Quote:
Originally Posted by Bill
Travis -
My experience with my old 2002 TM and 2002 Ford Explorer. The refrigerator is much closer to the TM battery than it is to the tow vehicle battery. That means that the current from the two batteries is unequal because of the unequal voltage drop in the wire runs. The refrig drew most of its current from the TM battery, with the result that when we arrived at a campground, the TM battery was dead. Annoying! I did a partial fix by installing an entire new run of #10, including a separate connector, from the Explorer's alternator directly to the TM battery (the Explorer's OEM wiring had quite a bit of #12, and several connectors, in the line). It helped quite a bit, but not enough....
One of our members did a much more elaborate fix by converting the nominal 12 VDC in the engine compartment to 24VDC, running a separate 24 volt wire back to the TM, and then converting it back down to battery charge voltage in the TM. That eliminated the voltage drop of course. He reported that it worked like a charm.
Bill
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Just a review for everyone else, you both know all of this.
While towing, at the TM's 12V distribution board (full of fuses), two sources of "+12V" power compete with each other (while power is being pulled away by the fridge at roughly 12A - it's a one-ohm resistance heater). "+12V" Battery Voltage will always be in balance on the board. At extremely low current, the Tow Vehicle would be preferred (with operating Voltage at about 13.5V). But, through really long wires and multiple connectors, with some of the wires quite small, the TV path suffers large and increasing 'Voltage Drop' as current begins to increase. The 10-AWG copper wire to the TM house battery can support much higher current, with lower Voltage Drop.
When the TM house battery is offering power of 100-110 Watts with a Voltage of 12.0 - 12.6V at the board, and at high current, some current WILL come from the higher-voltage TV path until 'Voltage Drop' on the TV path has increased to equal that Voltage along the battery path. But resistance increases so quickly on the thin wires, current is very small (under typical conditions). The TV is probably only 'contributing' 10-20 watts when the TM battery is above 70% charged.
But the TM battery gets drawn down pretty quickly. As it's supply voltage begins to fall (to perhaps 11.8 - 12.2) it's endpoint voltage (while supplying considerable current WITH some Voltage Drop) becomes less attractive at the board, and a larger proportion of current is pulled fro the TV. In a typical scenario, with decent but not 'exceptionally good' TV wiring, the TM batteries will be pulled down to about 50-60% SOC before the Tow-Vehicle path becomes dominant. The TM batteries should not be 'killed', but they will be unable to provide much power in camp. (However: with a badly compromised or broken TV 'trailer battery charge' wire path, TM batteries COULD be killed, because the Norcold fridge (running 12V mode) will never disconnect from 12V power due to low voltage.)
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Running 'high voltage with lower current' from the TV requires that a Solar Controller modify the voltage back down to a safe level (for the TM batteries to absorb). High Voltage does not go to fridge directly, or into the 12V distribution circuit board - it goes through the Solar Charge Controller, to the battery terminals, and from there it goes back to the 12V Distribution Board. (It also offers to charge the batteries, since much more more 140 watts is being offered. The batteries may or may not be in a position to accept a charging voltage above "Float".)
I currently use a boost to 36V, the other user of this modification (klpauba) is using 24V. When this circuit is active (disconnecting the genuine TM Solar Panels and connecting the Tow Vehicle 'trailer battery charge' instead, the TV appears as a single large solar panel.