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TowingDiscussions related to towing and towing products.
This is a discussion thread titled "Why do I have a large voltage drop from alternator to trailer battery?", within the Towing forum, part of the Technical & Vehicle Assistance Forums category.
Why do I have a large voltage drop from alternator to trailer battery?
I've read lots of threads that discuss problems encountered with charging trailer batteries while en route. It seems the source of this problem is typically wire of insufficient size in the tow vehicle, and that contributes to loss through resistance.
My 2005 Sequoia SR5 came with the factory tow package, and I looked underneath, and it looks like #10 wire going to pin 4 (the power circuit) and ground. I don't know if it's #10 all the way to the Sequoia battery or not, but I can see at least one connector right under the bumper.
Assuming a 20-foot run from the alternator to the hitch (40-foot roundtrip), I should only be losing 0.4V attributable to the wire resistance in the car. However, I am losing 1.3V between the alternator and the trailer battery with a 10 amp load on the trailer. Since the voltage at the car battery is about 13.6V, that pretty much maxes out all the power coming from that circuit at 10 amps. The trailer voltage measurement was taken at the battery leads with the batteries disconnected, which are about 20 feet behind the hitch.
Any ideas where I'm losing so much power? I cleaned the Bargman (the 7-pin) connectors....other owners of this trailer (Trailmanor) that have this problem seem to have isolated it to the vehicle, not the trailer.
I've read elsewhere that the recent Sequoias/Tundras have adequate wiring to deliver adequate power, but is that not the case? Will I have to run heavier wire to the hitch? If so, where should it be attached under the hood? I want to connect it to whereever the factory one is connected so the relay (i.e. battery isolator) is in the circuit.
Also, when I measure the voltage at the car battery, it is 14.1V as soon as I start the engine, but it gradually falls to 13.6V and stays there. Is that because the alternator isn't able to provide as much power after warming up?
Re: Why do I have a large voltage drop from alternator to trailer battery?
As stated there is a connection problem creating more resistance. Start at the power source and work your way through check the whole thing front to back.
If that does not result in a fix it maybe easier to run down grab some 10ga stranded THNN wire 1/4 wire loom and and run a new circuit.
Re: Why do I have a large voltage drop from alternator to trailer battery?
Thanks for all the replies!
Does anyone know exactly where I would connect a new wire? I know I could just connect it to the alternator or battery, but I at least want the relay in the circuit, so it has to be after that. It's difficult to tell exactly where the wires go since they are all bundled, and I don't have a wiring diagram. Anyone else do this before on Sequoia/Tundra and keep the relay in circuit?
Anyone know why the alternator voltage goes down after 20 minutes?
Re: Why do I have a large voltage drop from alternator to trailer battery?
Dave -
14.1 cold and 13.6 warm = good
Your battery and alternator are good shape. Voltage should go down as the battery warms up.
#10 wire has about 0.001 ohm per foot so 40 feet at 10 amps
is 10 x 40 x 0.001 = 0.4 volts (you're correct)
Connections typically have anywhere from 0.01 volt drop to 0.1 volt drop depending on current and corrosion. On old cars you figure 0.1 - on new figure 0.02 or so. But how many connections are there in the truck between the battery and the Bargman? 2? 5? 10? And how many in the trailer? If it were me, I'd file my meter probes to a sharp point so you can pierce the wire insulation. Then set the whole system up with all batteries connected and your 10 amp load with the engine idling. Start at the truck battery and read the voltage at as many points as you can find in that whole string back to the trailer batteries. You may find a considerable drop between point A and point B telling you where your major loss is. Or you may find it is pretty much equal amounts all the way down the string = no major culprit. I think 1.3 drop in 80 feet with 4-8 connections is not surprising. But the voltage reading at the Bargman should tell the story. If it is more than 0.7 below the truck battery then the truck wiring/connectors are the major contributor.
Re: Why do I have a large voltage drop from alternator to trailer battery?
Yes, moisture trapped in the copper strands can cause corrosion and compromise the conductivity of the wire over time. Unfortunately that method of testing is needed sometimes, so I make sure to dab a tiny amount of paint or silicone on the hole when done.
Re: Why do I have a large voltage drop from alternator to trailer battery?
