I have to make some assumptions to answer your questions; correct me if I am wrong on any of these:
1) You have a battery monitor with shunt in place already to monitor your batteries. A volt meter is NOT a good way to determine SOC in a working camper
2) You will have 2x 75AH or more of batteries. My experience was with 225AH of AGM batteries.
3) When you are at 50% SOC (the lowest you should take your batteries if you want them to live 3+ years) you will have 75AH left (not really usable), and 75AH to replenish.
4) You will drive for 2-3 hours to relocate to your next site, and want your batteries fully recharged in that time
5) Your truck's alternator (2011 F350) can output 60A or more for an hour or more without overheating
All that said, you can protect your alternator (and be ready for Lithium) by installing a DCDC charger between the truck and camper. These compensate for voltage/power loss and limit the current to the capacity of the DCDC charger. With AGM these are not necessary if the wire size (AWG) is thick enough to avoid more than 3% voltage drop. My Victron Orion limits charging current to 30A, which means we can use smaller guage wire too.
To calculate voltage drop we need to know the expected current over the wires. I saw 90A initial, 50-60A continuous with my AGM setup. If we size for 60A, then your truck can recharge your 150Ah battery bank from 50% to 100% in 75/60 = 1.25 hours. It will actually take longer than that because the AGM battery chemistry will resist charging as it fills, so the last bit of charging is actually quite slow.
So, you have two options... thinner wire and a DCDC charger, or no DCDC and thicker wire.
1) DCDC option - the converter is not 100% efficient, so to output 30A it will need 35-40A at the input. The distance from your truck engine bay to the camper is about 25' one way. The Victron "engine running" detection system is clever, and adjustable. Resting voltage of healthy AGM truck starting battery is 12.7V. 12.2V is 50% (at rest). Alternator running, the voltage output jumps to 14V or more. (Caveat: Smart alternators are another can of worms). So, imagine you are sitting down to start the truck, and have been using power all night. The camper batteries are depleted to 50%. Truck batteries are hopefully fine, since you have had no load on them overnight. The DCDC is monitoring the voltage input, and says... "hmmm, 12.6 input... truck isn't running, so I will NOT try to charge the camper" Now you start the truck, and the input voltage jumps to 14V or more. Now the DCDC says "Whoa, we've got good juice, let's fire up the charging process!" And it connects and starts to take 40A on the input side.... and if the wires are too small, the voltage drops too much, down to say 12.8V. (I can explain voltage drop too if you want). At that voltage, the DCDC says, "wait a sec, what happened to the juice?" and shuts down the charging process. As soon as the current drops back to nearly zero, the voltage pops up again the the DCDC repeats the process... off, on, off, on.... NOT optimal.
(By the way, this is the same issue we see with rigs using the stock battery separators and the stock 14g trailer wire... connect, disconnect, over and over, and not much charging happening).
So, how thick a wire do you need with the DCDC? Enough so that your 14V input does not drop below 12.8V when drawing 40A.
Let's start with 6 AWG and use an online voltage drop calculator (here's one -
Voltage Drop Calculator). Inputing the numbers above, that will result in a 7% voltage drop, and the DCDC seeing 13.02V at the input. Perfect. Trying again with 8AWG, we only get 12.44V on the input. This would result in the DCDC assuming the truck is NOT running, even though it is. So, no good. Just for giggles, if we change the AWG to 14, we get a laughable 7.8V on the input, and 44% voltage drop. No wonder our campers don't get charged using the stock setup!!!!
OK, so with DCDC, 30A max charge current, you need 6AWG.
Without the DCDC, the calculations are a bit different. You want to limit voltage drop to 3% or less. Looking at it another way, if you want to push a charge into the camper batteries, the voltage on their input needs to be higher than their voltage a present. So, if they are at 50% SOC, they are theoretically at 12.2V. The input voltage needs to be about 1/2 volt higher for current to flow, so 12.7. When they are close to fully charged at 12.6V, the input still needs to be 0.5V higher, or 13.1V in order for charging to happen.
Assuming the same 14V coming from your alternator, we need to see 12.7V on the input when drawing the max current we are aiming at. That was set at 60A above. I am going to suggest using at least
4 AWG in this case. Using the same voltage drop calculator, 60A nets us 13.07V when using 6AWG. Great, so it will start to charge. In fact, I still see 12.6V at 90A, so it may try to do that initially.
As the batteries get charged, their voltage rises. Using numbers from above, at 4AWG and 60A, 13.07 - 0.5V = 12.57V. That is the highest voltage you will likely get your camper batteries to using 4AWG without the DCDC. That is about 75% SOC. Playing around with the calculator, 2AWG gets you 13.43V on the input, or 13.43-0.5 = 12.93V. Fully charged!
So, without the DCDC, you need 2AWG.
A few more points:
- In both cases you need fuses/breakers on both ends, and a means to (dis)connect the truck camper. I used Anderson Power Pole connectors in the truck bed for that.
- DCDC protects the alternator, and has SMART charging parameters to properly charge the camper batteries.
- direct wires can charge the camper batteries faster, allow for solar panels on the camper to charge the truck batteries, and potentially for you to "jump" the truck using the camper batteries. Direct wires also have NO smarts, so you can potentially overcharge the camper batteries and boil them dry. I had voltage and amp meters in my truck cab when I ran this setup. I would disconnect the camper batteries when they got to 12.6V so as to avoid that, and then let the solar panels finish the charging process.
