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if you look at the current through the charge wire from an electrical stand point, you will see that the current through the wire is a variable based on the voltage difference between the plug (truck side) and the coach battery.
As the delta between the two decreases, so does the current. So in reality, the coach battery will never see a full charge when hooked to the truck. In theory, it will see the same state of charge as the start battery on the truck, but in reality there is some resistance and voltage drop at the rear plug so the state of charge on the coach is Limited by the voltage available at the plug - which will always be less than the alternator voltage....
Understood. The goal of this experiment is to get an idea of much current the +12V circuit can supply when the resting battery bank voltage is low (<12 V), the alternator is hot, and the engine is running at road speed.
I discharged the 158 Ah AGM battery bank (2x Group 24) down to 12.27 V open circuit (measured after a 4 hour rest), which is an ~50% state of charge.
After running errands and getting the truck engine up to full normal operating temperature, I plugged in the batteries and had my wife hold the engine speed steady at ~2000 rpm.
The camper battery voltage increased to 12.56 V, and the truck was supplying 6.3 A to the batteries.
Considering the small voltage difference between the batteries and the alternator regulator (<2 V), this may be about right. So, I’m glad that we have the 200 W PV system on the roof of the camper.
I must confess that I forgot to take readings at idle speed. I’ll do that when I do my next test in about three weeks, which will include running the 3-way fridge on DC.
The camper batteries are now hooked up to a bench charger.
No way. My 7 pin outputs a good 14.x volts to the trailer once I finish doing the Hokey Pokey and the truck recognizes the trailer.
IIRC, our truck does this as well if the camper batteries are fully charged. I’ll (try to remember to) test this condition once my camper batteries are recharged (my bench charger / maintainer is slow).
My (limited) knowledge on all this would lead me to expect a constant voltage (CV) within reason, but a variable current. So 14.4 volts observed at the trailer battery, may only be 3 amps. But in your case, with a discharged battery, I would expect both voltage and amps to be much higher.
My experience with multi-stage battery chargers is the bulk mode operates as a current source, and the battery voltage rises towards the target absorption voltage as the SOC increases. With limited current available from the truck’s +12 battery charge circuit, I would expect similar behavior.
I don’t know what impact (if any) the trailer light module might have on all this.
I was surprised at the low current reading. Frankly, I wonder if I used my new clamp-on ammeter Ac/DC correctly, although I did have it in DC mode, and I believe I zeroed it correctly. I may have to set up an experiment with a shunt and compare the results.
I dug out one of my old PV system test fixtures and hooked up a 1.0 Ohm 300 W resistor and a 50 mV 500 A shunt in series to a 12V AGM Group 34 battery.
Based on V/R =I, I was expecting ~12 V / 1 Ohm = ~12 A of test current
The voltage drop across the shunt was 1.2 mV, so (1.2 / 50) X 500 = 12 A.
I wonder if it would be helpful to agree on a test standard with a given battery voltage and then with the seven pin plugged in and volts and amps measure at the batteries. That might give us a range. I haven't seen 12 amps at my batteries.
This morning I hooked my Fluke clamp meter to the charge lead off my truck at my distribution panel and used alligators to my standard Fluke to track voltage. I also measured voltage at the truck battery. My camper battery was offline for part of the trials using my disconnect, but any parasitic loads were still active as I kept the charge lead connected to the distribution panel (i.e, propane detector and small plug in digital voltage meter were still active, .5 amps). Nothing else was turned on.
Voltage at the truck battery remained constant throughout at 14.1 VDC regardless of load Having measured it over idle, 1,000, 2,000, and 3,000 RPM and when loaded by the batteries plus parasitics or fridge on DC setting plus parasitics.
At beginning of run camper batteries were at 12.4 VDC
Closing disconnect switch charge lead voltage dropped to 13.25 - 3.47 VDC and incoming amps were 4.6 - 3.5.
This step added only the batteries to the load. Incoming amperage was the highest at idle and above idle went from 3.8 - 3.5 @ 3,000. RPM. 4.6 reading may have been before alternator was fully warmed up
Next step was to open the disconnect switch so current was coming only from the truck charge line. Putting fridge on DC setting resulted in a charge lead reading 12.5 amp/11.5 VDC. Adding furnace to load resulted in reading of 14 amps, but incoming voltage of 11.35 VDC.
So can truck alternator keep fridge cool on DC from charge lead alone? Sort of, but with reduction in wattage. However, if charge lead and batteries are employed together cooling should be improved. The DC element does operate continuously in DC setting rather than cycle.
Readings may vary by truck, equipment, connections, as well as combinations and permutations of the preceding.
I ran some additional tests. With no load, I measured 14.00 V at the truck’s 7-pin connector.
With a 0.75 Ohm load (close to the 0.68 Ohm load of the 3-way fridge’s DC heater) plugged in without a battery, the truck delivered 16 A (a 1.6 mV drop across a 50 mV / 500 A shunt) with an 11.57 V drop across the resistor.
A 16 A circuit current resulting in a 2.43 V drop means 0.152 Ohms of resistance in the wiring and connections. That’s roughly equivalent to 152 ft of 10 AWG wire.
That’s pathetic.
We have a 10-day camping trip planned stating in mid-September. I plan to collect our camper from its storage spot in mid-August, and I’ll run some more tests with the truck and the PV system supplying power.
We want to be able to do more boondock camping, and we’re planning to go to Alaska next year. We’ll need to be able to reliably run the fridge and charge the batteries. Accordingly, I’m going to start looking into using one of the high-current upfitter switches to run a separate low AWG +12 harness to the camper batteries.
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