a decent insulation, though.
Can a modern inverter generator charge directly to the LiFePo battery or does it have to go through a LiFePo specific 120vAC - 12vDC charger? The converter on the camper is NOT LiFePo compatible - what I've heard is that this means it will only charge the LiFePo batteries to 80% and in order to get the other 20% I'd need to go through a LiFePo specific generator that can produce the current necessary to fully charge the battery. And I have one of those, and it is a 30A model. So it seems to me that the battery is going to get 30A whether it comes from an inverter generator or a DC-DC charger.
A generator 12V output will probably have the same limitations as your alternator in this respect, so yes you're correct that the profile is unlikely to be ideal for charging. The generator having 120V gives you more flexibility in finding a good charger, that's what you'd be best off doing in that case.
Over on one of the RV forums I'm on I keep hearing people say "it's bad for the engine to idle an engine for too long" but that sounds an awful lot like some old wive's tale or the thing their grandpappy told their daddy back in the 1930's. Consider how many police/fire/EMS vehicles, construction vehicles, delivery vehicles, etc, sit idling for hours and hours at a time, not to mention the commuters who sit in traffic jams at idle or just barely above for hours at a time in most of our big cities. If the "it's bad for the engine to idle your vehicle too long" thing was true we'd see a lot more broke down vehicles than we do.
It's still not great to idle a gasoline engine like this. You want them running at their design RPM, roughly between 1500 RPM up to 4000 RPM. This is most efficient, e.g. peak power at ideal mixture. At idle you generally run slightly rich and this tends to dilute the engine oil as it washes the cylinder walls and creates carbon in the chambers.
Plus you'll be in park so there's no load on the engine from the drivetrain. It's not good to rev engines without being in gear, that's hard on crank bearings, the valves tend to float, etc. At idle it's not a big deal but just something to keep in mind.
Notice how a generator works. It might run at medium high idle or surge under light load but at rated load they peg to a fairly high RPM. But they get 1,000 watts from a small engine, it's all it has to do. You're asking a big engine to do almost no work, so there's a lot of wasted energy and wear and tear you don't need to do.
I think using a cop car as an example isn't great. They only last a few years and if you've ever owned one you know they are worked hard, put away wet. They're also often maintained on very regular schedules, like monthly oil changes, which you're unlikely to do. Also consider that a cop car is built with or upfitted with other things that make extended idling less harsh, like a cooling system that doesn't need air being pushed through it by moving down the road.
It's not like you're expecting to do this daily for the life of the truck, but it's also not going to be a rare situation. I'd personally prefer not to do this to my truck engine but if you follow a severe duty maintenance schedule it's probably not really any harder on the truck than you already are by pulling a trailer and such.