I have a 17' Casita travel trailer with the battery about 15' from the hitch. Your voltage drop to the 7-way plug (if I am reading your post correctly) is not out of bounds. The problem is that you do not have "just" 10 ga wire running to the 7-way from the alternator. You also have a relay, and (hopefully) a 30 to 50 amp fuse. The Casitas have the same problem keeping the trailer battery charged on the road especially when we run our refrigerators on 12V. This is due to the additional resistance caused by the extra 30 ft of 10 ga wire. The problem is not actually the voltage (current) that is transfered to the trailer battery but the fact that V=IR. This means that the voltage regulator in the alternator "senses" the additional resistance as part of the charge on the trailer battery and just stops sending current before the maximum voltage is transferred to the trailer battery. Modifications that have solved this problem are adding a 4 ga charging circuit from the truck battery to the trailer battery or just moving the trailer battery into the engine compartment (yes, I have seen this mod on a Tundra). Emergency vehicles (ambulance, fire trucks) solve this problem by having two alternators under the hood - one dedicated to charging the deep cycle batteries and very large gauge charging circuits.
As suggested, check the charging circuit especially in the engine compartment to determine how much voltage is actually being sent to the 7-way. Also, check the age of the deep cycle trailer battery. Lead-Acid deep cycle batteries wear out fast especially if they have been drained more than 50% repeatedly. I just replaced my trailer battery with a Cabelas AGM deep cycle. Expensive, but no fluid to check and the AGM is more tolerant to numerous drain/charge cycles.
If you want to add a charging circuit: attach it directly to the battery, not the alternator and remember to put a 30 amp fuse in the circuit. There are relays that can be added that will turn this high amp circuit on only when the ignition is on. Look at instructions for installing trailer electric brake controllers for more details.
I did not know that a battery isolator circuit came with the Tundra tow package. This would be yet another voltage drop in the circuit to the 7-way. I just unplug the trailer at the 7-way when stopped.
Re: Why do I have a large voltage drop from alternator to trailer battery?
Thanks for the replies. I had not thought about puncturing the wires to test voltage, and although I am certainly not crazy about the idea, I'm not sure how else to narrow down the bad connection since the load has to be connected when checking. And even if I do find that I'm losing power at a connector, then what? I guess I could solder that connection, but now I'm thinking it would be less hassle to run a heavy gauge wire (maybe #6 or #4) and solder it to the exiting #10 wire just a few inches from the Bargman. A few inches won't be that much resistance, and I don't want to break the weather tight seal on the connector. But even that isn't ideal.
RE: the isolator in the Sequoia....I know it's there. Check with a voltmeter to see if you have any power there when the ignition is off. I don't. It only is there with the key in the "On" position. I was under the impression that is what the relay did....when there is no power coming from the ignition, the switch in the relay opens to interrupt power from the truck battery back to the hitch. Sure I could put another relay and fuse in line with the new wire, but I'd prefer not to do that since it's already all installed.
For trailer batteries, I have two Trojan T-105 6v batteries less than 1 year old. They are true deep cycle batteries, unlike the "deep cycle" RV and marine batteries you see in many auto parts stores. The latter batteries are more of a hybrid, as they still have to be able to supply enough power to run a starter. My batteries don't, and as a result, can be regularly discharged past 50%. I've never taken them below that, but I've read you can regularly go as low as 20% without sustaining any damage. There is a 12v Everstart Trolling battery at Wal-Mart that falls into the same category, and apparently is quite good, but it is called a trolling battery, not a marine deep cycle battery.
Excellent idea! I was going to suggest one of those but I did not have any links or info to give you for an RV application. We use DC-DC units where I work for different applications (electronic controls).
A little pricey unless you order 500+ of them LOL.
Re: Why do I have a large voltage drop from alternator to trailer battery?
Upon closer inspection, the above DC input charger isn't going to work because it's output maxes out at 7.5A. Back to thinking....
Based on my calculations, I'm going to need #4 wire to supply a 15 amp load in the trailer, and that's going to be about $150. Ouch. Plus then I'll have 2 wire harnesses....
Re: Why do I have a large voltage drop from alternator to trailer battery?
Yes, I saw those. However, I don't see how they would work because they are not current limiting. Thus, if the trailer battery has low voltage (low charge), it is going to draw, and the converter will deliver, a very large amount of current at 13.8v. That could damage the battery, cause off-gassing, excess heat, or a combination of any of those conditions. And I also don't see how I can prevent it from overcharging the battery, either.
I called the company today to see if they had any suggestions. They didn't recommend using the DC/DC converter for my application. If the charger had a higher output, it would be perfect, but it's not.
Why do I have a large voltage drop from alternator to trailer battery?
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