Travelover
Elite Member
I don't know about the battery life effects, but too small a wire will not get you a full charge. Battery life is more affected by discharging too deeply (more than 50% on a deep cycle).
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how can I get "alternator only power" from truck?
- Thread starter jeffsw6
- Start date
- Views: 5797
I don't know about the battery life effects, but too small a wire will not get you a full charge. Battery life is more affected by discharging too deeply (more than 50% on a deep cycle).
I agree with the wire gauge, but not for the why. The large gauge is to minimize voltage drop at the trailer battery, not to keep the wire from melting. Lots of people wire their trailer charging circuits with light gauge wire. The result is the trailer battery doesn't charge properly, especially if they try to run the refrigerator on 12 volts at the same time. The wire doesn't melt, as it is only creating a couple volt drop at most.
The overiding factor in determining ampacity for 'chassis' wiring is derived from the heat generated. Voltage drop alone will yield a lower wire gauge. Put an undersized conductor in a cable bundle or under a carpet based soley on voltage drop and you risk a fire.
I don't know about the battery life effects, but too small a wire will not get you a full charge. Battery life is more affected by discharging too deeply (more than 50% on a deep cycle).
When you parrallel batteries without isolating them, the newer battery with the higher charge will be routinely overcharged and fail in very short order. This is why it is always recomended to change both batteries at once in a dual battery system.
Travelover
Elite Member
The overiding factor in determining ampacity for 'chassis' wiring is derived from the heat generated. Voltage drop alone will yield a lower wire gauge. Put an undersized conductor in a cable bundle or under a carpet based soley on voltage drop and you risk a fire.
OK, this is where I started from, as well. All our lives we learn to gauge wire based on making sure that it doesn't generate more heat in use than it can dissipate, other wise it will burn up.
If you just had say, a 12 volt coffee pot at the end of a long wire, no issue, base the wire gauge on the amperage that the appliance draws. When you have a battery at the end of a long wire you have another problem. The rate that the battery charges at is very sensitive to even small voltage drops. So, you can have a wire gauge that easily handles the amps, yet still drops too much voltage for the second battery to charge properly.
I'm not doing a very good job of explaining this, perhaps this will help:
Charging Trailer Batteries from the Tow Vehicle Alternator
buickanddeere
Super Member
Who ever said under charging a battery makes them last longer .......is dead wrong. A battery with 13.8 V applied to the terminals will last the longest. Some batteries do require 14.2V to fully charge in a reasonable duration of time. Temps above 77F, under charging, over charging, vibration and deep discharging all ruin batteries. A lead acid battery may say 100 amp hour on the label but you really don't want to use more than 25 AH worth before charging. As for battery isolates which use diodes, they sort of work and a simPle but..........loosing .7V doesn't sound like much until we consider 12V battery charging. The best working solution to get around somebody leaving the key on or running an engine out of fuel. Is to use an oil pressures switch and a HD relay.
The voltage-sensing relay device seems to do the same thing as an oil pressure switch would. It says the relay closes when the power supply goes over 13.4V and opens when it falls below 12.6V. DC battery isolator and split charge relay for RV, car, and truck applicationsThe best working solution to get around somebody leaving the key on or running an engine out of fuel. Is to use an oil pressures switch and a HD relay.
The fancy charging device has electronics to charge the trailer battery slowly, I guess by varying its output voltage, and it can step up a low supply voltage to whatever it decides is needed to charge the trailer battery. The maximum charging rate seems adjustable up to 7.5A which is small enough that it should not require a larger gauge supply wire. DC Input Battery Chargers for Lead Acid and SLA Batteries, for charging batteries from vehicles and other low voltage sources
So I will buy the relay and connect it to the 12V BAT black wire, and then connect the charger to the output of the relay. As long as the load of the charger doesn't cause the relay to repeatedly close and open (if the supply voltage fluctuated from > 13.4V to < 12.6V) it should work great.
Travelover
Elite Member
.......... The maximum charging rate seems adjustable up to 7.5A which is small enough that it should not require a larger gauge supply wire.............
You may want to measure the charging voltage at the second battery once installed to see if you are actually getting enough voltage to charge at the full, desired rate.
The charger has the capability to step up voltage. My only voltage drop concern would be if there was so much that the relay wouldn't stay closed. If I have that problem I will do what others have mentioned and increase the wire size for the black wire on the trucks.
I don't think it should be a problem. I used an ohm's law calculator that I assume to be accurate. It suggests with 36 feet of 12AWG wire and a 13.8V from the alternator, with a 20A load I would still have 12.66V at the relay, which is above minimum it needs to remain closed. My alternator voltage should be higher and wire run a little shorter than that, plus I will use something bigger than 12AWG from the trailer's spaghetti box to the relay.
I don't think it should be a problem. I used an ohm's law calculator that I assume to be accurate. It suggests with 36 feet of 12AWG wire and a 13.8V from the alternator, with a 20A load I would still have 12.66V at the relay, which is above minimum it needs to remain closed. My alternator voltage should be higher and wire run a little shorter than that, plus I will use something bigger than 12AWG from the trailer's spaghetti box to the relay.
Travelover
Elite Member
The charger has the capability to step up voltage. ............
Sorry, I'm a little slow. This is more sophisticated than I realized, thus the price.